EP4164895B1

EP4164895B1 - A method of removing a wax-resin adhesive from the canvases of wax-resin li ned paintings, a cleaning mixture and an organogel for use in this method and the method of producing this organogel

Info

Publication number: EP4164895B1
Application number: EP21756082.0A
Authority: EP
Inventors: Klaudia KANIEWSKA; Marcin KARBARZ; Elzbieta PILECKA-PIETRUSINSKA
Original assignee: Muzeum Narodowe W Warszawie; Uniwersytet Warszawski
Current assignee: Muzeum Narodowe W Warszawie; Uniwersytet Warszawski
Priority date: 2020-06-12
Filing date: 2021-06-14
Publication date: 2024-01-31
Anticipated expiration: 2041-06-14
Also published as: EP4164895A1; EP4164895C0; PL4164895T3; WO2021251840A1; PL434299A1

Description

The present invention relates to a method of removing of wax-resin adhesive from canvas, protected with it, nanocomposite organogel used in the method of removing of wax-resin adhesive from canvas protected with it in the conservation process and a method of producing this nanocomposite organogel.
The basic substructure of painting (canvas) plays the role of the carrier of the paint layers in in the easel painting and it is an integral part of it. Such a substructure, known as a support of painting, made of canvas, most often linen, was commonly used in easel painting since 16^th century. The canvas on the reverse of painting is exposed to long-term destructive effects of numerous factors, such as ultraviolet, air pollution and the growth of microorganisms, as a result of which its mechanical features are weakened.
Restoration of damaged canvases, especially the method of strengthening and securing the support's fabric, is a very important issue in the field of conservation and restoration of works of art. One of the most common methods of securing damaged paintings in Europe was the method of attaching a new canvas to the back of the existing one (so called lining of painting or relining) using an adhesive made of beeswax and natural resin. This method, also known as the Dutch Method, was popularised at the end of the 19^th century by Dutch restorers Johannes Albertus Hesterman and his sons Frederik Coenraad and Johannes Albertus Jr., however its development is attributed to the restorer Nicolaas Hopman and his son Willem Antonijo. The Dutch Method ensured hydrophobicity of the support, simultaneous strengthening of the old canvas and paint layers, and immunity to microorganisms. The wax-resin adhesive was also used for securing paint layers and attaching strengthening strips of canvas, as well as for consolidation and creation a moisture barrier on the reverse of wood supports and for transferring paint layer from wood support to canvas [1].
The wax-resin adhesive usually consists of: natural or bleached beeswax and natural resin, in various proportions. The traditional recipe (Hopman Jr., Hesterman) contained by volume: 4 parts beeswax, 3 parts rosin and 2 parts Venetian turpentine [2]. A wax-resin adhesive was also often used in the following proportion: 6 parts of beeswax, 4 parts of dammar gum and 1 part of Venetian turpentine [3].
The Dutch Method was criticised in the 1970s, mainly due to the issues considering darkening of painting and colour changes in paint layers [4], but also due to the increased weight of the lined painting, low adhesive strength, brittleness of the wax-resin adhesive, acidification of the canvas (as a result of the addition of rosin), and as a result, acceleration of the aging process of the canvas by destruction of cellulose. For this reason, in accordance with modern knowledge in the field of conservation, it is recommended to remove the old wax-resin lining that have ceased to fulfill their protective function, and replace them with new lining layers properly strengthening the canvas. Performing the new lining procedure, however, requires complete removal of the old lining and wax-resin adhesive from the reverse of the canvas. For example, the National Museum in Warsaw has many paintings that require an urgent replacement of lining layers.
The lining procedure is reversible because the wax-resin adhesive has a relatively low melting point (60-65°C) [5]. Removal (subtraction) of the lining canvas attached to the original with the use of wax-resin adhesive is an easy procedure thanks to the poor adhesion of this material to the fabric. It is much more difficult to remove the excess of wax-resin adhesive remaining on the reverse of the canvas, because the adhesive applied while warm seeps the painting in the entire volume [6].
Various methods of removing wax-resin lining adhesive are known in the art. One of the methods considers to soften the adhesive and then drain it by heating (e.g. ironing through Japanese tissue paper) or by using a vacuum or low-pressure table [7]. This procedure is sometimes complemented by mechanical treatment, e.g. using a scalpel, rubbing with sawdust [8]. An extraction procedure is also used, with a solvent or a mixture of solvents being applied in the form of a compress to the treated canvas. Often, the restorer develops his/her own procedure, combining several methods, e.g. solvent extraction alternating with heating, draining using filter paper and mechanical treatment. Unfortunately, multiple heating of the painting and poorly selected solvent are detrimental to the painting, while the mechanical treatment often destroys old and weakened canvas.
Among the known methods of removing the wax-resin adhesive from the reverse of the painting, the best results, in the opinion of restorers, can be achieved using the solvent method with use using trichlorethylene (so-called TCE), applied on cellulose-based carriers (e.g. lignin or sawdust) [9]. The use of other solvents (e.g. petroleum ether, kerosene/odorless Mineral Spirits, white spirit) shows a low surface efficiency of removing wax-resin adhesive. It should be noted that the organic solvents used so far in the process of removing wax-resin adhesive, especially TCE, are harmful to health. TCE has a strong narcotic [10], carcerogenic [11] and mutagenic [12] effects, therefore its use is not recommended. TCE is included in the list of chemicals for which occupational exposure limits are established [13]. TCE should only be used with proper ventilation, which is difficult to ensure when working with large-format paintings and when working in the field. Also, TCE has a low boiling point (87.2°C), thus, it evaporates quickly at room temperature, which increases solvent consumption, and the extraction procedure needs multiple repetitions, which causes a risk to the restorer. However, no effective method of removing wax-resin adhesive has been developed so far, the effectiveness of which would be comparable to the effectiveness of the use of TCE.
There is a well-known study by Maria Roznerska and Hanna Wójt from 2002 containing a detailed description of traditional methods of extraction and removal of wax-resin adhesive from the reverse of paintings, as well as the results of their own research aimed at development of new effective methods of extraction of wax-resin adhesive that would be safe for the restorer and works of art [14]. Organic solvents and mixtures of organic solvents were tested (i.a. turpentine, toluene, xylene, petroleum ether, kerosene, white spirit, trichlorethylene, benzene, dichloromethane, dichloroethylene, ethanol), which were applied on the reverse of the painting in the form of lignin, cellulose, sawdust and gel compresses (i.a. Klucel M, Tixogel), and then removed gently or by rubbing. The tests were carried out on prepared samples of wax-resin adhesive of various compositions (wax, dammar gum, mastic, elemi resin, Venetian turpentine, rosin), as well as on the real painting, where all use of solvents was complemented by heating and extraction of wax-resin material using a low-pressure table. It has been shown that gels soaked with solvents are more effective in removing wax-resin adhesive than the solvents themselves or the use of the solvents together with lignin and sawdust. Unfortunately, the presented method is not fully effective, because after the extraction was completed, it was observed that in the hollows between the weaves of the canvas an unextracted fraction of wax-resin adhesive remained, together with a white powder being a residue left after the evaporation of the gels. Moreover, it has been noticed that the extraction with use of gels is not fully controlled in terms of the amount of solvent used and the depth of its penetration.
The Department of Conservation of Painting and Polychrome Sculpture at the Nicolaus Copernicus University in Toruń uses a method of wax-resin adhesive extraction considering the use of a mixture of kerosene, ethyl alcohol and xylene. Unfortunately, the mixture of solvents used does not allow for proper full cleaning of the reverse of the painting. For this reason, the treatment using solvents is complemented by suction on a low-pressure table with use of absorbent blotting paper [14].
There are known various attempts to use gels soaked with mixtures of solvents for extraction of wax-resin adhesive, often complemented by the use of a low-pressure table. The use of soft gel carriers usually gives better results than the use of solvents alone, but the undoubted disadvantage of these methods is the contamination of the reverse of the original canvas with remains or fragments of gels used.
The selection of a gel carrier for the organic solvents used causes further problems. Organogels usually do not absorb large amounts of solvent and mainta in appropriate mechanical properties. Organogels made of polymers (e.g. polystyrene) cross-linked with multifunctional monomers (e.g. divinylbenzene) are known, which are able to absorb large amounts of organic solvents, but show low flexibility and low mechanical strength. On the other hand, hydrogels, usually showing excellent mechanical strength, absorb large amounts of aqueous solvents [15], but do not absorb organic solvents due to the high hydrophilicity of the material.
There is known a highly viscous Klucel M gel (hydroxypropyl cellulose) with a mixture of solvents such as xylene/isopropanol 1:3, kerosine/isopropanol 1:3, as well as toluene/ethanol 3:2 [16]. There is known a cellulose ether Ethylcellulose T-200 with a solvent mixture white spirit/xylene 1:1 [17]. There are also solvent gels developed by Richard Wolbers [18], which work well for removing layers of wax or natural resins [19] in the treatment of cleaning paint layers. There is known a PVA-borate gel able to uptake up to 20% of 1-propanol [20]. There is also known a 73PVAc gel, capable to uptake large amounts of solvents such as acetone, ethyl acetate, ethanol, propanol. However, all these gels have the disadvantage that it is difficult to remove their residues from the weaves of the canvas after removal of wax-resin lining adhesive.
There are also known gels which are capable to uptake mixtures and emulsions of water-containing solvents. For example, the hydrogel [semi-IPN p(HEMA)/PVP] accepts an oil-in-water microemulsion (water, sodium dodecyl sulfate, 1-pentanol, propylene carbonate, ethyl acetate), but does not accept non-polar solvents [21]. There are known hydrophilic acrylamide gels that are capable to uptake oil-in-water microemulsions [22]. There is known Tixogel (organophilic bentonite derivative) uptaking a non-polar solvent with a small amount of polar solvent [23]. However, some of these gels leave residues on the canvas in the form of a white coating.
The use of organic solvents and emulsions on gel carriers is also known in other areas of conservation and restoration of works of art. There are known methods of removing wax (without the addition of resins) from murals. In 1985, at Westminster Abbey Chapter House, wax layers were removed with a 4:1 mixture of white spirit/isopropanol using cotton wool and hot air [24]. In the Brancacci Chapel, an oil-in-water microemulsion was used, constituting a dispersion of dodecane in an aqueous solution of ammonium dodecyl sulfate (surfactant) and 1-butanol (co-surfactant) [25,26]. The successful treatment using such emulsion in removing wax from the porous surface was also achieved in combination with an agar gel (polysaccharide of natural origin) [27].
There are known methods of extracting synthetic polymers used in conservation of the art department, developed in Florence by a group led by prof. Piero Baglioni. An oil-in-water microemulsion based on an alkyl polyglucoside (APG) and alkyl polyglucoside sulfosuccinate (AGESS) was developed to remove acrylic resins (e.g. Paraloid B72) from the surface of murals [28]. The use of oil-in-water microemulsions (p-xylene in water) embedded in hydrophobically modified hydroxyethyl cellulose (HMHEC, 2 wt.%) was also tested [29]. During the research, magnetic acrylamide nanomatics (nanomagnetic sponges) [30] and gels based on polyvinyl alcohol cross-linked with borates were also developed [31]. For the removal of wax-resin lining adhesive (e.g. Plextol B500, Mowilith DM5), the researchers tested a hydrogel with partially interpenetrating polymer networks (semi-IPN p(HEMA)/PVP) based on poly(2-hydroxy-ethyl methacrylate) with polyvinylpyrrolidone (PVP), loaded with an emulsion consisting of water (73.3 wt.%), sodium dodecyl sulfonate (3.7 wt.%), 1-pentanol (7.0 wt.%), propylene carbonate (8.0 wt.%) and ethyl acetate (8.0 wt.%) [22]. While examining the properties of this hydrogel, Prof. Piero Baglioni with his group suggested the possibility of using it to remove waxes [26].
The research conducted by the group from Florence is complemented by the research carried out by Aurelia Chevalier-Menu in 2010, who tested the use of laser and nanogels based on methylcellulose and polyvinyl alcohol (PVA) and networks based on polyacrylamide cross-linked with N, N'-methylenebisacrylamide to clean linen support form lining adhesive materials, such as Plextol B500, Plexisol P550, Plextol B500 (ethyl acrylate copolymer and methyl methacrylate), Mowilith DMC2, Mowilith DM5 (copolymer of vinyl acetate and n-butyl acrylate), Beva 371 (product based on vinyl ethyl acetate), lead white containing oil coating and flour glue [32].
There is known a system containing partially hydrolysed polyvinyl acetate with borax, which is considered to be a very promising gel for the use in conservation. It can be a good carrier for water and water mixtures of organic solvents (e.g. ethanol, isopropanol, n-propanol, benzyl alcohol, acetone) in various proportions. It is used in the process of cleaning the surface of painting, but it does not work well on a porous surface, which disqualifies it's use to remove wax-resin lining adhesive from the weaves of the canvas [33].
Unfortunately, the existing methods of removing wax-resin lining adhesive from the support are insufficient for conservation purposes. The classical chemical method using TCE is effective, but poses a risk of losing health by the restorer. On the other hand, other methods, including methods using gel carriers, are significantly safer, but without the need for heating, draining or mechanical removal of residues arising during the extraction process, they do not provide sufficient efficiency.
The goal of the present invention is to provide a new method of removing wax-resin lining adhesive form the reverse of paintings, which would be free from the deficiencies known for the prior art. The priority is to eliminate the currently used solvents of high harmfulness to humans, such as TCE, while maintaining the safety for the antique paintings. The specific goal is to develop a new method of extracting a wax-resin adhesive, using mixtures of non-toxic solvents immobilised in a gel carrier with high mechanical strength, ensuring safety for the work of art and reduced harmfulness to humans.

Summary of the invention

The invention is defined in the amended claims and relates to a nanocomposite organogel comprising a cleaning mixture for dissolving a wax-resin lining adhesive and removing it from the canvases of wax-resin lined paintings, containing organic solvents, is characterised in that it contains an alcohol component, a hydrocarbon component and a ketone component forming together an AHK mixture (alcohol/hydrocarbon/ketone), which is chemically inert towards polymerised oils present in the painting and has physicochemical properties determined by Teas parameters: fd = 67÷69 (dispersion force), fp = 11÷19 (polar force) and fh = 15÷21 (hydrogen bonding forces), characteristic of the area common to waxes, natural resins and synthetic resins.
Preferably, the cleaning mixture contains 30-40 vol.% of the alcohol component, 40-50 vol.% of the hydrocarbon component and 15-25 vol.% of the ketone component. Preferably, the mixture contains 30-40 vol.% of isopropanol, 40-50 vol.% of isooctane and 15-25 vol.% of acetone, preferably 35 vol.% of isopropanol, 45 vol.% of isooctane and 20 vol.% of acetone, which corresponds to Teas parameters of: 68.8-12.0-19.3. Alternatively, the mixture contains 30-40 vol.% of ethanol, 40-50 vol.% of isooctane and 15-25 vol.% of acetone, preferably 34 vol.% of ethanol, 47 vol.% of isooctane and 19 vol.% of acetone, which corresponds to Teas parameters of: 68.2-12.2-19.6. Alternatively, the mixture contains 30-40 vol.% of methanol, 45-55 vol.% of isooctane and 15-25 vol.% of acetone, preferably 30 vol.% of methanol, 50 vol.% of isooctane and 20 vol.% of acetone, which corresponds to Teas parameters of: 68.4-13.0-18.6. Alternatively, the mixture contains 30-40 vol.% of isopropanol, 40-50 vol.% of n-hexane and 15-25 vol.% of acetone, preferably 35 vol.% of isopropanol, 45 vol.% of n-hexane and 20 vol.% of acetone, which corresponds to Teas parameters of: 68.8-12.0-19.3. Alternatively, the mixture contains 30-40 vol.% of isopropanol, 40-50 vol.% of isooctane and 15-25 vol.% of cyclohexanone, preferably 37 vol.% of isopropanol, 42 vol.% of isooctane and 21 vol.% of cyclohexanone, which corresponds to Teas parameters of: 68.7-11.8-19.5. Optionally, the mixture contains water in an amount not exceeding 10 vol.%, preferably not more than 5 vol.%, in regard to the volume of the AHK cleaning mixture (alcohol/hydrocarbon/ketone).
One aspect of the invention is a nanocomposite organogel pNIPA-LAP-AHK containing nanoporous polymer matrix pNIPA-LAP with a structure based on poly(N-isopropylacrylamide) containing Laponite XLS (92.32 wt.% of Mg_5.34Li_0.66Si₈O₂₀(OH)₄Na_0.66, 7.68 wt.% Na₄P₂O₇) as crosslinking agent, filled with the AHK cleaning mixture (alcohol/hydrocarbon/ketone) described above.
The method of manufacturing the nanocomposite organogel pNIPA-LAP-AHK, is characterised in that the process of free radical polymerisation is carried out where Laponite XLS is mixed with water and sonicated until a clear solution obtained and further mixed with N-isopropylacrylamide, and then the mixture is stirred and subjected to deoxidation in an ice bath for ca. 1 h followed by the addition of the accelerator of the free radical polymerisation process, and the solution obtained is transferred to the molds and left to react, and after 24 h the hydrogel obtained is several times rinsed with water to remove the unreacted reactants, dried to constant mass and then inserted into the AHK (alcohol/hydrocarbon/ketone) cleaning mixture described above and left to swell, to receive the pNIPA-LAP-AHK organogel. Laponite XLS is used in an amount of 20-150 mg per 1 ml of water, preferably 60 mg per 1 ml of water. N-isopropylacrylamide monomer is used in a concentration of 0.5-2.5 M, preferably 1 M. As the accelerator of free radical polymerisation process TEMED (N,N,N',N'- tetramethylethylenediamine) is used in an amount of 5 ml per 1 ml of solution and as an initiator of free radical polymerisation sodium persulfate is used in a concentration of 2 mM.
Another apsect of the invention is a method of removing a wax-resin lining adhesive from the canvases of wax-resin lined paintings, considering dissolution and removal of the adhesive with a cleaning preparation containing a carrier and a solvent, is characterised in that to dissolve and remove the wax-resin lining adhesive the pNIPA-LAP-AHK organogel described in claim 1 is used, comprising the AHK cleaning mixture (alcohol/hydrocarbon/ketone) described above, where said mixture is immobilised in the nanocomposite polymer matrix pNIPA-LAP described above, with shape corresponding to the portion of the treated canvas, being the support of the pa inting, wherein the organogel is applied on the surface of the treated canvas and is allowed to stay in contact with the reverse of the painting for at least 20 minutes to extract the wax-resin ad hesive towards the cleaning mixture immobilised in the organogel, and then the organogel is being removed from the surface of the treated canvas and subjected to regeneration by its immersion in the fresh portion of the cleaning mixture to wash of the extracted wax-resin adhesive of the organogel, and then this procedure is being repeated using the regenerated organogel until the wax-resin adhesive completely removed from the reverse of the painting, even from the hollows between the weaves of the canvas being the support of the painting. According to the invention, the pNIPA-LAP-AHK organogel is applied to the reverse of the painting being restored, preferably more than 5 times, each time for a period of 30-60 minutes. Preferably, the duration of the application of pNIPA-LAP-AHK organogel to the reverse of the painting is variable, preferably longer in each consequent iteration. Preferably, the pNIPA-LAP-AHK organogel applied to the reverse of the painting is additionally covered with Melinex film (made of Polyethylene terephthalate having low permeability and high dimensional, thermal and chemical stability) to reduce the loss of the cleaning mixture due to evaporation, and additionally load it with a weight to increase the contact surface of the organogel with the cleaned canvas. According to the invention, the cleaning mixture contains optionally water in an amount not exceeding 10 vol.%, preferably not more than 5 vol.%, in regard to the volume of the AHK cleaning mixture (alcohol/hydrocarbon/ketone).
The invention guarantees a reduction in the harmfulness and toxicity of the process of removal of wax-resin adhesive in comparison to the classical solvent methods, thanks to the use of non-toxic solvents and immobilising them in a gel carrier. At the same time, the preparation is safe for the cleaned canvas and paintings. A single local application of the preparation allows to remove wax-resin adhesive from the reverse of the painting without the need for high temperature or vacuum treatment, which could harmful the work cleaned work of art. The invention is described below in details with reference to the drawings:

Fig. 1

presents the Teas diagram with the black outlined area of overlapping parameters of natural resins, waxes, synthetic resins, on the basis of which the composition of the cleaning mixture used in the invention is selected;

Fig. 2

presents the synthesis of pNIPA-LAP-AHK gel;

Fig. 3

presents the images of pNIPA-LAP gels:

A) gel containing water,
B) gel dried at room temperature,
C) gel soaked in POA mixture (isopropanol/isooctane/acetone),
D) the structure of the gel after lyophilisation (SEM image);

Fig. 4

presents images of pNIPA-LAP-POA gel in various perspectives;

Fig. 5

presents results of testing the mechanic properties of pNIPA-LAP-POA gel:

A) elongation diagram of pNIPA-LAP-POA gel and photographic documentation,
B) compression diagram of pNIPA-LAP-POA gel and photographic documentation:

Fig. 6

presents the stages of the process of extracting the wax-resin adhesive from the canvas using pNIPA-LAP-POA gel:

A) view of the gel soaked with POA mixture before the application to the canvas,
B) view of the gel soaked with POA mixture applied to the canvas (extraction),
C) view of the gel soaked with POA mixture after 1 hour of extraction,
D) view of the gel after regeneration in the fresh portion of POA mixture;

Fig. 7

presents the results of the study of reproducibility of the process of extraction of wax-resin adhesive from the canvas with use of a single piece of pNIPA-LAP-POA gel:

A) view of the canvas after extraction in 9 places using one piece of pNIPA-LAP-POA gel duration of a single extraction: 1 h, duration of regeneration in POA mixture: 1 h,
B) zoomed view of the canvas before the extraction (optical microscope, x25),
C) zoomed view of the canvas after the extraction (optical microscope, x25);

Fig. 8

presents the results of the study of the rate of the process of extraction of wax-resin adhesive from the canvas with use of a single piece of pNIPA-LAP-POA gel:

A) view of the canvas after extraction in 7 places using one piece of pNIPA-LAP-POA gel duration of the extractions: 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min,
B) zoomed view of the canvas before and after extraction (optical microscope, x25);

Fig. 9

presents the results of the treatment using pNIPA-LAP-POA-H2O gel containing POA mixture with the addition of 5 vol.% of water, extraction time 1 h:

A) general view of the canvas after the extraction,
B) zoomed views of the canvas before and after extraction (optical microscope, x25);

Fig. 10

presents the results of the tests and the subsequent stages of the process of extracting the wax-resin adhesive from the canvas using pNIPA-LAP-AHK gel containing cleaning mixtures with alternative compositions: MOA (methanol/isooctane/acetone), PHA (isopropanol/hexane/acetone), POC (isopropanol/isooctane/cyclohexanone):

A) view of the gel soaked with cleaning mixture before the application to the canvas,
B) view of the gel soaked with cleaning mixture applied to the canvas (extraction),
C) view of the gel soaked with cleaning mixture after 1 hour of extraction,
D) view of the canvas after extraction using gels with 3 different cleaning mixtures;

Fig. 11

presents the results of a study of the resistance of cellulose canvas against the treatment with cleaning mixtures, verified by comparative infrared spectroscopy measurements; the reference measurement was carried out on canvas not treated with solvents (the reference spectrum was marked with a red line):

A) FTIR spectrum of the canvas soaked in POA mixture, 30 min,
B) FTIR spectrum of the canvas soaked in POA mixture, 6 weeks
C) FTIR spectrum of the canvas soaked in isopropanol/white spirit 1:1 mixture, 30 min,
D) FTIR spectrum of the canvas soaked in isopropanol/white spirit 1:1 mixture, 6 weeks

Fig. 12

presents the results of a study of the resistance of aged oil paints (pigment + linseed oil) against the treatment with POA cleaning mixture, verified by comparative measurements of gas chromatography - mass spectroscopy (GCMS); the reference measurement was carried out on paints not treated with solvents:

A) GCMS spectrum of lead white paint,
B) GCMS spectrum of lead white paint treated with POA mixture, 10 minutes,
C) GCMS spectrum of cyprus umber paint,
D) GCMS spectrum of cyprus umber paint treated with POA mixture, 10 minutes.

Fig. 13 presents the results of a test removal of wax-resin adhesive from the reverse of oil painting "Zinnias in a Blue Vase" using pNIPA-LAP-POA organogel:

A) view of the face of the painting before cleaning,
B) view of the reverse of the painting before cleaning,
C) view of the face of the painting after removing of wax-resin adhesive,
D) view of the reverse of the image after removing of wax-resin adhesive,
E) subsequent stages of applying the pieces of organogel on the reverse of the painting
F) illustrations of the cleaning process: view of the reverse during clean ing, view of the used organogel, view of the applied organogel, cleaned canvas;

Fig. 14 presents the results of a test relining of the reverse of oil painting "Zinnia in a Blue Vase" cleaned with use of pNIPA-LAP-POA organogel:

A) view of the face of the painting: before cleaning, after cleaning, after relining,
B) view of the reverse of the painting: before cleaning, after cleaning, after relining.

Detailed description of the invention.

The goal of the present invention is to design new means for the extraction of wax-resin lining adhesive, being an alternative to toxic trichlorethylene currently used, i.e. to design new cleaning mixtures and new carrier exhibiting high elasticity and high mechanical strength, capable to uptake large quantities of organic solvents.
The effectiveness of the use of cleaning mixtures employed in the present invention was compared to the effectiveness of solvents commonly used in conservation practice. Solvents such as petroleum ether, white spirit, STARWAX wax remover, Shellsol HNC-04 [34] as well as TCE were tested. Such tests were also carried out with two-component mixtures of solvents, such as petroleum ether/acetone (4:1), white spirit/isopropanol (1:1), white spirit/ethanol (1:1), isooctane/isopropanol (1:1), white spirit/isopropanol (4:1), isooctane/acetone (2:3) and then their effectiveness was compared to trichlorethylene (TCE). Composition of cleaning mixtures of the present invention has been designed on the basis of the analysis of the literature, conducted discussions [35], simulations and scientific experiments. According to the restorers' practice, solvents of a significant toxicity has not been taken into consideration, i.e. toluene or xylene, and solvents with strong and long-term retention, such as turpentine, ethylene glycol, dimethyl sulfoxide (DMSO), and those requiring elevated temperature (heating) or the use of low pressure table.
The tests have been carried out using small samples of the lining canvas from the 1947 lining of painting "Battle of Grunwald" by Jan Matejko. The canvas was soaked with a wax-resin lining adhesive. Its composition was known from conservation documentation: beeswax, rosin, Venetian turpentine in a volumetric proportion of 10:10:1 [36].

Searching and testing the cleaning mixtures for extraction of a lining adhesive

During the search of the alternative solvents, multi-component solvent mixtures were tested. Their proportions were designed to receive mixtures capable of simultaneous dissolving of waxes and natural resins. For this purpose, the methodology developed by Jean P. Teas in 1968 [37] was employed, which assumes that each solvent is characterised by three parameters determined mathematically on the basis of Hansen parameters. Teas' parameters define dispersion force (fd), polar force (fp), and hydrogen bonding force (fh) [38,39], the sum of which is constant and equals 100 [37,40]: $fd + fp + fh = 100$
Although the Teas' methodology is not a perfect tool [41], it allows to predict the properties of novel solvent mixtures to some extent [42].
The Teas' method was used to develop three-component mixtures capable of dissolving waxes and resins simultaneously. Commonly used available interactive applications were used: "Modular Cleaning Program" [43] and "TriSolv" [44]. The analysis of the Teas graph allowed to determine the parameters of the cleaning mixtures in the range safe for paintings, so that their properties cover the area common to waxes and resins, beyond the area of polymerised oils which can be found in paintings. According to the invention, the mixtures show the value of dispersion force fd = 67÷69, polar force fp = 11÷19 and hydrogen bonding force fh = 15÷21 (Fig. 1). Unexpectedly, it turned out that the developed non-toxic cleaning mixtures exhibit parameters similar to the parameters of TCE (68-12-20) and other currently used toxic solvents, e.g. chloroform (67-12-21), dibutyl ketone (67-16-17), 1,2-dichloroethane (67-19-14) [39].
Potential three-component mixtures were preselected, the use of which would ensure effective removal of wax-resin adhesive and an increase of the conservators' safety. The components of the mixtures were selected from cheap, low-toxicity organic solvents commonly used in conservation and chemical laboratories (e.g. ethanol, isopropanol, hexane, isooctane, acetone, petroleum ether, white spirit, etc. [45]). Toxic solvents were excluded, even if their effectiveness in removing the wax-resin adhesive (e.g. xylene, toluene, etc.) was known. The search was conducted with the aim of maximising the affinity of the cleaning mixture to the components of the wax-resin adhesive, containing hydrocarbons (including fatty acids, alcohols and esters) having mainly hydroxyl, carbonyl and carboxyl groups. Three-component AHK cleaning mixtures containing alcohol, hydrocarbon and ketone components were selected for further research.
Two three-component mixtures containing components from the classes defined above were preselected: ethanol/isooctane/acetone (EOA) and isopropanol/isooctane/acetone (POA), which compositions were designed to meet the requirements of Teas' parameters (Table 1). Then, an experimental verification of the earlier predictions was performed in order to determine the optimal composition of these ternary mixtures. A number of compositions of the ternary mixtures corresponding to the boundary and intermediate values of Teas' parameters were selected (Table 2) and tested in the removal of wax-resin adhesive.

Comparative tests of extraction of wax-resin lining adhesive were carried out according to the procedures traditionally used in conservation of paintings to determine the effectiveness of EOA and POA mixtures in comparison to classically used solvents. A four-layer sheet of lignin moistened with a solvent was placed on the canvas and covered with Melinex foil. The procedure was carried out for 60 minutes. Wood sawdust was not used since mechanical removal of the residue from the hollows between the weaves of the canvas would damage the weak fibres of the fabric.

Table 1. The compositions of ternary cleaning mixtures AHK capable of dissolving wax-resin adhesive, selected using the Teas' methodology. Composition is given in volumetric precents.

mixture	ethanol	isopropanol	isooctane	acetone
EOA	30-36%	-	42-48%	15-22%
POA	-	32-40%	41-46%	17-24%

Table 2. Exemplary compositions of preselected alternative ternary cleaning mixtures AHK capable of dissolving wax-resin adhesive, with the given values of Teas parameters.

symbol	composition in volumetric precents	value of Teas' parameters
symbol	composition in volumetric precents	fd	fp	fh
EOA	ethanol (11%), isooctane (40%), acetone (49%)	67.0	17.7	15.4
EOA	ethanol (25%), isooctane (43%), acetone (32%)	67.0	14.7	18.2
EOA	ethanol (16%), isooctane (43%), acetone (41%)	68.0	16.0	16.0
EOA	ethanol (30%), isooctane (44%), acetone (26%)	67.0	13.7	19.3
EOA	ethanol (21%), isooctane (44%), acetone (35%)	68.0	15.0	17.0
EOA	ethanol (30%), isooctane (45%), acetone (25%)	67.6	13.4	19.1
EOA	ethanol (26%), isooctane (45%), acetone (29%)	68.0	14.0	18.1
EOA	ethanol (34%), isooctane (46%), acetone (20%)	67.6	12.5	19.8
EOA	ethanol (31%), isooctane (46%), acetone (23%)	68.0	12.9	19.1
EOA	ethanol (34%), isooctane (47%), acetone (19%)	68.2	12.2	19.6
EOA	ethanol (35%), isooctane (48%), acetone (17%)	68.6	11.7	19.7
EOA	ethanol (22%), isooctane (46%), acetone (32%)	69.0	14.2	16.8
POA	isopropanol (11%), isooctane (39%), acetone (50%)	67.0	17.8	15.2
POA	isopropanol (20%), isooctane (40%), acetone (40%)	67.0	16.0	17.0
POA	isopropanol (40%), isooctane (43%), acetone (17%)	67.4	11.8	20.8
POA	isopropanol (39%), isooctane (44%), acetone (17%)	68.0	11.7	20.3
POA	isopropanol (31%), isooctane (45%), acetone (24%)	69.0	12.0	18.4
POA	isopropanol (39%), isooctane (46%), acetone (15%)	69.0	11.0	19.9
POA	isopropanol (35%), isooctane (47%), acetone (18%)	68.0	12.0	20.0
POA	isopropanol (35%), isooctane (45%), acetone (20%)	68.8	12.0	19.3
POA	isopropanol (26%), isooctane (45%), acetone (29%)	68.0	14.0	18.0
POA	isopropanol (32%), isooctane (49%), acetone (19%)	69.0	12.0	19.0
POA	isopropanol (27%), isooctane (48%), acetone (25%)	69.0	13.0	18.0
POA	isopropanol (23%), isooctane (47%), acetone (30%)	69.0	14.0	17.0
MHC	methanol (30%) n-hexane (46%) cyclohexanone (23%)	68.3	13.2	18.5
MHC	methanol (34%), n-heptane (46%), cyclohexanone (20%)	67.2	13.1	19.7
MHC	methanol (33%), n-heptane (47%), cyclohexanone (20%)	67.9	12.9	19.2
MHC	methanol (32%), n-heptane (48%), cyclohexanone (20%)	68.6	12.6	18.8
MWA	methanol (27%), white spirit (48%), acetone (25%)	67.9	13.9	18.2
MWA	methanol (34%), white spirit (49%), acetone (17%)	67.0	12.9	19.9
MWA	methanol (35%), white spirit (49%), acetone (16%)	67.0	12.8	20.2
MOA	methanol (30%), isooctane (50%), acetone (20%)	68.4	13.0	18.6
MNC	methanol (30%), n-nonane (46%), cyclohexanone (23%)	68.3	13.2	18.5
EHC	ethanol (35%), n-heptane (42%), cyclohexanone (23%)	67.3	12.7	20.0
EHA	ethanol (35%), n-hexane (45%), acetone (20%)	67.0	12.7	20.3
EHA	ethanol (35%), n-hexane (48%), acetone (17%)	68.6	11.7	19.7
PHA	isopropanol (35%), n-hexane (45%), acetone (20%)	68.7	12.0	19.3
PHC	isopropanol (39%), n-hexane (41%), cyclohexanone (20%)	68.0	11.8	20.0
PHC	isopropanol (35%), n-heptane (40%), cyclohexanone (25%)	68.1	12.6	19.3
POC	isopropanol (37%), isooctane (42%), cyclohexanone (21%)	68.7	11.8	19.5

Effective extraction was observed using TCE, STARWAX, Schellsol A, four binary mixtures: white spirit/isopropanol (1:1), isooctane/isopropanol (1:1), isooctane/acetone (2:3), white spirit/isopropanol (4:1) and two previously designed ternary AHK mixtures: EOA and POA. After the use of these solvents, a characteristic lignin colour was observed, originating from the dissolved wax-resin adhesive, and the structure of the linen fibre was uncovered, although the wax-resin adhesive remained in its deeper parts and in clusters on the surface, in the areas where it was previously present in a thicker layer. A limited extraction was observed using Shellsol Reiniger 153 HNC-04 solvent and white spirit/ethanol (1:1) mixture, where no uniform colour of the lignin was observed, but only clusters of the extracted mass, with its significant softening. On the other hand, a negligible extraction effect was observed using petroleum ether and its mixture with acetone. The conducted experiment confirmed the effectiveness of EOA and POA mixtures in removal of wax-resin lining adhesive, which was comparable with the effectiveness of classically used, toxic TCE.

Design of an alternative carrier for the cleaning mixture

Successful attempts were made to design a amphiphilic gel carrier that could act as a carrier for organic solvents used in extraction of the wax-resin lining adhesive from the reverse of paintings, especially the developed AHK (alcohol/hydrocarbon/ketone) ternary cleaning mixtures. The desired organogel must be capable of absorbing large volumetric quantities of solvent, otherwise the gel carrier would not be able to provide the appropriate extraction capacity. The desired organogel should also exhibit high elasticity, cohesiveness and mechanical strength so that it would neither break during extraction procedure nor contaminate the canvas.
The usability of commonly known cross-linked polymers such as polyacrylamides, polyacrylic acid and poly (N-isopropylacrylamide) was tested. The gels were synthesised in two ways, using the classic cross-linking agent N, N'-methylenebisacrylamide or the nanostructured factor Laponit XLS (92.32 wt.% M_95.34Li_0.66Si₈O₂₀(OH)₄Na_0.66, 7.68 wt.% Na₄P₂O₇).
Gels based on poly (N-isopropylacrylamide) were obtained: pNIPA-BIS cross-linked with N,N'-methylenebisacrylamide and pNIPA-LAP cross-linked with Laponite. They were synthesised by free radical polymerisation in the presence of a polymerisation initiator (e.g. Na₂S₂O₈) and a polymerisation accelerator (e.g. N,N,N',N'-tetramethylethylenediamine) according to the procedure presented in Example 1. The optimal pNIPA-LAP material was obtained using 1 M N-isopropylacrylamide and 60 mg/ml Laponite. The scheme of the synthesis is shown in Fig. 2.

Tests of compatibility and strength of polymer gels with organic solvents

Compatibility tests of the developed pNIPA-BIS and pNIPA-LAP gels and the classically used polyurethane gels with organic solvents were performed. The tests employed the solvents selected during the previous attempts if dissolving the wax-resin adhesive, i.e. TCE, white spirit, STARWAX, Shellsol Reiniger HNC-04, Schellsol A, four binary mixtures: white spirit/isopropanol (1:1), isooctane/isopropanol (1:1), isooctane/acetone (2:3), white spirit/isopropanol (4:1) and three ternary AHK cleaning mixtures (EOA and POA) according to the invention. The tests' results are shown in Table 3.

Table 3. Results of compatibility tests of the gels and organic solvents.

solvent / mixture	pNIPA-BIS	pNIPA-LAP	elastic polyurethane	rigid polyurethane
TCE	-	-
white spirit	-	-
HNC-04			-	-
Schellsol A			-	-
STARWAX	-	-
white spirit / isopropanol (1:1)	+	-
isooctane / isopropanol (1:1)	+	-
Isooctane / acetone (2:3)	-	-
white spirit / isopropanol (4:1)	-	-
EOA	+	+	-	-
POA	+	+	-	-

Satisfactory results were obtained for the pNIPA-BIS and pNIPA-LAP gels. It was observed that the gels did not swell (did not show compatibility) in classical solvents (TCE, white spirit, some binary mixtures), however, they swelled very well in previously developed ternary mixtures of EOA and POA. Mixtures white spirit/isopropanol (1:1) and isooctane/isopropanol (1:1) showed compatibility with pNIPA-BIS gel while with pNIPA-LAP gel showed no compatibility. For comparison, the classically used polyurethane gels did not swell in any of the solvents tested, and they dissolved in Shellsol A (both rigid and elastic gels).
Photos of the samples of pNIPA-LAP gel after synthesis and cleaning, drying and swelling in the POA solution are shown in Fig. 3, with additionally presented scanning electron microscopy image (SEM) of the freeze-dried hydrogel, which shows the porous structure of the obtained material. The swelling capacity of the polymer gel in the POA mixture is similar to the swelling capacity of this material in water. The POA content in the pNIPA-LAP-POA organogel was estimated at about 87%.

Tests of gel carrier with EOA and POA solvents

Tests were carried out to extract an old wax-resin lining adhesive from canvas using pNIPA-BIS-AHK and pNIPA-LAP-AHK organogels soaked in EOA (ethanol 34%, isooctane 47%, acetone 19%, Teas parameters: 68.2-12.2-19.6) and POA (isopropanol 35%, isooctane 45%, acetone 20%; Teas parameters: 68.8-12.0-19.3). The preparation was applied to the reverse of the painting, covered with a Melinex film, loaded with two pieces of glass of a flat surface, and then the entire setup was covered in a Petri dish.
The effect of the extraction with use of pNIPA-BIS-EOA and pNIPA-BIS-POA organogels was very clear after only 30 minutes, because there was observed a characteristic colour of the gel caused by a wax-resin adhesive. After 60 minutes, the canvas showed brightening and exposed fabric fibres. The observation of the canvas under a microscope confirmed the effectiveness of the extraction of wax-resin adhesive also from the hollows between the weaves of the canvas. Unfortunately, the mechanical strength of the organogels based on the pNIPA-BIS matrix was low and the gel showed a tendency towards breaking.
The effect of the extraction with use of pNIPA-LAP-EOA and pNIPA-LAP-POA organogels was significant in both systems. pNIPA-LAP-POA system removed wax-resin lining adhesive, leaving clean canvas, while pNIPA-LAP-EOA system removed the adhesive leaving a delicate white residue on the canvas (removable by repeated application of organogel or washing), which may cause a certain inconvenience. Organogel pNIPA-LAP-AHK showed very high mechanical strength and cohesiveness, not contaminating the canvas. Use of Laponite XLS as a cross-linking agent instead N,N'-methylenebisacrylamide increased the mechanical strength, cohesiveness and elasticity of the gel.
The obtained pNIPA-LAP-AHK material was subjected to mechanical tests using a tensile machine. An organoleptic evaluation was also performed. The material showed no tendency towards sticking together, it underwent multiple rolling into a roll (Fig. 4), which is important for practical reasons as it provides the convenience of storage, transport and other activities prior to the use of the gel for conservation purposes. Compression tests with relatively high pressure have shown that the organogel undergoes elastic deformation and that after force ceased the gel retains its original shape (Fig. 5). Stretch tests showed that it is possible to stretch the organogel by several hundred percent without causing mechanical damage (Fig. 5). The release of a liquid mixture of solvents from pNIPA-LAP-AWK organogel under the applied force was also not observed, which can be observed when using classic carriers (e.g. sawdust, lignin).

Practical tests of pNIPA-LAP-POA organogel

Tests for the removal of the wax-resin adhesive from the canvas were carried out, as well as tests of regeneration of pNIPA-LAP-POA organogel (Example 5, Fig. 6). During the extraction of wax-resin adhesive, the organogel clearly stains yellow. Regeneration of the organogel in POA solution washes wax-resin adhesive of the gel, which is visible by the disappearance of the yellow colour. Regeneration of the gel can be carried out numerous times without noticeable changes of the cleaning ability maintaining the mechanical properties.
Repeatability tests of the extraction process were carried out by cleaning 9 different areas of the canvas with the same piece of pNIPA-LAP-POA gel for 1 hour, with regeneration after each cleaning (Example 5, Fig. 7) and tests of the extraction time by extracting at varying extraction times of 5 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes and 60 minutes with the same piece of pNIPA-LAP-POA gel subjected to regeneration after each extraction (Example 6, Fig. 8). Effective removal of wax-resin adhesive from the reverse of painting was observed, comparable with TCE. Satisfactory removal of wax-resin adhesive even from between the fibres of the canvas, was observed with the extraction time exceeding 20 minutes. In the places of gel's direct contact with the fabric, visible brightenings appeared, proving the removal of wax-resin adhesive, which was confirmed by microscopic examination.
The cleaning efficiency of organogel contaminated with water in the amount of 5 vol.% was also tested. (Example 7, Fig. 9). It was observed that pNIPA-LAP gel absorbs the hydrated cleaning mixture as well as pure POA cleaning mixture, and the extraction efficiency of the wax-resin adhesive was comparable to that of TCE. It is important because it allows to carry out conservation works without risk of contamination of the cleaning mixture with water.
Despite the lower boiling point of the components of POA mixture (83°C, 99°C, 56°C) compared to TCE (87°C), the use of pNIPA-LAP-POA system is much safer for the restorer than using TCE on lignin, because depositing cleaning mixture POA in the organogel, it reduces its evaporation, and thus, the restorer's exposure to. The process of evaporation of POA mixture during the extraction of wax-resin lining adhesive can be additionally minimised by the use of a Melinex foil cover. The extraction of wax-resin lining adhesive with pNIPA-LAP-POA organogel allows for a significant reduction in the consumption of solvents.

Tests of the alternative AHK cleaning mixtures (alcohol/hydrocarbon/ ketone)

Compatibility tests of pNIPA-LAP gel with a number of ternary AHK cleaning mixtures of a general composition of alcohol/hydrocarbon/ketone were performed, as well as their ability to extract wax-resin adhesive. Examples of the compositions of the mixtures a re presented in Table 2, while detailed results of the tests are presented in Example a and Fig. 10. Experimentally, it was found that the theoretically predicted overall composition of ternary cleaning mixture AHK (alcohol/hydrocarbon/ketone), according to the invention, in proportions that ensure reaching Teas parameters (fd = 67÷69, fp = 11÷19, fh = 15÷21), ensures achieving the goal of the invention by all embodiments of the solvent mixtures, i.e. the efficiency of extraction of wax-resin adhesive is comparable to the efficiency of using TCE, and also the ability to be embed in the structure of pNIPA-LAP gel to form pNIPA-LAP-AHK organogel. Among the tested pNIPA-LAP-AHK systems, the optimum system for removing wax-resin adhesive from the reverse of painting is pNIPA-LAP-POA. pNIPA-LAP gel with EOA is similarly effective, as well as MOA (methanol/isooctane/acetone), PHA (isopropanol/n-hexane/acetone) and POC (isopropanol/ isooctane/cyclohexanone). This does not exclude the possibility of using mixtures of other compositions with the claimed properties.

Test of aggressiveness of pNIPA-LAP-POA for canvas and painting

The safety of treatment the reverse of paintings with pNIPA-LAP-POA organogel for the face of the painting has been investigated, since the wax-resin adhesive permeates throughout the painting [46], therefore the neutrality of the organogel for the paint layers, including pigment and binder, is important.
The influence of POA mixture on linen canvas was studied. Comparative research using reflection infrared spectroscopy (ATR-FTIR) was carried out by Dr. Magdalena Wróbei-Szypula from the Laboratory of the National Museum in Warsaw. The samples of the canvas were exposed to POA for 30 minutes and for 6 weeks. In the obtained spectra, no changes in the canvas material were found in relation to the canvas untreated with solvents. The results presented in details in Example 8 and Figure 11.
The influence of POA mixture on degraded oil paints was studied. Comparative research using gas chromatography coupled with mass spectrometry (GC-MS) were carried out by Dr. Bartfomiej Witkowski from the Faculty of Chemistry, University of Warsaw, using samples of aged linseed oil polymerised with lead white paint or cyprus umber paint. No influence of using AHK mixtures on the content of fatty acid in the binder was observed. The results are presented in details in Example 9 and Figure 12.

Tests of pNIPA-LAP-POA organogel in conservation practice

A test process of relining of canvas was carried out, preceded by the removal of wax-resin adhesive mass from the reverse of painting using pNIPA-LAP-POA organogel. The process was performed by conservator Dr. Elibieta Pilecka-Pietrusińska in her own studio on the painting " Zinnias in a Blue Vase" (Polish painter, ca. 1930, oil, canvas size 53x43 cm, area 2279 cm², private collection), which was lined using the Dutch method on a rigid substrate around 1950.
The wax-resin adhesive was first mechanically thinned and then completely removed from the reverse of the painting using pNIPA-LAP-POA organogel according to the procedure described above. The treatment was preceded by a successful test of the strength of the paint layers against POA cleaning mixture. Tests of strength of the painting against the cleaning preparation used should precede the cleaning procedures according to the invention. As a result of the extraction, the canvas regained its elasticity and natural colour. Removing the wax-resin adhesive soaking through the painting also resulted in lightening of the face of the painting and restoration of its proper colour. The result of the removal of wax-resin adhesive is presented in detail in Example 11 and in Fig. 13.
The canvas cleaned with pNIPA-LAP-POA organogel was then successfully relined using a modern transparent lining material: BEVA^®371 film and a glass fibre fabric [47,48]. The result of the relining is presented in detail in Example 11 and Fig. 14.

Summary

There was presented an experimentally confirmed method of designing the composition of low toxicity ternary AHK cleaning mixtures (alcohol/hydrocarbon/ketone), based on the analysis of Teas' parameters ensuring the ability of said AHK mixtures to simultaneously dissolve waxes and organic resins. According to the invention, selected AHK cleaning mixtures are immobilised in a novel polymeric gel carrier exhibiting amphiphilic properties, as well as high elasticity, cohesiveness and mechanical strength, thanks to which the gel carrier does not undergo mechanically degradation neither during the process of removing wax-resin lining adhesive, nor during the regeneration of the gel carrier.
Wax-resin lining adhesive consists of a number of components with various properties. Natural wax comprises fatty acids and alcohols with hydroxyl groups and esters with carboxyl and carbonyl groups. Rosin, mastic and dammar gum contain organic acids (derivatives of terpenes) with hydroxyl, carbonyl and carboxyl groups, and Venetian turpentine contains abietic acid (a component of rosin) with a hydroxyl group. Ternary AHK (alcohol/hydrocarbon/ketone) cleaning mixtures according to the invention have an optimal composition allowing the dissolution of the wax-resin adhesive due to the synergy of the interaction of hydrocarbon, hydroxyl and carbonyl groups of the solvent with the components of the adhesive. At the same time, the cleaning mixture is chemically inert towards the canvas and polymerised oils present in painting. The invention includes a countless number of combinations and proportions of the components of AHK cleaning mixture, as long as its composition meets the Teas parameters: fd = 67÷69, fp = 11÷19 and fh = 15-21, characteristic for the area common for waxes, natural and synthetic resins, excluding the area of polymerised oils (Fig. 1). The compositions and parameters of exemplary AHK cleaning mixtures are presented in Table 2.
pNIPA-LAP polymer based on poly(N-isopropylacrylamide) cross-linked with Laponite in the presence of Na₂S₂O₈ as an initiator of free radical polymerisation and TEMED agent as an accelerator of free radical polymerisation shows both chemical compatibility with AHK cleaning mixtures (alcohol/hydrocarbon/ketone) and absorbs them in large amounts, creating pNIPA-LAP-AHK organogel, maintaining durability and mechanical flexibility allowing its use during the extraction of wax-resin lining adhesive even from the hollows between the weaves of the canvas, leaving no residue on the reverse of painting being cleaned. Extraction with pNIPA-LAP-POA organogel has the effectiveness comparable with the effectiveness of classical extraction using TCE, while providing safety for a work of art and reducing harmfulness to humans.
The invention guarantees reduction of the harmfulness and toxicity of the extraction process in comparison to the classical process employing TCE, thanks to the replacement of TCE by less harmful solvents and embedding them in a gel carrier limiting their volatility.
The use of AHK cleaning mixture (alcohol/hydrocarbon/ketone) reduces the costs of conservation in comparison to the costs of using TCE. For example, POA mixture has a price comparable to that of TCE, but the cost of using POA is significantly lower than the cost of using TCE due to differences in actual consumption during the extraction of wax-resin lining adhesive. TCE is a fast-evaporating solvent, and its consumption in the classical lignin method is high, unlike POA mixture which is less volatile and it is additionally protected against evaporation by pNIPA-LAP gel carrier. Moreover, pNIPA-LAP-POA organogel can be regenerated many times with a small amount of POA mixture.
Cleaning the canvas from wax-resin adhesive considers dissolving said adhesive by ternary AHK cleaning mixture (alcohol/hydrocarbon/ketone), with the simultaneous absorption of impurities by the organogel. The impurities are removed from the surface of the canvas together with the piece of organogel (e.g. pNIPA-LAP-POA), and then washed and diluted in a cleaning mixture (e.g. POA) during the regeneration of the used piece of organogel. pNIPA-LAP-AHK material maintain its cleaning properties even when the cleaning mixture is contaminated with water (up to co. 5 vol.%).
Already a single (more than 20 minutes) local application of the organogel soaked in the AHK cleaning mixture allows for good cleaning of the canvas from wax-resin lining adhesive. Short time of treatment is highly beneficial because it allows you limit time of exposure of the painting's support to the cleaning agents. The transparency of pNIPA-LAP-AHK material provides a possibility for visual control of the condition of the canvas during the cleaning process without the need of its interruption, as in case of use of lignin or sawdust known from classical methods. Moreover, during the extraction process there is no need to apply drastic conditions (e.g. high temperature, vacuum), which might be harmful to work of art being cleaned. The invention provides materials effectively functioning under standard pressure and temperature conditions.
The very good mechanical strength of pNIPA-LAP-AHK organogel enables cleaning of works of art without leaving any residual pieces of gel matrix on the cleaned surface, ensuring introduction of no additional impurities to the conserved painting. In addition, the very good mechanical strength of pNIPA-LAP-AHK gel facilitates conservation works that require its numerous applying, pressing and taking off. pNIPA-LAP-AHK material is reusable. It can be easily regenerated by washing in the AHK cleaning mixture. The material can be dried and swollen again in the cleaning mixture without deteriorating its properties.
pNIPA-LAP-AHK organogel is safe for canvas and painting when used on the reverse of painting, because it is neutral against cellulose components as well as for polymerised oils contained in paint layers, which has been confirmed in conservation tests as well as FTIR and GCMS measurements. After cleaning with pNIPA-LAP-AWK organogel, the paintings regain their shine and colour, while the reverse of paintings is ready, without further preparation, for applying a new lining layer.
The method of removing a wax-resin adhesive from the canvases of wax-resin lined paintings, the cleaning mixture and an organogel for use in this method and the method of producing this organogel has been presented below in the working examples.
Example 1. Determination of the composition of AHK cleaning mixture. Using the computer programs "Modular Cleaning Program" [42] and "TriSolv" [43], compositions of low toxicity ternary AHK cleaning mixtures (alcohol/hydrocarbon/ketone) has been designed (Table 2), having Teas parameters in the appropriate ranges (fd = 67÷69, fp = 11÷19, fh = 15÷21) presented in Fig. 1, being similar to parameters of TCE (68-12-20), allowing for simultaneous dissolution of waxes, natural resins and synthetic resins, and being inert towards polymerised oils present in painting. For example, the following mixtures has been designed: EOA (ethanol 34%, isooctane 47%, acetone 19%; 68.2-12.2-19.6), POA (isopropanol 35%, isooctane 45%, acetone 20%; 68.8-12.0-19.3), MOA (methanol 30%, isooctane 50%, acetone 20%; 68.4-13.0-18.6), PHA (isopropanol 35%, n-hexane 45%, acetone 20%; 68.8-12-19.3) and POC (isopropanol 37%, isooctane 42%, cyclohexanone 21%; 68.7-11.8-19.5).
Example 2. Synthesis of pNIPA-LAP-POA nanocomposite organogel. pNIPA-LAP gel matrix was synthesised by free radical polymerisation. 20-150 mg of Laponite XLS (92.32 wt.% Mg_5.34Li_0.66Si₈O₂₀(OH)₄Na_0.66; 7.68 wt.% Na₄P₂O₇) were mixed with 1 ml of water and sonicated until a clear solution obtained. N-isopropylacrylamide monomer in a concentration of 0.5-2.5 M was added to the obtained solution. The solution was deoxygenated and stirred in an ice bath for about 1 h, and then TEMED free radical polymerisation accelerator (5 µl per 1 ml of the solution) was added. After removal from the ice bath, sodium persulfate (Na₂S₂O₈) was added as a free radical polymerisation initiator to receive its concentration of 2 mM. The solution was poured into a rectangular mold to obtain the desired gel pieces. After 24 h, the obtained hydrogel was cut into pieces and washed several times with water to remove unreacted reagents. The purified gel was dried to constant weight in air at room temperature. Then, the dried gel was placed in a POA ternary solvents mixture with a volumetric composition of 7/9/4, having Teas parameters of: 68.8-12.0-19.3, as in Example 1. The synthesis scheme is shown in Fig. 2, and the obtained pNIPA-LAP-POA organogel is shown in Fig. 3. It was found that the optimal parameters of pNIPA-LAP gel can be obtained during the synthesis using 60 mg of Laponite and 1 M NIPA. Gels with lower content of Laponite absorb more solvent but have lower mechanical strength. On the other hand, gels with higher content of Laponite are more compact and exhibit lower absorption ability.
Example 3. Properties of pNIPA-LAP-POA organogel. pNIPA-LAP-POA organogel obtained as in Example 2 was subjected to mechanical strength tests. The tested organogel was stable when handled and underwent multiple rolling without damaging (Fig. 4). The tested organogel also endured 700% stretching and 80% compression and afterwards returned to its original shape (Fig. 5).
Example 4. Test of wax-resin lining adhesive removal using pNIPA-LAP-POA. The research was conducted using the lining canvas from the 1947 lining of painting "The Battle of Grunwald" by Jan Matejko. The canvas was soaked with a wax-resin lining adhesive, comprising: beeswax, rosin, Venetian turpentine (10:10:1). The lining canvas was cut into the samples of a size of 4x10 cm and tested towards removal of wax-resin adhesive using pNIPA-LAP-POA organogel obtained as in Example 2. A piece of pNIPA-LAP-POA organogel with dimensions of 2x3 cm was placed on a fragment of the lining canvas for 1 h. During this time, the piece of gel turned into characteristic yellowish colour, indicating the extraction of wax-resin adhesive. Then, the used piece of gel was regenerated by placing it for ca. 1 h in POA solution obtained as in example 1. During regeneration, the dissolved wax-resin adhesive migrated into the solution, and the piece of organogel became transparent. The successive steps of the extraction process are shown in Fig. 6.
Example 5. Reproducibility of the extraction of wax-resin adhesive from the canvas. A piece of pNIPA-LAP-POA organogel of a size of 4×4×1.5 cm was applied to 9 areas of the lining canvas of a size of 15×20 cm, as in example 4. The extraction time was 1 h, and after each extraction the piece of gel was regenerated during 1 h in POA solution, as in Example 4. A series of 9 extractions was carried out. A photo of the cleaned canvas is shown in Fig. 7A. In the places of direct contact of the gel with the fabric, brightening was visible, indicating the removal of wax-resin adhesive. The fabric was examined under an optical microscope on the cleaned and unclean areas, and a comparison of these areas is shown in Figs. 7B-7C, where removal of wax-resin lining adhesive is clearly visible even from the hollows between the weaves of the canvas.
Example 6. Rate of the extraction of wax-resin adhesive from the canvas. A piece of pNIPA-LAP-POA organogel of a size of 3×4×1 cm was applied to 7 areas of the lining canvas of a size of 10x20 cm, as in example 4. The extraction time was variable: 5 minutes, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes and 60 minutes, and after each extraction the piece of the gel was regenerated during 1 h in POA solution, as in Example 4. A series of 7 extractions was carried out. A photo of the cleaned canvas is shown in Fig. 8A. In the places of direct contact of the gel with the fabric, brightening was visible, indicating the removal of wax-resin adhesive. The fabric was examined under an optical microscope on the cleaned and uncleaned areas, and a comparison of these areas is shown in Fig. 8B, where removal of wax-resin adhesive is clearly visible even from the hollows between the weaves of the canvas, in the case of extraction longer than 20 minutes.
Example 7. Test of extraction of wax-resin lining adhesive using POA containing water. A piece of pNIPA-LAP-POA/H2O organogel of a size of 1.5×3×1 cm soaked with POA solution with 5% water content, was applied to the lining canvas of a size of 3×8 cm, as in example 4. The extraction time was 1 h as in example 4. A photo of a fragment of the cleaned fabric is shown in Fig. 9A. At the place of direct contact of the gel with the fabric, brightening was visible, indicating the removal of wax-resin lining adhesive. The fabric was examined under an optical microscope on the cleaned and unclean areas, and a comparison of these areas is shown in Figs. 9B-9C, where the removal of wax-resin adhesive is clearly visible even from the hollows between the weaves of the canvas
Example 8. Test of removal of wax-resin adhesive using other cleaning mixtures. Three pieces of pNIPA-LAP organogel of a size of ca. 2×2×0.5 cm were soaked with MOA, PHA and POC solutions form Example 1 (Fig. 10A) and applied to pieces of the lining canvas of a size of 3x10 cm as in Example 4 (Fig. 10B). The extraction time was 1 h, and during this time all the pieces of gel turned into a characteristic yellowish colour, indicating the extraction of wax-resin adhesive (Fig. 10C). In the places of direct contact of the gel with the fabric, brightening was visible, indicating the removal of wax-resin lining adhesive form the canvas (Fig. 10D).
Example 9. FTIR test of tolerance of linen canvas towards POA and PWs solutions. The tests were carried out using attenuated total reflection infrared spectroscopy (FTIR-ATR) using an Alpha FTIR spectrometer by Brucker equipped with QuickSnap ATR module with a diamond crystal. The spectra are presented in absorbance mode. Samples of decatised linen canvas were treated with POA mixture (7/9/4 v/v) and PWs mixture (isopropanol/white spirit, 1:1) for a period of 30 minutes or 6 weeks. Subsequently, the tested samples of the canvas were subjected to FTIR tests and compared with the spectrum of pure linen. The results are shown in Fig. 11. The spectra of the canvases treated with the cleaning mixtures do not show any significant differences from the spectrum of the clean canvas, thus, none of the substances used causes changes in the structure of the canvas.
Example 10. GC-MS test of tolerance of pigments towards POA solution. The tests carried out using GC-MS spectrometer allowed to determine the influence of the extraction using POA mixture (7/9/4, v/v) on the chemical composition of oil paints. The paints were prepared with linseed oil and two pigments: lead white and cyprus umber. The fresh paints were aged at 80°C with use of UVB-Vis radiation for 24 hours. Then, from each paint, two samples weighing ca. 2 mg were collected, one of which was exposed to a 10-minute POA treatment, and the other was used as a reference sample. The results of the analyses are presented in Fig. 12. The spectra of samples treated with POA and the reference samples do not differ significantly, which means that the solvent extraction does not significantly affect the content of palmitic acid, stearic acid and azelaic acids, which are the main components of degraded oils drying in oil paintings.
Example 11. Test of using pNIPA-LAP-POA in conservation practice. The painting "Zinnias in a blue vase" (Polish painter, c.a. 1930, oil, canvas size 53x43 cm, area 2279 cm², private collection) was subjected to cleaning with use of pNIPA-LAP-POA organogel in order to remove wax-resin adhesive from the reverse of painting before relining. POA cleaning mixture (7/9/4 v/v) was used during cleaning.
Before the treatment, the strength of the paint layers against POA cleaning mixture was tested, which is recommended before cleaning each painting. A piece of the face of the painting was exposed to POA and neither discoloration of the cleaning mixture nor changes in the painting was observed. Only after the neutrality of the cleaning mixture for the painting was confirmed, it was decided to start the process of removal of wax-resin adhesive.
The extraction of wax-resin adhesive from the reverse of the painting was preceded by the mechanical removal of the secondary rigid lining substructure and the excess wax-resin adhesive. The extraction according to the invention was carried out using four pNIPA-LAP-POA oragonogel pieces: I (9x6x0.5 cm), II (6x6x0.5 cm), III (11.5x5x0.2 cm) and IV (15x10x0.3 cm). The total area of the organogel (297.5 cm²) was 7.6 times smaller than the area of the cleaned painting (2279 cm²). 1 litre of POA mixture was used. The organogel was put on the reverse of the painting, covered with Melinex foil and loaded with sand bags (small bags of a size of 9x11 cm and weight of 200 g, large bag of a size of 20x20 cm and weight of 800 g) to ensure even light pressure (2 g / cm2) of organogel towards canvas. The system was covered with a larger sheet of Melinex foil or a petri dish to reduce solvent evaporation. First, the wax-resin adhesive was removed from the canvas with piece I, using it 120 times, with the interaction time: 15, 20 or 30 minutes. With use of this piece, a significant part of the residual wax-resin adhesive was removed from the surface 42 times larger than itself. Then, with use of pieces III and IV the surface was further cleaned and the interface accumulations of wax-resin adhesive, formed during the treatment, were removed. Piece III was used 85 times, while piece IV was used 45 times. For final cleaning, a fresh piece II was used, which extracted only small remnants of the adhesive. The organogel was applied in two regimes: a single application and rinsing, or a double application with turning upside down and rinsing - both methods were effective. A positive result was also obtained when repeatedly rinsing in POA mixture, which allowed to reduce the amount of the cleaning mixture consumed. During the works, the condition of the face of the painting was checked regularly. Wax-resin adhesive has been completely removed from reverse of the painting, even from the hollows between the weaves of the canvas, leaving clean canvas ready for relining. As a result of removing the wax-resin adhesive, changes in the appearance of the face of the painting were also observed, which regained its shine and colours. The result of cleaning is shown in Fie. 13.
The canvas cleaned with use of pNIPA-LAP-POA organogel was then successfully subjected to relining procedure using a modern transparent lining material: BEVA^®371 film and glass fibre fabric [47,48]. The lining was made by hand, covering with an adhesive the lining glass fibre fabric stretched around stretcher bars, which was further joined to the reverse of the painting. The adhesive was activated by heating with a lukewarm iron with a temperature of 50-60°C. The result of relining is shown in Figure 14.

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Claims

Nanocomposite organogel pNIPA-LAP-AHK containing nanoporous polymer matrix pNIPA-LAP with a structure based on poly(N-isopropylacrylamide) containing Laponite XLS (92.32 wt.% of Mg_5.34Li_0.66Si₈O₂₀(OH)₄Na_0.66, 7.68 wt.% Na₄P₂O₇) as a crosslinking agent, filled with a cleaning mixture for dissolving a wax-resin lining adhesive and removing it from the canvases of wax-resin lined paintings, containing an alcohol component, a hydrocarbon component and a ketone component forming together an AHK cleaning mixture (alcohol/hydrocarbon/ketone), which is chemically inert towards polymerised oils present in the painting and has physicochemical properties determined by Teas parameters: fd = 67÷69 (dispersion force), fp = 11÷19 (polar force) and fh = 15÷21 (hydrogen bonding forces), characteristic of the area common to waxes, natural resins and synthetic resins.
The nanocomposite organogel pNIPA-LAP-AHK according to claim 1, wherein the cleaning mixture contains 30-40 vol.% of the alcohol component, 40-50 vol.% of the hydrocarbon component and 15-25 vol.% of the ketone component.
A method of manufacturing a nanocomposite organogel pNIPA-LAP-AHK, characterised in that the process of free radical polymerisation is carried out where Laponite XLS is mixed with water and sonicated until a clear solution obtained and further mixed with N-isopropylacrylamide, and then the mixture is stirred and subjected to deoxidation in an ice bath for ca. 1 h followed by the addition of the accelerator of the free radical polymerisation process, and the solution obtained is transferred to the molds and left to react, and after 24 h the hydrogel obtained is several times rinsed with water to remove the unreacted reactants, dried to constant mass and then inserted into a cleaning mixture for dissolving a wax-resin lining adhesive and removing it from the canvases of wax-resin lined paintings, containing an alcohol component, a hydrocarbon component and a ketone component forming together an AHK cleaning mixture (alcohol/hydrocarbon/ketone), chemically inert towards polymerised oils present in the painting and having physicochemical properties determined by Teas parameters: fd = 67÷69 (dispersion force), fp = 11÷19 (polar force) and fh = 15÷21 (hydrogen bonding forces), characteristic of the area common to waxes, natural resins and synthetic resins,
and left to swell, to receive the PNIPA-LAP-AHK organogel.
The method according to claim 3, wherein Laponite XLS is used in an amount of 20-150 mg per 1 ml of water, preferably 60 mg per 1 ml of water.
The method according to claim 3, wherein N-isopropylacrylamide monomer is used in a concentration of 0.5-2.5 M , preferably 1 M.
The method according to claim 3, wherein as the accelerator of free radical polymerisation process TEMED (N,N,N',N'-tetramethylethylenediamine) is used in an amount of 5 ml per 1 ml of solution and as an initiator of free radical polymerisation sodium persulfate is used in a concentration of 2 mM.
A method of removing a wax-resin lining adhesive from canvases of wax-resin lined paintings, comprising the dissolution and removal of the adhesive with a cleaning preparation containing a carrier and a solvent, characterised in that to dissolve and remove the wax- resin lining adhesive the pNIPA-LAP-AHK organogel defined in claim 1 is used, comprising the AHK cleaning mixture immobilised in the nanocomposite polymer matrix pNIPA-LAP having a shape corresponding to the portion of the treated canvas, being the support of the painting, wherein the organogel is applied on the surface of the treated canvas and is allowed to stay in contact with the reverse of the painting for at least 20 minutes to extract the wax-resin adhesive towards the cleaning mixture immobilised in the organogel, and then the organogel is removed from the surface of the treated canvas and subjected to regeneration by being immersed in a fresh portion of the cleaning mixture to wash off the extracted wax-resin adhesive from the organogel, and then this procedure is repeated using the regenerated organogel until the wax-resin adhesive is completely removed from the reverse of the painting, even from the hollows between the weaves of the canvas being the support of the painting.
The method according to claim 7, wherein the pNIPA-LAP-AHK organogel is applied to the reverse of the painting being restored more than 5 times, each time for a period of 30-60 minutes.
The method according to claim 8, wherein the duration of the application of pNIPA-LAP-AHK organogel to the reverse of the painting is variable, preferably longer in each consequent iteration.
The method according to claim 7, wherein the pNIPA-LAP-AHK organogel applied to the reverse of the painting is additionally covered with a film made of poly(ethylene terephthalate) having low permeability and high dimensional, thermal and chemical stability to reduce the loss of the cleaning mixture due to evaporation, and additionally load it with a weight to increase the contact surface of the organogel with the cleaned canvas.
The method according to claim 7, wherein the cleaning mixture also contains water in an amount not exceeding 10 vol.%, preferably not more than 5 vol.%, in regard to the volume of the AHK cleaning mixture (alcohol/hydrocarbon/ketone).