WO1995009131A1

WO1995009131A1 - Cobalt glass compositions for coatings

Info

Publication number: WO1995009131A1
Application number: PCT/GB1994/002105
Authority: WO
Inventors: Peter John Smith; Harbans Kaur Mangat
Original assignee: Cookson Matthey Ceramics & Materials Limited
Priority date: 1993-09-28
Filing date: 1994-09-28
Publication date: 1995-04-06
Also published as: EP0721430A1; GB9319971D0; US5747395A; DE69419928D1; EP0721430B1; AU7703694A; DE69419928T2

Abstract

A glass composition which is essentially free from lead and cadium comprises SiO2: 30 to 70 wt%, B2O3: 1 to 25 wt%, Al2O3: 0 to 10 wt%, Bi2O3: 0 to 5 wt%, CoO: 0.1 to 30 wt%, ZnO: 0.1 to 20 wt%, at least one of Li2O, Na2O or K2O: 1 to 30 wt% total, ZrO2 up to 10 wt% and other optional ingredients. The glass composition can be used as a blue coloured on-glaze enamel which relies upon the presence of Co2+ in a network-forming position in the glass to produce the blue colour, with oxygen donors such as ZnO¿2?, alkaline earth oxides and alkali oxides increasing the amount of cobalt occupying network-forming positions, and ZrO2 to provide chemical durability.

Description

COBALT GLASS COMPOSITIONS FOR COATINGS

The present invention relates to glass

compositions and, in particular to glass compositions which are of minimal toxicity and are substantially free of lead and cadmium and which can be used either as blue coloured glasses, for mixing with pigments to provide other colour compositions for the decoration of ceramic materials and/or glass, or as pigments per se.

Occasional episodes of lead poisoning have resulted from the use of improperly formulated and fired lead-containing glazes and/or decoration enamels on ceramic ware. Whilst lead containing glazes and decoration enamels can be prepared which are safe, and meet current reguirements for

permissible lead release to food with which they come into contact, the problem of lead poisoning is avoided if lead itself is avoided. In addition, various pollution controls regarding the use of lead and limits on the content of lead in waste water can be avoided by the use of lead-free glazes and

decoration enamels.

Consequently, glaze and enamel decorating

suppliers have strived to provide lead-free products which meet the necessary technical requirements, particularly the more exacting demands on

durability. However, the development of a durable and intense royal blue colour has proved to be an exception. In the past, royal blue on-glaze enamels were generally produced using high-leaded,

transparent glassy fluxes and pigments based on the crystalline CoO-ZnO-SiO system. For the new

lead-free colours, transparent lead-free borosilicate fluxes have been used in place of the leaded

versions. However, as the CoO-ZnO-SiO phase is soluble in mild acid, replacement of leaded fluxes with lead-free equivalents does not fully address the durability problem. Moreover, the standard practice of increasing colour intensity by increasing the pigment loading serves only to degrade the chemical resistance still further. Thus, the conventional pigmented flux approach is incapable of producing a durable and intense blue enamel without recourse to elaborate coating techniques for the pigment. Hence, an alternative method of producing a durable intense blue enamel for tableware decoration is required.

The use of a durable lead-free intense blue glass in place of a conventional pigmented flux is herein disclosed.

Various attempts have been made to produce blue glasses for enamel decoration of jewellery, glassware and tableware. However, the widespread use of on-glaze blue glass enamels for tableware has been limited by shortcomings in certain key areas.

In addition to being lead-free and cadmium-free, a coloured glass to be used for on-glaze enamel decoration must simultaneously satisfy a number of key requirements. Firstly, the softening point of the glass should be such as to allow maturation within the temperature range of from 750° to

900 C, which are typical on-glaze enamelling

temperatures for decoration on bone china,

earthenware, hotelware or porcelain pieces.

Secondly, the coloured glass should be compatible with a wide variety of glazes.

Thirdly the coloured glass should display good resistance to pronounced opacity or mattness during a conventional enamel firing schedule in order to promote good colour development with a glossy finish.

Fourthly, the linear coefficient of thermal expansion (CTE) of the coloured glass should be compatible, or made compatible through additions of filler materials, with that of the glaze coated thereon to prevent crazing and/or spalling.

Finally, the durability of the coloured glass should be such as to enable the fired colours to withstand attack by acidic foodstuffs and alkali detergents. With controls on permissable levels of metal release into acidic and alkali solutions becoming more stringent, more exacting demands are being placed on the durability requirements of decorated pieces.

In the light of the present invention, it is important to stress that it is the durability of the final product, i.e fired colour, which is of concern, not that of the glassy frit which may change in use.

We have now developed a glass composition which can be used as a blue coloured on-glaze enamel which simultaneously meets, or at least substantially meets, the aforementioned key requirements. It relies upon the presence of Co ^{2 +} in a network- forming position in the glass to produce the blue colour, with oxygen donors such as ZnO₂, alkaline earth oxides and alkali oxides increasing the amount of cobalt occupying network-forming positions, and ZrO₂ to provide chemical durability.

Accordingly the present invention provides a glass composition which is essentially free from lead and cadmium and which comprises the following

components:- Weight per cent

SiO₂ 30-70

B₂O₃ 1-25

Al₂O₃ 0-10

Bi₂O₃ 0-50

COO 0.1-25

At least one of

Li₂O 0-10

Na₂O 0-20 1-30 (in total)

K₂O 0-20

Optionally one or more of MgO, CaO, SrO or BaO

0-15

ZnO 0.1-20

ZrO₂ up to 10

TiO₂ 0-1 optionally one or more of SnO₂, WO₃, MoO₃ or

Nb₂O₅

0-5 optionally one or more of Y₂O₃, La₂O₃ or

CeO₂

0-5

P₂O₅ 0-5

The glass of the present invention relies on the presence of Co²⁺ i.n the glass network to provide a blue colour.

The glasses of the present invention may be non-crystallizing or "stable". However, the intensity of the blue colour of the glass may be further increased by altering the glass chemistry to promote precipitation of the (Co,Zn)₂SiO₄ phase either during enamel firing or by a prior thermal treatment of the glass. Optionally a nucleating agent such as ZrO₂, P₂O₅ or other agents known to those

skilled in the art of glass ceramics may be employed to assist in the (Co,Zn)₂SiO₄ precipitation.

Preferred compositions of the present invention are detailed below:

Preferred Range More Preferred Range wt% wt%

SiO₂ 35-60 35-55

B₂O₃ 1-20 2-10

Al₂O₃ 1-10 2-6

CoO 1-20 see below

R₂O 1-20 2-15

ZnO 1-15 see below

ZrO₂ 1-7 2-5

R₂O + ZrO₂ 1-25 2-20

For non-crystallizing or "stable" glasses ZnO is preferably included in an amount of from 1 to 5 wt%, more preferably 1 to 3 wt% , and the preferred range for CoO is 10 to 20 wt%, more preferably 10 to 15 wt%. For crystallizing glasses which favour the precipitation of (Co,Zn)₂SiO₄, the preferred

range for ZnO is 5 to 15 wt%, more preferably 7 to 12 wt%, and the preferred range for CoO is 5 to 15 wt%, more preferably 5 to 12 wt%.

For glasses with CTE's below 7.0 X 10^-6/°C, the presence of Bi₂O₃ is generally required in order to provide a glossy fired appearance. The preferred range is 5-25 wt%., the more preferred range is 3-15 wt%.

The composition of the present invention contains ZrO₂ in order to improve the resistance of the

compositions to attack by alkali detergent solutions.

The glasses of the invention can be prepared from natural (benificated) rocks and minerals e.g.

limestone, silica sand etc. as well as "fine

chemicals".

The various minerals and/or oxides used in the preparation of the compositions of the present

invention are usually in the form of fine powders. Precursors of these oxides can also be useful,

providing that they decompose to form the required oxides at temperatures below the melting temperature of the glass. Suitable "fine chemical" precursors are the nitrites, nitrates, carbonates, metal organic salts, for example citrates, acetates, etc.

The glass of the present invention may be

prepared by mixing together materials, charging the mixture of materials to a glass melting furnace at a temperature sufficiently high to produce the fluxed glass and then fritting the glass by pouring it into water, or by passing it through water cooled rolls. It may be preferred to carry out the melting and fritting operation in an oxidising atmosphere, or to include a oxygen rich component in the mixture which is melted and fused. The frit may be ground into a powder by conventional grinding techniques.

The glasses of the present invention are

preferably fluxes which are used in combination with one or more pigments as a colour on ceramic substrates or glass.

It will be understood by those skilled in the art that for use as an enamel on tableware the glass compositions must mature or yield a smooth continuous surface at temperatures below the melting

temperatures of the glazes on the ceramic articles on which they are to be used. Generally, the glass compositions will melt at a temperature of below 1350 C, preferably at a temperature in the range of from 1150°C to 1300°C.

The present invention thus includes within its scope a composition for the decoration of ceramic substrates and/or glasses which comprise a blue glass frit as hereinbefore described on its own or in admixture with one or more pigments, or other

colourants, or filler material such as fused silica, β-spodumene or β-eucryptite. The compositon for the decoration of ceramic materials and/or glass may additionally contain a plastic binder therein.

The glasses of the present invention may be made into decoration printing inks or pastes and applied to ceramic or glass substrates using standard

decoration methods. These include silk-screen printing, pad printing, or by application of

decalomania transfers (decals) to the substrates. To make the printing inks or pastes, a dry powder of the glass is dispersed in an oil-, water-, solvent- or wax-based medium with the aid of shear mixers and triple roll mills. For silk-screen printing, oil-based media are generally used, whilst for pad printing, wax-based inks are more commonly used because of their thermoplastic nature.

In silk-screen printing, the ink or paste is applied to the substrate by using a sharply-edged rubber squeegee to press the ink through the open areas of a fine mesh stretched on a frame. The mesh is generally no longer made of silk but is more commonly made of either metal or polyester. The substrate is positioned below the screen and as the two contact, the ink adheres in a precise pattern relating to the open area of the design of the screen. Precise control over the thickness of the ink or paste is achieved through adjustment of the ink or paste rheology, the diameter of the mesh filament or pores, the distance of the screen from the substrate and the pressure on the squeegee.

Pad printing involves transferring an ink design from a metal or plastic engraved cliche (plate) or from a screen printed image (an image deposited on to a flat silicone plate), to a substrate by means, for example, of a pliable silicone pad. For application using a cliche, depending on the machine type the first step in the process involves pouring the ink into or directly on to the etched plate. The cliche is flooded with ink and then cleaned off with a doctor blade, leaving the ink only in the etched impression of the image. Using appropriate pressures and cycle times, the pad is pressed on to the etched portion of the plate, where it picks up the ink. The pad then rises up and travels laterally towards the substrate. It is lowered (with appropriate pressure and cycle time) on to the substrate to deposit the image. Pad printing has the advantage over

silk-screen printing of enabling printing on curved and discontinuous surfaces, but traditionally is less suited for printing large surface areas.

For application using decalomania transfers

(decals), the decoration or design containing the glass enamel, is formed independent of the surface to be decorated, usually on a transfer paper or other carrier. Subsequently, as and when required, the decal is transferred on to the substrate surface.

For all printing methods, the organic components of the printing inks or pastes either evaporate or burn away during thermal fusion of the decoration to the substrate during the firing cycle.

The glasses of the present invention are

compatible both with conventional glazes on ceramic materials and with the bismuth based glazes which are described in our European Patent Application No.

91303072.2.

The glasses of the present invention give good colour development, i.e. the fired colour is bright and "wet".

They can be made suitable for either bone china, fine china, hotelware, earthenware and porcelain by altering the thermal expansion of the glass to match that of the substrate. For the low-expansion

hotelware and porcelain ware, the addition of a clear "soft" glassy flux may be required to boost the gloss of the blue glass with appropriately lowered CTE.

Alternatively, filler material such as fused silica or /3-spodumene may be added to the glass such that the net thermal expansion of the fired mixture is similar to that of the tableware piece.

For on-glaze enamel firing at 800-850°C, the softening point (S.P.) of the glass should be in the region glass 450-600 C, while for enamel firing at 950°C, it may be as high as 700°C. To ensure

that the fired enamels do not develop stress-related cracks, the CTE's of the enamels should lie ideally in the range 7.0 - 9.5 X 10^{- 6}/ºC for use on bone china and in the range 4.5 - 5.6 X 10^{- 6}/ºC for use on porcelain. However, for thin enamel coatings (<10μm), the thermal expansion mismatch with the substrate can be greater.

The glass compositions of the present invention, when fired as an on-glaze enamel on tableware preferably develop a blue colour defined by the co-ordinates L(lightness) = 30 to 80, preferably 30 to 60, C(intensity) = 10 to 50, preferably 30 to 50, and H(hue) = 255 to 300, preferably 270 to 300.

The glass compositions of the present invention may also be used in vitreous enamel coating

compositions for application to metallic substrates, with the particular glass composition being chosen with a thermal expansion which is compatible with that of the metallic substrate.

In another embodiment of the present invention the crystallizing glasses may be thermally treated, for example, at a temperature in the range of from 750° to 950 C, and subsequently ground to the

desired particle size range of from 0.5 to 500

micrometres, preferably 0.5 to 50 micrometres. The particulate pigments may be dispersed in paints, polymers, ceramic bodies or glazes to provide colour in a manner analogous to conventional pigments, the particle size of the pigment being chosen having regard to the desired application.

The present invention will be further described with reference to the following non-limiting Examples.

Examples 1 to 55

For the examples provided, glass frits were prepared by fusing together raw materials of the listed oxides in the amounts as stated in parts by weight at 1200 C, and fritting the glasses

so-produced by pouring into water. Following

fritting, the coarse frits were ground to give a particle size distribution of 90% less than 16

micrometres, silk-screen printed and fired onto either bone china (BC) or porcelain (P), at a heating rate of 13°C per minute to 820°C, with a dwell time of 20 minutes. All of the glasses fired to give a glossy appearance. For comparative purposes, a commercially available enamel, 33UL19, taken from Cookson Matthey Ceramics (previously Cookson Ceramics and Minerals) unleaded Pathfinder Range of on-glaze colours for tableware decoration, was chosen. The 33UL19 blue enamel is a conventional pigmented flux based on an unleaded borosilicate flux and a

cobalt-containing oxide pigment.

The fired enamels were assessed in terms of colour development and acid and alkaline durability. The developed colour was assessed using an ICS

Texicon Q.C. Spectrophotometer in the reflectance mode. The colour was defined in terms of the

co-ordinates L(lightness), C(intensity) and H(hue). On bone china the 33UL19 colour was defined as L = 43.48, C = 37.90 and H = 88.68, while on porcelain, 33UL19 developed a colour defined as

L = 47.14, C = 36.16 and H = 285.66.

The glasses of Examples 1 to 5 and 29 to 51 are stable glasses, whilst the glasses of Examples 6 to 28 are crystallizing glasses in which the blue colour is intensified by the formation of (Co,Zn)₂SiO₄ crystallites during the enamel firing cycle.

For the alkali test, the test conditions were immersion for 7 hours in a concentrated solution of a sodium phosphate - based commercial detergent held at 77ºC, giving a pH = 13. The acid test involved a 100 hour immersion in a 4% acetic acid solution at 25ºC, giving a pH = 4.

Following completion of the test, acid and alkali durabilities were evaluated by assigning a number according to the following convention:

Durability Rating

1 = No visible loss of colour or gloss.

2 = Some loss of gloss and possible fading of

colour.

3 = Gloss removed or some loss of colour.

4 = Gloss removed and some loss of colour.

5 = Considerable attack causing removal of colour. For the standard 33UL19, the acid durability was rated at 2 and the alkaline durability was judged to have a rating of 3.

Ex 10 11 12 13 14 15 16 17 18 19 20

S iO₂ 36.28 53.78 49.05 39.02 59.18 45.13 35.02 45.00 56.71 40.00 36.36 B₂O₃ 5.83 6.74 5.55 10.62 7.42 5.66 4.62 6.48 7.11 5.00 6.61 Al₂O₃ 2.84 3.30 2.71 3.90 3.62 2.76 2.23 9.87 3.47 5.00 2.31

CoO 8.55 7.55 8.13 8.29 10.87 8.29 8.67 11.07 2.45 11.00 6.98 Bi₂O₃ 30.17 11.50 18.98 19.35 - 19.35 32.98 11.07 12.14 20.00 34.24

Li₂O 0.73 - - 0.71 - 0.71 0.58 - - 1.00 -

Na₂O 2.51 4.39 3.62 3.69 4.83 3.69 0.55 4.22 4.63 11.00 -

K₂O - - - 3.69 - 3.69 1.71 - - - 1.78

ZnO 10.56 9.88 8.13 8.29 10.87 8.30 8.07 9.50 10.42 3.00 8.38

MgO - - - - - - - - - - 2.07

ZrO₂ 2.51 2.90 3.83 2.44 3.20 2.44 5.51 2.79 3.06 4.0 2.48

La₂O₃ - - - - - - - - - - P₂O₅ - - - - - - - - - - -

TiO ₂ - - - - - - - - - - - SrO₂ - - - - - - - - - - -

Substrate P BC BC BC BC BC P BC BC P -

Colour: L 39 .49 53 .76 47 .52 43. 13 40.71 49 .44 44 .75 - 76. 42 62 . 94 -

C 33 .95 33 .32 32 .61 35. 85 52.17 37 .88 33 .50 - 10. 98 12 . 59 -

H 269 .13 268 .54 279 .96 276. 55 272.17 278 .04 259 .94 - 259 . 94 276. 0 3 -

Acid

Durability 4 - 4 - 1 3 4 - - - -

Alkaline

Durability 3 2 1 2/3 1 2 2 - - 2 -

Ex 21 22 23 24 25 26 27 28

SiO₂ 37.27 38.25 51.16 44.26 43.02 46.72 44.69 43.84 B₂O₃ 6.78 5.04 6.82 5.55 5.39 5.86 5.60 5.50Al₂O₃ 2.37 2.43 4.54 2.71 2.63 2.86 2.73 2.68 CoO 7.16 5.22 9.09 8.13 7.90 8.59 8.21 8.06 Bi₂O₃ 35.10 36.03 11.36 18.97 18.44 20.03 19.16 18.79

Li₂O 0.62 0.63 2.27 0.70 0.68 - 0.71 0.69

Na₂ O - 0.60 - 3.62 3.52 3.82 3.65 3.58

K₂O 1.82 1.87 - 3.62 3.52 - 3.65 3.58

ZnO 8.59 8.82 11.36 8.13 7.91 8.59 8.21 8.06 MgO - - - - - - - - ZrO 0.24 0.24 3.41 2.39 2.33 2.53 2.42 2.37

- - - 1.91 - - - -

La₂O₃

P₂O₅ - - - - 4.65 - 0.97 2.84 TiO₂ - 0.87 - - - - - -

SnO₂ - - - - - 1.0 - -

Substrate BC B C BC I 3C BC BC BC BC

Colour: L 46.58 45. 64 51. 05 62. .77 - 44.53 - -

C 40.32 41. 15 23. 77 13. .19 - 29.64 - -

H 278.42 281. 17 266. 06 276.98 - 270.41 - -

Acid

Durability 2 2 - - 5 - -

Alkaline

Durability 2 2 1 - 3 - -

Ex 29 30 31 32 33 34 35 36 37 38

SiO₂ 51.40 42.44 48.63 53.40 50.79 49.82 49.58 49.58 46.00 49.00 B₂O₃ 8.52 8.00 8.46 8.85 8.42 8.25 8.21 8.21 8.00 8.00Al₂O₃ 2.71 4.00 2.69 2.82 2.68 2.63 2.61 2.61 3.00 3.00

CoO 12.65 12.79 12.56 9.25 10.75 10.54 12.20 12.20 15.00 12.00Bi₂O₃ - 10.00 5.90 - - - - - - - Li₂O - - 0.98 - - - - - - -

Na₂O 10.88 6.00 6.77 11.30 10.75 10.54 10.49 10.49 12.00 12.00K₂O - 2.00 - - - - - - - - ZnO 2.97 3.00 2.44 3.08 2.93 4.79 2.86 2.86 3.00 3.00

SrO 7.68 7.77 7.63 7.19 9.77 9.59 6.68 6.68 9.00 7.00

ZrO₂ 3.96 4.00 3.93 4.11 3.91 3.83 3.82 3.82 4.00 4.00

CeO₂ - - - - - - - 4.29 - - - - - - - - 4.29 - -

Nb₂O₅ - - - - - - - - - - 2.00

P₂O₅

Substrate BC BC BC BC BC BC BC BC BC BC

Colour : L 37.38 35.53 - 42.72 41.98 46.35 58.12 62.14 51.79 -

C 43.28 13.68 - 36.37 39.04 31.90 29.85 18.40 17.54 -

H 289.79 284.50 - 288.45 289.87 286.87 286.39 283.75 282.29 -

Acid

Durability 1 - - - 2 2 - - - -

Alkaline

Durability 2 2 2 1 2 2 - - - -

Ex 39 40 41

SiO₂ 49.65 49.65 54.70

B₂O₃ 7.61 7.61 8.61

Al₂O₃ 2.74 2.74 2.74

CoO 12.00 12.00 9.00

Li₂O - - 3.00

Na₂O 12.00 12.00 5.00

ZnO 3.00 3.00 3.00

SrO 7.00 7.00 19.00

ZrO₂ 4.00 4.00 4.00

P₂O₅ - - -

WO₃ 2.0 - -MnO₃ - 2.00 - I Substrate BC BC BC

Colour: L 50.20 51.85 55.19

C 27.36 26.78 28.47

H 284.14 284.35 285.15

Acid

Durability - - -

Alkaline

Durability - - -

Ex 42 43 44 45 46 47 48 49 5500 5511

SiO₂ 53.64 53.71 55.54 39.80 38.07 42.82 45.68 50.75 52.00 52.98

B₂O₃ 8.96 8.16 8.43 17.30 19.03 14.28 11.42 6.35 9.00 8.04

Al₂O₃ 1.68 1.77 1.82 2.69 2.69 2.69 2.69 2.69 - 1.74

CoO 10.69 9.68 10.01 12.57 12.57 12.57 12.57 12.57 13.00 9.54

Bi ₂O₃ - 0.71 - - - - - -

Li₂O 5.11 5.38 5.56 0.98 0.98 0.98 0.98 0.98 - 5.30Mn₂O 11.56 8.98 9.28 12.66 12.66 12.66 12.66 12.66 11.00 8.85H₂O - 2.10 2.17 - - - 2.07ZnO 2.30 2.43 2.51 2.44 2.44 2.44 2.44 2.44 3.00 2.39CaO - 4.63SrO 4.70 4.94 2.48 7.63 7.63 7.63 7.63 7.63 8.00 2.36 BaO -

ZrO₂ 1.36 2.14 2.20 3.93 3.93 3.93 3.93 3.93 44..0000 2.10

Substrate BC BC BC BC BC BC BC BC BBCC BC Colour: L 49.76 59.48 66.85 46.63 46.79 41.48 43.66 55.43 - C 39.75 29.09 17.97 33.80 32.86 42.24 40.35 21.04 - H 285.50 283.36 286.26 283.21 283.51 286.89 285.13 288.40 - Acid

Durability 2 - - - - - - - - -

Alkaline

Durability 2 - - 2 2 - - 3 -

The coefficients of thermal expansion of certain of the glass compositions tabulated above are given below:

Example No. CTE X 10^-6/°C

21 7.19

23 4.92

15 7.32

29 8.83

10 6.58

22 6.38

Examples 52 and 53

The glasses of the compositions of

Examples 15 and 17, in powder form having a particle size of 90% less than 14 micrometres were heated separately at 13 C/minute to 850°C, held at this temperature for 10 minutes and cooled down to room temperature at 13°C/min. The resulting fused materials were pulverised and passed through a 100 micrometre sieve. The intensity of the blue colour was increased in each case by the thermal treatment, with X-ray diffraction analysis confirming the presence of (Co,Zn)₂SiO₄ in each case.

Claims

CLAIMS :

1. A glass composition which is essentially free from lead and cadmium and which comprises the following components:-

Weight per cent

SiO₂ 30-70

B₂O₃ 1-25

Al₂O₃ 0-10

Bi₂O₃ 0-50

COO 0.1-30

At least one of

Li₂O 0-10

Na₂O 0-20 1-30 (in total)

K₂O 0-20 optionally one or more of MgO, CaO, SrO or BaO

0-15

ZnO 0.1-20

ZrO₂ up to 10

TlO₂ 0-1 optionally one or more of SnO₂, WO₃, MoO₃ or

Nb₂O₅

0-5 optionally one or more of Y₂O₃, La₂O₃ or

CeO₂

0-5

P₂O₅ 0-5

2. A glass composition as claimed in claim 1 which when fired as an on-glaze enamel on tableware develops a blue colour defined by the co-ordinates L = 30-80, preferably 30-60, C = 10-50, preferably 30-50, and H = 255-300, preferably 270-300.

3. A glass composition as claimed in claim 1 or claim 2 which when fired as an on-glaze enamel on tableware shows no visual evidence of attack

following immersion in 4% acetic acid solution at 25°C for 24 hours.

4. A glass composition as claimed in any one of claims 1 to 3 which when fired as an on-glaze enamel on tableware displays resistance to attack during immersion in a solution of sodium phosphate (pH 13) at 77°C for 7 hours.

5. A glass composition as claimed in any one of the preceding claims which contains CoO in an amount of from 1-20 wt%, preferably 2-15 wt%.

6. A glass composition as claimed in any one of the preceding claims which contains ZnO in an amount of from 1-15 wt%, preferably 2-15 wt%.

7. A glass composition as claimed in claim 5 or claim 6 which is a non-crystallizing composition and which comprises from 10 to 20 wt% CoO and from 1 to 5 wt% ZnO.

8. A glass composition as claimed in claim 7 which comprises from 10 to 15 wt% CoO and from 1 to 5 wt% ZnO.

9. A glass composition as claimed in claim 5 or claim 6 which is a crystallizing composition and which comprises from 5 to 15 wt% CoO and from 5 to 15 wt% ZnO.

10. A glass composition as claimed in claim 9 which comprises from 5 to 12 wt% CoO and from 7 to 12 wt% ZnO.

11. A glass composition as claimed in any one of claims 1 to 6 which contains Al₂O₃ in an amount of from 1 to 10 wt%.

12. A glass composition as claimed in any one of the preceding claims which contains ZrO₂ in an amount of from 1-7 wt%, preferably 2-5 wt%.

13. A glass composition as claimed in any one of the preceding claims, optionally in admixture with a filler material, for firing onto bone china which has a coefficient of thermal expansion in the range of from 7.0 to 9.5 X 10^-6/°C.

14. A glass composition as claimed in any one of the preceding claims, optionally in admixture with a filler material, for firing onto porcelain which has a coefficient of thermal expansion in the range of from 4.5 to 5.6 X 10^-6/°C.

15. A glass composition as claimed in claim 14 which contains Bi₂O₃ in an amount of from 5-25 wt%, preferably 3-15wt%,

16. A printing ink or printing paste which comprises a powder of a g}.ass composition as claimed in any one of the preceding claims dispersed in an oil-, water-, solvent- or wax-based medium.

17. A decalomania transfer which comprises a carrier material which has formed thereon a

decoration or design which is formed wholly or in part from a glass composition as claimed in any one of claims 1 to 15.

18. A decalomania transfer as claimed in claim 17 wherein the carrier material is a transfer paper.

19. An article which is decorated with a glass composition as claimed in any one of claims 1 to 15, a printing ink or printing paste as claimed in claim 16, or a decalomania transfer as claimed in claim 17 or claim 18.

20. An article as claimed in claim 19 which is an article of bone china, fine china, hotelware, earthenware or porcelain.

21. A vitreous enamel coating composition for application to metallic substrated which comprises a glass composition as claimed in any one of claims 1 to 12, 15 or 16, the said glass composition having a co-efficient of thermal expansion compatible with that the intended metal substrate.

22. A method for preparing a pigment composition which method comprises thermally treating a glass composition as claimed in any one of claims 1 to 6 or 9 to 15 which is a crystallizing glass composition, at a temperature in the range of from 750 to

950 C and comminuting the treated glass composition to a particle size in the range of from 0.5 to 500 micrometres.

23. A method as claimed in claim 22 wherein the treated glass composition has a particle size in the range of from 0.5 to 50 micrometres.

24. A pigment composition which comprises particles of a thermally treated crystallizing glass composition as claimed in any one of claims 1 to 6 or 9 to 15 having a particle size in the range of from 0.5 to 500 micrometres.

25. A pigment composition as claimed in claim 24 which has a particle size in the range of from 0.5 to 50 micrometres.

26. A paint, a polymer material, a ceramic body or a glaze which has a pigment as claimed in claim 24 or claim 25 dispersed therein or therethrough.