IE66701B1 - Process for minimizing the migration of lead from a crystal flask towards a liquid located in said flask - Google Patents

Abstract

Process for the treatment of the surface of a solid object made of a material which has a vitreous structure and especially crystal, lead glass and the like and containing a certain content of heavy metals and especially lead, this surface being liable to come into contact with media of acidic nature and especially alcohols such as brandy, whisky, wines or fruit juices and similar food products, with a view to preventing the said heavy metals from migrating towards the said liquids, characterised in that it consists in bringing about, in a surface region of the said surface intended to come into contact with the liquid, and after having partly removed the lead ions present in this surface region, the formation of a silicoaluminous diffusion barrier opposing this migration towards the liquid, by ion exchange between the said solid material with vitreous structure and a coating temporarily applied to the said surface, this coating itself consisting of a silicoaluminous crust resulting from the heating of a layer of kaolin applied to the said surface in the form of a thixotropic slip of pure hydrated aluminium silicate.

Description

The present invention concerns a process for treating the surface of a solid object made from a material with a vitreous structure and having a certain heavy metal content, this surface being liable to come into contact with liquids with an acidic nature, with a view to preventing the said heavy metals from migrating towards the said liquids.

In the present description, material with a vitreous structure will refer essentially to lead glass, crystal and other materials of the same nature used in their solid form, that is to say not having any particular surface characteristics, and heavy metals will refer to metals such as lead, cadmium, barium and others which may represent a danger to health if they come into contact with drinks or food for a long time or permanently.

Liquid with an acidic nature should be taken to mean alcohols such as cognac, whisky, clear spirits, wines or fruit juices and the like, as well as very corrosive viscous products such as mustards and other acidic foodstuffs.

By way of detailed illustrative example of the process according to the invention, there will hereinafter be described the application of this process of the treatment of receptacles made of crystal or lead glass, intended notably to contain alcohol for example, with a view to preventing the migration towards the said alcohol of the lead from the internal surface of these receptacles in contact with the said alcohol.

In this example, the term lead glass will refer to any glass containing a certain proportion of lead, including of course crystal properly speaking, that is to say containing at least 24% lead oxide.

It is known that any liquid contained in a receptacle tends to dissolve, whatever the material of this receptacle may be, a certain proportion of the chemical substances forming this material. This proportion is, of course, generally extremely small. For example, water contained in a glass comprising sodium and calcium dissolves a certain proportion of the Na2O contained in this glass. A standardised test (DIN 1211) makes it possible to compare the hydrolytic resistance of glasses comprising sodium and calcium (dissolution of 30 to 1000 pg of Na20 per g of glass comprising sodium and calcium reduced to a powder, placed in water at 90°C for one hour).

This general phenomenon, though it needs to be taken into account in analytical chemistry and in certain branches of fine chemistry, does not generally affect practical life, except in very particular cases, such as that of certain heavy metals such as cadmium, barium and lead which tend to migrate into foodstuffs when they form part of the materials used for cooking utensils or when they are in contact with liquids. Even more particularly, crystal contains lead oxide (AFNOR standard : NF 30.004), and it has been found that quantities of lead, which may be as high as a few ppm, may 20 migrate into acidic media (with a pH of about 3.5) when these are kept in crystal receptacles for a fairly long time.

This phenomenon has, furthermore, been the subject of detailed studies, which have led to the drafting of control standards with a view to ensuring that the population is protected 25 against the possible dangers due to contact of foodstuffs with the surfaces of articles made of glass used for the preparation, serving and. storage of food and drink (International Standard ISO 7086/1-1982).

Thus, in certain countries, the competent authorities in health matters now tend to recommend a maximum lead content for alcoholic beverages, which may pose a problem both to the manufacturers of bottles and carafes made of crystal and to those dealing in expensive alcoholic beverages such as cognac, whisky and the like, who wish to offer their products in bottles .

A first solution to this problem was offered by the Applicant itself in the form of a process which is the object of French patent application FR-A-2 2S7 768 and leads to a receptacle made of crystal, intended to contain all products of an acidic nature, and which is characterised in that, with a view to preventing the migration of the lead in the glass towards the said products, the said receptacle has on its internal surface a continuous thin film of lead-free glass, forming a screen.

In its implementation, this process consists, before the shaping of a parison of crystal, of taking a very small quantity of lead-free glass, then of gathering, on top, the crystal parison, which then adheres to the lead-free glass, so that, during shaping, the glass is greatly stretched and covers the whole of the inside of the piece in a very thin layer.

Although the above solution is perfectly satisfactory in its results, research has been pursued with the purpose of finding yet another solution.

The invention consists, in the application which is the object of the present example, of a new process aimed at achieving the same goal, that is to say to prevent the migration of the lead ions from the lead glass or crystal of which a receptacle is made, towards an acidic product or medium such as a liquid contained in this receptacle. This process is based on a principle which is completely different from the process set out above and which consists not of isolating the glass from the liquid but of creating, in the superficial area of the glass intended to come into contact with the liquid, and after partly removing the lead ions contained in this superficial area, a diffusion barrier preventing the lead ions contained in the crystal from passing towards the medium.

The means used to bring about this phenomenon belongs to the technique, already an old one, of the exchange of ions, and more specially here, of the exchange of cations between the glass containing lead and alkaline cations and a substance containing H+ and A13t cations brought into contact with the crystal, under given conditions.

It is in fact known that the hydrolytic attack of glass in an acidic medium may be represented by an ionic exchange between the excess H+ ions in a solution and the mobile cations of the vitreous lattice, the silica remaining practically unattacked. Such an exchange is in practice limited to lattice modifiers (alkaline and alkaline earth metals) and to certain heavy metals such as lead, cadmium or barium (in so far as they are present in the solid).

Starting from this basic data, attempts have already been made to limit the migration of the lead, by various methods which consist of removing the lead from a superficial diffusion layer, of variable thickness, cf the wall of an article by means of acidic aqueous scluttcns, prior to the use of this article. The drawback of these methods lies in the additional operations and handling to which they give rise and especially in the mediocrity of the results, if it is desired at the same time to comply with the constraints relating to safety and production times compatible with industrial manufacture.

Indeed, in the field of the compositions of lead glass and crystal, the kinetics of icn exchange between the lead of the wall and the cations of the liquid is governed by Fick's second law, namely that the quantity of lead passing into solution is proportional to the square root of the duration of contact, whilst, for its part, the diffusion coefficient is an Arrhenius function of the temperature. Moreover, the lixiviation of the receptacles by acidic solutions can be effected only at relatively low temperatures, below the boiling points of the liquids used, under the conditions of use and at durations of contact limited by constraints of industrial exploitation. Furthermore, safety constraints in the workshop make the use of very concentrated acids problematic in practice.

The result is that, in every case, the quantity of lead effectively extracted is relatively very small and that a further contact of an acidic liquid winh a receptacle thus treated again results in a diffusion of lead into the liquid, which is measurable after only a few days: the speed of diffusion remains approximately the sane as before treatment and there is only a small difference in the time of the phenomenon, this difference being insufficient with respect to storage periods. Indeed, for example, in the case of alcohol kept in crystal carafes, these periods may be as long as several years.

The present process is fundamentally different from these methods, and makes it possible no prevent the migration, from a wall made of lead glass or crystal, of the lead which would be liable to migrate towards the liquid, either in the course 20 of prolonged contact, as in the case of alcohol, or in the course of short contacts with highly acidic media such as preparations containing vinegar (mustards, sauces).

To achieve this improvement in the result sought, the process according to the invention is based co the one hand on the use 25 of high temperatures, and on the other hand on the use of compounds able to exchange and Al3- ions up to these temperatures.

In practice, the present example concerns a process for the elimination of lead in a superficial area of a receptacle made of lead glass or crystal, already finished or in the process of being manufactured, this process consisting of depositing, on the surface of the said receptacle intended to be in contact with the acidic medium, a thixotropic slip in a viscous liquid state, emptying the said receptacle, allowing a continuous deposit of the said liquid adhering to this surface to remain on the wall to be treated, heating the said body having this deposit to a temperature between 300°C and the softening temperature of the lead glass or crystal of which the said receptacle is made for a few hours, and then, after cooling, eliminating, by washing, the silico-aluminous skin resulting from the heating of the said deposit.

Slip here refers to a thixotropic suspension of compounds such as the phyllosilicates or tectosilicates, to which belong, for example, clays, kaolins, montmorillonites and zeolites, that is to say essentially a hydrous aluminium silicate which keeps some constituent water up to a high temperature, at which moment the diffusion of the cations of the crystal by exchange with the cations of the clay occurs at a sufficiently high speed, compatible with reasonable production times in industrial manufacture.

A first possible explanation of this phenomenon, on which the process according to the invention is based, would be the following: These crystalline solids are all characterised by structural arrangements in tetrahedral or octahedral layers, the electrical charge balance of which is deficient in positive charges following the substitution, for the Si4* cations, of other, less charged, cations such as Al3*. The excess of negative charges is then compensated for by other cations, mainly monovalent ones (H*, alkaline metals) or sometimes bivalent (alkaline earth metals). These compensatory cations can be exchanged for other kinds: the number of 30 negative charges thus compensated for by the exchangeable cations is referred to as cation exchange capacity (CEC) or base exchange capacity (BEC).

It should be noted that, as early as 1945 (cf The Glass Industry, Vol 26 No 6, June 1945, No 7, July 1945 and No 6, June 1947), some authors had indeed shown a possibility of high-temperature ion exchange between the H* ions of metakaolin and the alkaline metals of glass comprising sodium and calcium and had even envisaged an application for them, namely an improvement in durability or in certain (mechanical and electrical) properties of glass comprising sodium and calcium. However, the additional handling occasioned by such a process caused the authors to abandon it, particularly since the same results could be achieved by other, more convenient means.

These earlier studies were therefore limited to the exchange between the sodium of the glasses comprising sodium and calcium and the H+ ion of metakaolin, it being noted, in addition, that the difference between such a high-temperature ion exchange and the lixiviation at normal temperature mentioned above would occur in the kinetics of the process on the one hand and in the state of the surface obtained after treatment on the other hand.

Thus it has been possible to establish that, with the process according to the invention, at the temperatures indicated, between 300°C and the softening temperature of the lead glass or crystal of which the said article is made, contact of only a few hours is sufficient to cause the diffusion, towards the said silico-aluminous skin, of a certain quantity of lead from the glass or crystal.

In reality, surface analyses by ESCA and SIMS spectrometry have revealed that, in the course of the said process, a complex exchange is in addition established between the K* and Pb2’ ions of the crystal on the one hand and the Α13φ ions of the kaolin on the other hand, this exchange, which is therefore more complex than the exchange on which the previous hypothesis was based, producing, in the wall of a receptacle made of crystal, a superficial silico-aluminous layer with a thickness of about 100 nanometres (1000 angstroms) and which forms an interface barrier resisting the interdiffusion of the Pb21· ions of the solid and the H* ions of the acidic liquid in contact. The alumina contributed by the kaolin participates in the formation of this very stable superficial layer, which persists after a heat treatment at 120°C for seventeen hours, corresponding to ageing at ambient temperature of several decades, in ambient atmosphere without contact with a liquid.

The silico-aluminous skin remaining on the wall at the end of the process is eliminated, for example by washing in water, supplemented by a finishing process, for example washing with sand, ultrasonic treatment or the like.

An advantage of this process lies in the precise nature of the conditions of its application, which make it operational on industrial machines, on any body made of lead glass or crystal.

The invention will now be illustrated with reference to a certain number of these practices demonstrating the efficacy of the process.

Example 1: This example refers to a particular application of the process according to the invention to the treatment of crystal carafes with 30% lead intended to contain cognac.

These carafes having a capacity of 750 ml, a slip is prepared consisting of kaolin, which is commercially available and sold in France by Lambert Riviere under the name Kaolin Heavy, with a high degree of purity, the upper limits of the impurities being: Calcium ...................

Heavy metals ..............

Chlorides .................

Sulphates .................

Acid-soluble substances ... 250 ppm 2 5 ppm 250 ppm 0.1% by weight 10 mg The other physical and chemical properties being in accordance with the purity required by the 1974 European Pharmacopoeia.

After about 53% water and 47% kaolin have been mixed intimately in a mortar, the slip is introduced into the carafe and the latter is agitated so as to create a film of slip over the whole of its internal surface. The excess is emptied by gravity.

The thixotropy of the slip thus enables the entire surface of the bottle to be covered with a uniform layer without there being any subsequent flow, notably at the re-emptying stage.

The carafe is then taken to an oven and the temperature is progressively raised to 400°-440°C and maintained at a constant level for 4-6 hours, and then cooled slowly.

The slip is then eliminated by washing, first in water, which 20 removes most of it, and then with a sand suspension to detach any traces or film which may still remain, so that the article recovers its initial appearance.

It should be noted that the temperatures used are close to the stress point of crystal (about 440°C), but lower, to avoid impairment of the surface or excessive adhesion of the kaolin after cooling.

More generally, it should be noted that the proportion of water in the slip may vary quite considerably, according to numerous factors such as: - the nature of the kaolin used, - its particle size, - the method of application (paste applied with a brush or fluid suspension), - the additives providing its thixotropy, - the geometrical shape of the article (flat or hollow).

A range of about 40% to 60% by weight appears reasonable.

In order to subject the receptacle thus treated to a comparative durability test, it is filled with an aqueous solution with 4% acetic acid, and the lead concentration in this solution is measured by atomic absorption, at intervals of about 24 hours.

At the same time, identical measurements are made on a carafe which is identical but has not undergone the treatment according to the invention.

After 750 hours (one month) of contact, the following results are observed: Carafe Pb content of the solution Treated not detected Non-treated 1.70 mg/1 The efficacy of the process is therefore demonstrated.

In addition, the quantity of lead which had migrated into the kaolin during treatment was measured. This quantity is from 2 to 3 mg.

The aim of the following examples is to demonstrate that the efficacy of the same treatment is sufficient for the liquids kept in the treated bottles to continue to conform to health requirements .

To this effect, in a series of tests carried out over 6, 12, 24, 36 and 60 months (5 years) using, as a liquid, 4% acetic acid (pH = 2.30) and a commercially available alcohol (pH = 3.5), the results obtained were as follows, as shown in Table 1 below, and illustrated by the curves shown in the diagram in 1o Figure 1: TABLE 1 A = tests with 4% acetic acid according to the process of Example 1.

B = tests with alcohol (with an inherent initial lead content of 16.8 Mg/1) DURATION OF THE TEST Lead content (pig/l) of the liquid 6 months 12 months 24 months 36 months 5 years A (Tests with acetic acid) 31.0 43.8 62.0 76 98 B (Tests with alcohol) 13 18.3 25.9 31.6 40 These results (Table 1 and the corresponding curves A and B) call for the following comments: Firstly the five-year test is an almost superfluous safeguard. Indeed, the age of an alcohol such as cognac as shown on a crystal bottle, does not in any way signify that it has remained in this bottle throughout its ageing, which, in fact, has normally taken place in a demijohn or in an oak cask: the carafe is filled only when the cognac is to be sold, whatever its age, and the effective period of contact between the alcohol and the crystal is, in reality, only the period during which it is for sale on the shelves of the retailer, and then the period of consumption by the buyer, which, logically, rarely exceeds a few months, and at most a few years. That is why the tests set out below were carried out in real time up to 12 months, then in accelerated tests up to 36 months and finally extrapolated to 5 years in accordance with Fick's law, which is universally accepted in the field of ion migration, the extent of which is assessed as a function of the square root of the period of time.

Secondly, it will be noted that the quantities of lead passing into solution are substantially smaller in the alcohol than in the acetic acid. This is explained by the higher pH and the presence of other constituents in the alcohol.

Finally, and most importantly, it is found that the highest lead content in the alcohol, reached at the end of five years, is only 40 /zg/1, that is to say less than half the maximum content recommended to date.

It can therefore be stated that the process according to the invention, applied in the most extreme circumstances, guarantees compliance with the most stringent legislation.

In conclusion, it is clear that the process according to the invention makes it possible to achieve the result sought effectively, that is to say to prevent the migration of the heavy metal cations from a vitreous substance such as crystal towards a liquid in contact with a surface made of crystal, for example an alcohol such as cognac contained into a crystal bottle .

It is, of course, understood that the present invention is based on the experimental observation of the results of the implementation of the process which has just been described and illustrated by examples and cannot be limited by any hypothetical theory or explanation. Thus, in the present state of experimentation, all that can be done is to state the different plausible hypothetical components of an explanation of the efficacy of this process, these components being: )5 1. The simple H/Pb2+ exchange between the crystal and the kaolin. This superficial exhaustion of the lead in the crystal would effectively minimise its subsequent migration towards the liquid. 2. A much more complex exchange between the principal cations 10 of the crystal and the kaolin, which would be the cause of the formation of a barrier of silico-aluminous diffusion in the superficial zone of the crystal, this barrier resisting the subsequent migration of any lead still present in the crystal.

It is perfectly plausible that these components are 15 concomitant, the first prevailing at the beginning of the treatment, that is to say there would indeed be, initially, in the absence of any barrier, a migration of the lead towards the kaolin, this superficial exhaustion of the crystal being in itself a kind of favourable result, and then, as the operation proceeds, the creation and development of the barrier, resisting any subsequent migration of any lead still remaining in the crystal towards the liquid.

Claims (7)

1. Process for treating the surface of a solid object made from a material with a vitreous structure and in particular crystal, lead glass and the like and having a certain heavy 5 metal, and in particular lead, content, characterised in that it consists of causing, in a superficial area of the said surface intended to come into contact with the liquid, and after partly removing the lead ions contained in this superficial area, the formation of a silico-aluminous 10 diffusion barrier, opposing this migration towards the liquid, by an exchange of ions between the said solid material with a vitreous structure and a coating applied temporarily to the said surface, this coating itself consisting of a silicoaluminous skin resulting from the heating, at a temperature 15 between 300°C and the softening temperature of the glass or crystal, for a few hours, of a layer of pure hydrous aluminium silicate applied to the said surface in the form of a thixotropic slip.

2. Process according to Claim 1, characterised in that the 20 slip consists of a viscous aqueous suspension of a compound chosen from among the phyllosilicates and tectosilicates, such as kaolin, or any other compound containing essentially hydrous aluminium silicate which is as pure as possible.

3. Process according to Claims 1 or 2, characterised in that 25 the slip consists of a suspension of kaolin in water and the film of this slip deposited on the said surface is treated at 400°-440°C for 4-6 hours.

4. Process according to Claim 3, characterised in that the proportion by weight between the water and the kaolin is 30 between 40% and 60% water.

5. Process according co any one of Claims 1 co 4 for Che elimination of lead in a superficial area of the wall of a hollow article made of crystal or lead glass which is finished or in the process of being manufactured, characterised in that .. 5 it consists of: 5 - filling this hollow body with a thixotropic slip in a viscous liquid state, - emptying the said body while allowing a continuous deposit of the said- viscous liquid adhering to this surface to remain 10 on its inner wall, - heating the said body bearing this deposit for a few hours at a temperature between 300°C and the softening temperature of the glass or crystal from which the said article is made, - allowing the said body to cool, 15 - eliminating, by washing in water and then in sand or by ultrasonic treatment, the silico-aluminous skin charged with lead ions resulting from the heating of the said deposit.

6. A process substantially as hereinbefore described, with reference co che Examples and drawings. 20

7. Arcicle made of crystal or lead glass and which has been treated by the process according co any one of Claims 1 to 6.