GB2069479A - Lead-free and Cadmium-free Frits and Production of Glazes thereform - Google Patents

Abstract

A lead- and cadmium-free frit which has a coefficient of thermal arcs expansion (20-300 DEG C) of from 65 to 75 ô 10<-7>/C DEG , and which comprises, in weight percent on the oxide basis, as calculated from the batch: from 35 to 47 SiO2, from 5.5 to 9 B2O3, from 24 to 42 BaO, from 1.5 to 4 TiO2, from 6 to 10 ZrO2, from 1 to 5 Li2O, from 0 to 5 MgO, from 0 to 4 CaO, from 0 to 8 SrO, from 0 to 10 ZnO, from 0 to 8 Bi2O3 and from 0 to 10 MgO+CaO+SrO+ZnO+Bi2O3 is disclosed.

Description

SPECIFICATION Lead-free and Cadmium-free Frits and Production of Glazes Therefrom This invention relates to lead-free and cadmium-free frits and to the production of glazes therefrom.

The use of glass, glass-ceramic and ceramic bodies in food service applications, i.e. as culinary ware and tableware, is widely recognised. Such bodies, especially those prepared from glasses and glass-ceramics, have occasionally been marketed in the plain state, i.e. without external decoration.

Nevertheless, the great majority of the commercially available food service products relies upon the aesthetic appearance of the products to promote customer appeal. Consequently, research and development to provide decorative designs and finishes for the surfaces of such bodies have been quite extensive. The bulk of this effort has been directed to developing attractive glazes and enamels.

Reduced to the most fundamental terms, glazes may be considered clear glasses and enamels as glazes containing pigments to impart colour thereto. Both products are applied to the surface of the glass, glass-ceramic or ceramic body to be decorated in the form of very finely-divided particles, customarily referred to as "frit", and this frit then fired to fuse to and form a strongly-adherent, continuous film on the surface of the body.

Most of the commercially-marketed frits for decorating glass, glass ceramic and ceramic ware have included substantial amounts of lead oxide (PbO) and, in several instances, cadmium oxide (CdO).

These compounds have been incorporated into the frit compositions for two principle reasons: (1) to soften the frit, i.e. to lower the melting point thereof, so it could be fused onto the surface of the ware at a sufficiently low temperature that thermal deformation of the ware could be avoided; and (2) to raise the refractive index thereof. CdO has also been utilized as a colourant in certain frits. Cadmium and lead, however, possess the inherent disadvantageous characteristic of being highly toxic, such that glazes and enamels containing these metals which come into contact with food must display extremely good resistance to attack by acidic and basic materials, thereby preventing the release of significant amounts of these metals.The danger arising from the release of these metals, when compositions containing such come into contact with food, has been recognised by the U.S. Food and Drug Administration (FDA). That agency, in its Compliance Guidance Manual, issued June 13, 1974, established maxima for cadmium and lead release which a food-contacting surface must not exceed.

It is quite apparent that the potential for cadmium and/or lead toxicity could be completely removed by preparing frits wherein these metals were totally absent. The inability to satisfy the following combination of four criteria required in an acceptable glaze or enamel has forestalled the simple elimination of these metals, particularly lead, from the frits.

Firstly, the firing or maturing temperature of the frit, viz. the temperature at which the frit will flow sufficently to produce a smooth homogeneous coating, must be low enough to preclude thermal deformation of the ware being coated.

Secondly, the coefficient of thermal expansion must be compatible with that of the ware to inhibit crazing and/or spalling of the resultant glaze or enamel. Most advantageously, the frit will demonstrate a coefficient of thermal expansion somewhat less than that of the ware being coated. Such practice places the decorating coating in a state of compression with respect to the ware after being fired thereon.

Thirdly, the frit must exhibit excellent resistance to attack by acids and bases and, in food service applications, to attack by detergents used in commercial dishwashers. Corrosion of the surface may result in loss of gloss, the development of haze or iridescence in the glaze or enamel, the formation of porosity, or other effects deleterious to the aesthetic appearance or physical character of the decorative coating.

Fourthly, the frit must demonstrate good glass stability, i.e. the frit must resist devitrification during firing.

Although not mandatory, a fifth desirable property for the frit is a refractive index high enough to yield a coating displaying a clear high gloss.

While compositions have been explored which have satisfied several of these criteria, research has been constant to develop frits free from lead and cadmium which would answer the full combination of these requirements.

It will, of course, be appreciated that frits may be utilized for purposes other than as bases for decorative glazes and enamels. In such applications, a high gloss, resulting from the high refractive index of the composition, may not be necessary. Also, in certain decorative practices, a highly glossy surface coating is not deemed desirable. Nevertheless, under either of the circumstances, the frits will still advantageously display the first four of the above-mentioned criteria. Thus, the firing temperature of the frit will be low enough to avoid thermal deformation of the substrate being coated. The coefficient of thermal expansion of the frit must be compatible with that of the substrate. The frit must demonstrate excellent chemical durability. Also, to ensure the development of a transparent coating, the frit will exhibit good glass stability.

Coloured glasses have been the subject of extensive research with every colour of the visible spectrum being produced. Coloured Glasseq W. W. Weyl, Dawson's of Pall Mall, London, 1959, is a monograph devoted to a study of the effects of various cations and anions in developing colours in glasses. The interactions between ions to form colours are essentially limitless and are still under investigation even today.

The use of iron with or without the inclusion of titanium to obtained yeilow-to-brown colourations in various glass compositions, especially those glasses containing lead, has been practiced for many years. Weyl, supra, devotes a number of pages to such phenomena, e.g. page 108 ff and page 212 ff. Likewise, the development of a bright red colouration in glasses (ruby glasses) via the inclusion of cadmium sulphoselenide in the composition is well known. Inasmuch as glazes are essentially glasses applied as a thin surface coating on a substrate, pigments containing iron and, optionally, titanium have been formulated and sold commercially for additions to frit compositions, as have pigments containing cadmium sulphoselenide.

Serious difficulties have been encountered, however, in attempting to pigmentize glazes containing cadmium oxides or lead oxides. The degree of the release of these toxic oxides from glazes, or, stated differently, the chemical durability of the glazes, is a function of the firing temperature employed in the glazing operation. Thus, as a general rule, the higher the firing temperature, the lower will be the level of lead and cadmium release. However, when frits were prepared of lead-containing compositions for colouring with iron+titanium pigments and fired to temperatures in excess of 7000C, the desired strong yellow-to-brown colouration was not developed. Instead, only a washed-out, pale yellow colouration was produced.In like manner, the firing of lead-containing frits with conventional cadmium sulphoselenide pigments at high temperatures did not yield the desired, customary bright red colouration, but only dull grey hues.

An object of the present invention is to produce frit compositions useful as base materials for glazes and enamels which, because the toxic heavy metals cadmium and lead are essentially absent therefrom, are eminently suitable for decorating food service ware.

Another object of the present invention is to produce such frits which demonstrate good glass stability, high indices of refraction, firing temperatures of from 700 to 9500C, coefficients of thermal expansion (20--300CC) of from 65 to 75x 10-7/CO and excellent resistance to attack by acids and bases, especially attack by detergents since a primary application envisaged for the present frits is in the field of food service ware decoration, and, where transparent coatings displaying high gloss are desired, the frits will exhibit high refractive indices.

It is a further object of the present invention to provide coloured glazes produced on the basis of the present frits and providing strong colours particularly yellow-to-brown or red colourations, which are stable at elevated temperatures.

It has now been found that frits satisfying these objects may be produced from compositions within the Li2O-BaO-TiO2-ZrO2-B2O3-SiO2 system, wherein at least one member selected from MgO, CaO,SrO,ZnOandBi2O3) is optionally present. Compositions falling within the operable ambit of the present frits consist essentially, expressed in weight percent on the oxide basis as calculated from the batch, of:: SiO2 35-47 B203 5.5-9 BaO 24-42 TiO2 1.5-4 ZrO2 6-10 Li2O 1-5 MgO 0-5 CaO 0--4 SrO 0--8 ZnO 0--10 Bi203 0--8 SrO+MgO+CaO+ZnO+Bi203 0-10 As is readily apparent, the potentially toxic heavy metals cadmium and lead form no part of the present compositions, thereby recommending the utility thereof in ware having contact with food.The above-recited components may be combined to provide frits displaying good glass stability, and, where desired, high indices of refraction to yield glazes and enamels exhibiting high gloss, firing (fusing) temperatures of from 700 to 9500C, coefficients of thermal expansion (20--3000C) of from 65 to 75x10-7/CO and excellent resistance to attack by acids and bases, in particular attack by detergents such as are employed in commercial dishwashers.

ZrO2 and TiO2 serve two functions. Firstly, the combination imparts the sxceptional chemical durability to the frits. Secondly, those ingredients (along with BaO) are useful in achieving a high index of refraction. However, the inclusion of such substantial amounts of those components, especially a large quantity of ZrO2 raises the softening temperature and the viscosity of the frits to a considerable extent, thereby requiring extensive adjustments to be made to the other constituents to secure the desired combination of the other parameters, particularly glass stability and low firing temperature.

BaO has a profound effect upon the glass stability and refractive index of the present frits.

Consequently, where glazes and enamels displaying very high gloss are desired, large amounts of BaO will be employed. When such is not required, the amount of BaO may be reduced.

Preferably, the present frits will be limited in composition to the components recited above.

Nevertheless, the presence of minor amounts of compatible metal oxides and fluoride may be tolerated to modify the melting behaviour of the frits and/or the physical properties thereof. However, the total of all such extraneous additions will not exceed 5%. Examples of such additions include very minor amounts of Na2O and/or K20 and/or F to soften the frit. The fluoride content will normally not exceed 3%. It will be recognized, of course, that pigments may be dissolved and /or suspended in the frits to yield an enamel for providing a coloured decoration. The colourants and pigments which have been most widely used commercially have been the transition metal oxides, e.g. CoO, NiO, MnO, Fe203, Cur203 and V205. More recently, various rare earths, particularly praeseodymium, erbium and neodymium, have been utilized for this purpose.Normally, the amount of such pigments will comprise less than 20%, by weight, and, frequently, less than 5%.

The most preferred compositions will demonstrate coefficients of thermal expansion (20- 3000C) of less than 70x 1 0-7/CO and may be fired to mature glazes at temperatures of from 700 to 850at. The low coefficients of thermal expansion enable the present frits to be compatible with a large proportion of commercially-marketed glass, glass-ceramic and ceramic bodies utilized in food service applications. As an added advantage, such low coefficients of thermal expansion result in the decorative coating being in compression with respect to the substrate, consequently endowing the coated ware with improved mechanical strength. The low maturing temperatures permit the frits to be fired onto commercially-marketed food service ware without the thermal deformation thereof.These most preferred frit compositions consist essentially, expressed in weight percent on the oxide basis as calculated from the batch, of: SiO2 40-45 B203 6-8 BaO 31-39 TiO2 2-3 ZrO2 8-9.5 Li2O 1-2.5 MgO 0--4 CaO 0--4 SrO 0--8 ZnO 0--6 Bi203 0-7 MgO+CaO+SrO+ZnO+Bi203 2-8 It has also been discovered that strong thermally-stable, yellow-to-brown colours, most commonly orange-to-brownish red colours, may be developed with such frits by additions thereto of from 0.5 to 10%, by weight, of Fe203 and from 0 to 4%, by weight, of TiO2.Furthermore, a bright red colour may be produced with such frits when mixed with from 2 to 10%, by weight, cadmium sulphoselenide and fired in a non-oxidizing or inert atmosphere. Hence, according to a further embodiment of the present invention, these additions are made to the present frit to provide strong yellow-to-brown or red colouration, even after firing at elevated temperatures.

In particular, glazes exhibiting strong yellow-to-brown colouration may be obtained by applying a mixture of the present frit and from 0.5 to 10%, by weight, Fe203+from 0 to 4%, by weight, TO2, to a glass, glass-ceramic or ceramic body and firing to from 700 to 9500C for a period sufficient to cause the frit to flow and produce a smooth homogeneous coating.

In a similar manner, glazes having a bright red colouration may be obtained by adding to the present frit from 2 to 10%, by weight, cadmium sulphoselenide and carrying out the firing in a nonoxidizing or inert environment.

United States Patent No. 4,084,976 is directed to lead-free glazes to be utilized in conjunction with alumina bodies. Such glazes consist essentially, in weight percent, of SiO2 50-54 Al2O3 5-8 B203 6-12 CaO 4-6 MgO 2-8 BaO 2-5 SrO 5-8 ZnO 1-2 Li2o+Na2o+K2O 4-6 The working examples reported were fired at 11 600 C. Such glazes are outside the scope of the present compositions.

United States Patent No.4,120,733 is also drawn to lead-free glazes useful with alumina bodies and statedly provides glazes which are an improvement upon those described in U.S. Patent No. 4,084,976, supra, in that they mature at temperatures from 26 to 380C (from 80 to 1 000F) lower.

Such glazes consist essentially, in weight percent, of SiO2 48-54 Awl203 7-11 B203 16.5-20 BaO 11-14 CaO 2-3 ZnO 2-2.5 Na2O 4.25-5.25 K20 0.4-1 The working Examples reported were fired at from 1038 to 1 200 C. Such compositions are quite remote from the present glazes.

United States Patent No. 4,224,074 discloses frits suitable for decorative glazes and enamels which are particularly desirable for food service ware because the potentially toxic metals cadmium and lead are absent therefrom. Such frits have compositions within the R20-Al203-B203-ZrO2-SiO2-F system, wherein R20 consists of Na2O with, optionally, Li2O. As such, those frits are outside the purview of the present compositions.

Table I below reports a group of frit compositions, expressed in parts, by weight, on the oxide basis, as calculated from the batch, illustrating the present invention. Inasmuch as the sum of the individual constituents equals or closely approximates 100, for all practical purposes, the various components may be considered to be present in weight percent. The actual batch ingredients employed may be various materials, either the oxide or other compound, which, when melted together with the combination of the several constituents, will be converted into the desired oxide in the proper proportions.

Batches for the recorded exempiary compositions were compounded in the indicated proportions and ball-milled to assist in securing a homogeneous melt. Each batch was deposited into a platinum crucible and then introduced into a furnace operating at from 1200 to 13000 C. After melting for from two to four hours, one portion of the molten batch was formed into a glass patty having dimensions of about 1 5x 5x 1 cms (6" x2" xO.37 5") and that patty immediately transferred to an annealer operating at from 500 to 6000 C. The remainder of the melt was poured into a bath of water (drigaged) to yield finely-divided glass particles which, after drying, were dry ball-milled to pass a No. 325 United States Standard Sieve (44 microns).

Table I also lists the softening point (Soft.), annealing point (Ann.), each reported in OC, and coefficient of thermal expansion (Exp.) over the interval 20-3000 C, reported in terms of xl 0-7/CO, as determined from the above-described annealed patties. Each of the above determinations was undertaken utilizing measuring methods conventional in the glass art.

Table I 1 2 3 4 5 6 SiO2 40.92 41.88 44.38 41.68 42.33 42.52 BaO 33.74 34.48 31.93 38.65 30.53 35.05 B203 6.49 6.60 7.00 6.61 6.71 6.74 ZrO2 8.25 8.50 9.00 8.40 8.53 8.57 TiO2 2.45 2.50 2.65 2.49 2.53 2.54 Li2O 1.73 1.34 1.42 2.17 1.35 1.36 By203 6.43 - - - - MgO - - 3.62 - - ZnO - 4.70 - - - SrO - - - - 8.00 CaO - - - - - 3.20 Soft 744 761 766 746 769 767 Ann. 598 613 608 599 619 606 Exp. 69.2 66.1 64.6 72.3 70.7 70.8 As was discussed above, the drigaged glass particles were ground to a very finely-divided powder. The powder was hand pressed into a cylindrical shape having a height of about 1.3 cms (0.5") and a diameter of about 1.3 cms (0.5"). Each cylinder was placed onto a platinum foil having a thickness of about 0.06 cms (0.025") and the foil introduced into a electrically-fired furnace operating at a desired temperature, the specimens maintained at that temperature for about 10 minutes and then withdrawn from the furnace to cool to room temperature in the ambient environment.

This firing of the cylindrical mass causes the frit to fuse and flow into the shape of a button, the diameter of which is a practical indication of viscous flow. Thus, laboratory experience has revealed that a button diameter of at least about 1.4 cm (-9/1 6") is necessary to provide good flow of the fused frit and wetting of a particular substrate which is to be coated. The flow buttons were peeled from the foil and thereafter exposed to tests to measure resistance to chemical attack. It will be apparent, of course, that a particular frit-substrate combination would be tested as such and examined for the degree and wetting of the coating, as well as for the presence of pinholes, crazing and spalling.

To ascertain the resistance of the frits to acids, the fusion buttons were weighed and then immersed into an aqueous solution containing 10%, by weight, citric acid, these immersions were undertaken for 24 hours in a stirred solution operating at a temperature of 960C. A weight loss of no more than 0.02% was considered to indicate good resistance to acid attack. Each of the fusion buttons was also subjected to another test which had been devised to indicate the durability of the frits against alkaline detergents such as are employed in commercial dishwashers. In this practice, weighed fusion buttons are immersed into a 0.3% by weight, aqueous solution of an alkaline detergent marketed by Economics Laboratories, St. Paul, Minnesota, U.S.A. under the trade mark "Super Soilax".The immersion is continued for 24 hours at 960C, following which the samples are withdrawn from the solution, rinsed in tap water, dried, examined visually and re-weighed to measure any weight loss. The exposure period of 24 hours was considered to represent an accelerated equivalent of several years' actual use in food service applications. A weight loss of no more than 0.02% is deemed to manifest excellent detergent resistance. The buttons were examined visually after each test to note any change in glossy appearance.

Table li below reports the firing temperature utilized (by), the % weight loss after 24 hours in the citric acid solution and the % weight loss after 24 hours in the "Super Soilax" solution.

Table II Example Firing % Loss Change of % Loss Change of No. Temp. Detergent - Gloss Ctiric Acid Gloss 1 8000 0.002 None 0.01 None 2 850 0.004 " 0.02 3 850" 0 ,, 0.02 4 8500 0.007 " 0.009 5 8500 0.006 ,, 0.002 6 8500 0.004 " 0.003 As may be seen from Table II, each of the frits displayed excellent resistance to attack by both acids and bases. Each of the samples manifested highly glossy surfaces after the tests. This desirable feature is highly important where decorative glazes and enamels are the products sought. For example, glazes have been prepared which demonstrate very low loss of weight after immersion into acids and bases, but which are subject to severe loss of gloss as a result.Such products would obviously have to utility where highly decorative food service is desired.

To study the utility of the present frits in combination with various substrate materials, the above examplary compositions were fritted, ground to pass a No. 325 United States Standard Sieve, applied to the surface of the substrate and then fired to maturity.

Thus, the powdered frits of Examples 1 and 2 were blended with about 5%, by weight, of commercial pigments, the mixture silk screened onto dinner plates of "Pyroceram" ware, a glass-ceramic tableware product marketed by Corning Glass Works, Corning, New York, U.S.A., having a coefficient of Thermal expansion (0-3000C) of about 97x10-7/C0, and then fired for 15 minutes at 8750and 8250C., respectively. The fired glazes developed good gloss and easily passed both the above-described acid text and an exposure of 96 hours to the detergent test.Inasmuch as the coefficient of thermal expansion of Example 1 is 69.2x 1 O-7/C0 and that of Example 2 is 66.1 xl d-7/C0, the glazes produce a surface compression layer on the dinnerware which imparts improved mechanical strength thereto.

Likewise, sufficient powdered frit of Example 3 was suspended in distilled water to yield a solids content of about 65%, by weight, this slurry sprayed onto 0.6 cm (0.25") diameter rods of a glassceramic having a coefficient of thermal expansion (O-3000C) of about 1 00 x 10-7/CO and then fired at 9000C for 10 minutes. Again, the fired glaze exhibited good gloss and easily passed both the acid and detergent tests. The lower coefficient of thermal expansion inherent in the Example 3 composition caused the mechanical strength of the rods, as represented in terms of modulus of rupture to be increased from about 844 kg/sq.cm (12,000 psi) to 1 575 kg/sq.cm (22,400 psi).

Slurries of powdered frit of Examples 5 and 6 were then prepared in like manner to that described above for Example 3 and sprayed onto 3.2 cm (1.25") squares of a glass-ceramic having a coefficient of thermal expansion (O-3000C) of about 95x 1 O-7/C0. Upon firing for about 10 minutes at temperatures of from 800 to 9000 C, a smooth glaze resulted which displayed high gloss and excellent resistance to acid and detergent. Yet again, the lower coefficient of thermal expansion of the glazes with respect to the substrate imparted improved mechanical strength to the sample squares.

The following Examples illustrate the advantageous aspects of the coloured glazes according to the present invention.

A frit exhibiting a coefficient of thermal expansion (20--3000C) of about 66.4x 1 0-7/CO was prepared having the following approximate composition, expressed in weight percent on the oxide basis, as calculated from the batch, of BaO 28.57 MgO 2.50 Li2O 3.35 SiO2 46.19 B203 7.33 ZrO2 9.30 TiO2 2.76 and comminuted to pass a No. 325 United States Standard Sieve (44 microns).The resultant powder was blended with 1%, by weight Fe2O3 and 2%, by weight, TiO2 in powder form, silk screened onto dinner plates of "Pyroceram" ware, a glass-ceramic tableware marketed by Corning Glass Works, Corning, New York, U.S.A., having a coefficient of thermal expansion (0--3000C) of about 97x10-7/CO and the coated plates fired at 7500C for 1 5 minutes. The glaze exhibited high gloss and a strong brownish red colour.

Where a conventional lead-based frit was combined with Fe2O3 and TiO2 in like amounts to the above illustrative Example and fired onto dinnerware plates of "Pyroceram" ware under the same conditions, only a very pale yellow hue was observed.

A frit exhibiting a coefficient of thermal expansion (2O-3000C) of about 66.8x10-7/CO was prepared having the following approximate composition, expressed in weight percent on the oxide basis as calculated from the batch, of BaO 33.86 SiO2 40.80 Li2O 1.31 ZnO 4.37 B203 6.47 ZrO2 8.22 TiO2 2.44 CdO 2.53 and comminuted to pass a No. 325 United States Standard Sieve (44 microns). The fine particles were then mixed with 5%, by weight, of a powdered commercial cadmium sulphoselenide pigment, silk screened onto dinner plates of "Pyroceram" ware and the coated plates fired at 8500C for 1 5 minutes in a nitrogen atmosphere. The resulting glaze displayed high gloss and a bright red colour.

Firing in other non-oxidizing or inert atmospheres, e.g. argon and helium, or slightly reducing environments, e.g. forming gas (hydrogen+nitrogen) and mixtures of CO and CO2, will also yield bright red colours. Yet. when the glazing is undertaken in air or other oxidizing atmosphere, the red colouration is not thermally stable with only dull grey hues being produced.

A lead-containing frit, an alkali borosilicate frit and a zinc borosilicate frit were each blended with 5%, by weight, of the cadmium sulphoselenide pigment and fired onto "Pyroceram" brand dinner plates in a nitrogen atmosphere in like manner to that described above. Only dull grey colours resulted evidencing the uniqueness of the present frits for the purpose of providing strongly coloured glazes.

Claims (14)

Claims

1. A lead- and cadmium-free frit which has a coefficient of thermal expansion (20--3000C) of from 65 to 75x 1 0-7/C0 and which comprises, in weight percent on the oxide basis, as calculated from the batch: from 35 to 47 SiO2, from 5.5 to 9 B2O3, from 24 to 42 BaO, from 1.5 to 4 TiO2, from 6 to 10 ZrO2, from 1 to 5 Li2O, from O to 5 MgO, from O to 4 CaO, from O to 8 SrO, from O to 10 ZnO, from 0 to 8 Bi203 and from 0 to 10 MgO+CaO+SrO+ZnO+Bi203.

2. A frit as claimed in claim 1 having a coefficient of thermal expansion (20--3000C) of from 65 to 70x 1 0-7/C0 and comprising, in weight percent on the oxide basis, as calculated from the batch: from 35 to 47 SiO2, from 6 to 8 B203, from 31 to 39 BaO, from 2 to 3 TiO2, from 0 to 4 MgO, from 0 to 4 CaO, from O to 8 SrO, from O to 6 ZnO, from 8 to 9.5 Zoo2, from 1 to 2.5 Li2O, from 0 to 7 By203 and from 2 to 8 MgO+CaO+SrO+ZnO+Bi203.

3. A frit as claimed in claim 1 or claim 2 substantially as herein described.

4. A glaze which exhibits a strong, yellow-to-brown colouration which is stable at temperatures of from 700 to 9500C and which comprises from 0.5 to 10%, by weight, Fe203 and from 0 to 4%, by weight TiO2 and a frit as claimed in any of claims 1 to 3.

5. A glaze which exhibits a bright red colouration which is stable at temperatures of from 700 to 9500C in a non-oxidizing or inert environment and which comprises from 2 to 10%, by weight, cadmium sulphoselenide and a frit as claimed in any of claims 1 to 3.

6. A glaze as claimed in claim 4 or claim 5 substantially as herein described.

7. A process for the production of a frit as claimed in any of claims 1 to 3 substantially as herein described.

8. A frit as claimed in any of claims 1 to 3 when produced by a process as claimed in claim 7.

9. A process for the production of a glaze as claimed in any of claims 4 to 6 substantially as herein described.

10. A glaze as claimed in any of claims 4 to 6 when produced by a process as claimed in claim 9.

1 A method for coating a glass, glass-ceramic or ceramic body with a glaze exhibiting a strong yellow-to-brown colouration which compises: (a) mixing a powdered glaze as defined in claim 4; (b) applying a mixture to a surface of a glass, glass-ceramic or ceramic body; and (c) firing the body at from 700 to 9500C to cause the glaze to flow and produce a smooth homogeneous coating.

12. A method for coating a glass, glass-ceramic or ceramic body with a glaze exhibiting a bright red colouration which comprises: (a) mixing a powdered glaze as defined in claim 5; (b) applying the mixture to a surface of a glass, glass-ceramic or ceramic body; and (c) firing the body at from 700 to 9500C in a non-oxidizing or inert environment to cause the glaze to flow and produce a smooth homogeneous coating.

13. A method as claimed in claim 12 in which the inert environment is a nitrogen atmosphere.

14. A method as claimed in any of claims 11 to 13 substantially as herein described.

1 5. A glass, glass-ceramic or ceramic body when coated by a method as claimed in any of claims 11 to 14.