LEAD-FREE GLAZE FOR CERAMIC ARTICLES

Abstract: A lead-free glaze for ceramic articles, which is firable at a peak firing temperature of less than 1150 DEG C. The glaze comprises a borosilicate with greater than 3 % Li2O, and a total Na2O and K2O content of between 0.3 and 8 %.

Description: LEAD-FREE GLAZE FOR CERAMIC ARTICLES This invention concerns glazes, and particularly but not exclusively firable glazes applicable to ceramic articles; and also firable colours applicable to ceramic articles. There is concern over the release of toxic metals from tableware, and particularly the release of lead and cadmium. Legislation has been introduced in many areas with regard to the upper permissible limits of metal release, and these limits are generally being reduced. Accordingly, much emphasis is being placed on the elimination of lead from tableware glaze formulations, and also the removal of lead from working environments. The production of satisfactory lead free glazes and decorations has however failed to gain universal acceptance due to the limitation of the range of colours available and the poor durability of many unleaded systems. Unleaded glazes and colours often require higher firing temperatures than their lead containing equivalents, and this can cause deterioration in certain ceramic pigments. All percentages quoted in this specification are mol % of oxides. For the avoidance of doubt, the peak firing temperature of a glaze is the maximum temperature attained during firing of the article, which firing produces a continuous glaze layer. According to the present invention there is provided a lead free glaze, the glaze being firable at a peak firing temperature of less than 1150°C, the glaze comprising a borosilicate with greater than 396 Li 2 O, and a total Na 2 O and K 2 O content of between 0.3 and 8%. Preferably the glaze comprises less than 596 Li 2 O. The total content of Na 2 O and K 2 O is preferably less than 5% and desirably less than 2%. The glaze preferably comprises between 55 and 85% SiO 2 , and preferably comprises up to 15% Al 2 O 3 . The ratio SiO 2 :Al 2 O 3 is preferably between 7:1 and 13:1, and desirably between 8:1 and 10.5:1. The glaze preferably comprises less than 8% B 2 O 3 . The glaze preferably comprises between 0.5 and 15% CaO, and desirably between 5 and 8%. The glaze preferably comprises MgO and up to 5% thereof. The glaze preferably comprises BaO and up to 5% thereof. The glaze preferably comprises SrO and up to 5% thereof. The glaze preferably comprises ZnO and up to 8% thereof. The glaze preferably comprises Bi 2 O 3 and up to 5% thereof. The glaze preferably comprises V 2 O 5 and up to 2% thereof. The glaze preferably comprises WO s and up to 2% thereof. The glaze preferably comprises MoO 5 and up to 2% thereof. The glaze preferably comprises ZrO 2 and up to 8% thereof. The glaze preferably comprises TiO 2 and up to 2% thereof. The glaze preferably comprises a 2 O 3 and up to 2% thereof. The glaze preferably comprises CeO 2 and up to 2% thereof. The glaze preferably comprises GeO 2 and up to 2% thereof. The glaze preferably comprises SnO 2 and up to 8% thereof. The glaze may comprise partially or wholly fritted components to provide the required composition. The glaze is preferably made by mixing or milling together constituent components with a suspending agent which may be water, and perhaps also binders and/or stains. The glaze is preferably firable at a peak firing temperature of between 1050 and 1150°C. The invention also provides a firable colour, the colour comprising a mixture of a pigment, and a material with a composition the same as a glaze according to any of the preceding twenty two paragraphs. The material is preferably fritted. Embodiments of the present invention will now be desribed by way of example only. The table below shows the composition of seven glaze formulations according to the invention. These example glazes were produced as 90% frits and 10% china clay batches and were fired in electric intermittent laboratory kilns at 100°C per hour to peak firing temperature with a one hour dwell. The glazes were applied to vitreous hotelware biscuit plates. These glazes are not intended however to be exclusive to one body type. Test decoration fires were carried out on the glazed plates at 100°C /hour to a dwell at 800°C for thirty minutes. EXAMPLE % SiO 2 66.50 77.36 74.78 72.50 72.54 TiO 2 — 0.02 0.03 0.03 0.50 Al 2 O 3 8.00 7.93 7.50 7.50 7.20 Fe 2 o 3 — 0.09 0.05 0.05 0.04 CaO 7.50 8.20 1.91 7.00 7.00 MgO 1.00 0.12 0.60 1.00 0.75 κ 2 o 2.50 0.32 0.30 0.30 0.30 Na 2 O 3.50 0.39 0.20 0.20 0.75 ZrO 2 1.00 — — — 0.60 B 2°a 2.00 1.54 7.50 3.00 3.50 Li 2O 4.50 3.93 4.50 4.50 4.00 Bi 2 O 3 — — — — 0.30 BaO 2.00 — 1.00 2.50 1.00 ZnO — — 0.50 — — SrO 1.50 — 1.77 1.50 1.50 Mol % Oxide SiO 2 75.00 74.50 TiO 2 — — Al 2 O 3 6.25 6.00 Fe 2 O 3 — — CaO 5.00 5.50 MgO 1.25 1.00 κ 2 o 0.50 0.50 Na 2 O 1.50 1.50 ZrO 2 — 0.50 B A 3.00 3.00 Li O 3.50 3.50 Bi O BaO 2.50 2.00 ZnO SrO 1.50 2.00 Example 1 This example gave a good finish when fired to 1090°C. This glaze formula has an intermediate SiO 2 content between conventional fritted glazes for use at 1080°C and porcelain or stoneware glazes firing at 1200-1400°C. The glaze formula was made more complex by the addition of alkaline earths to reduce any tendency toward devitrification. This sample had a good appearance and produced good durability. Example 2 This formula has a high SiO level and was found to mature well at 1100°C and to have a good durability. The glaze is characterised by low Na 2 O, K 2 O and B 2 O 3 , relying on the low temperature fluxing power of Li 2 O. High temperature fluxing to enable fritting was provided by CaO. Example 3 This glaze gave good gloss after firing at 1100°C. The gloss and durability were improved by the inclusion of ZnO and the alkahne earths BaO, SrO. Example 4 This glaze matured well, firing around 1080°C, it shows increased BaO and CaO compared to example 3 giving good high temperature fluxing for fritting. The B 2 O 3 level was reduced, relying on low temperature fluxing from Li O on gloss firing. The glaze had very good durability. The absence of ZnO gave very good underglaze colour development. Example 5 This glaze gave similar properties to example 4 but the formula was complexed by the small additions of Bi 2 O 3 , TiO 2 and ZrO 2 to reduce any tendency toward devitrification on enamel firing. Examples 6 and 7 These glazes gave a very good gloss when fired at 1080°C. Glazes according to the present invention have higher Si0 2 and Al 2 O 3 content than would generally be the case for glazes firable at below 1150°C. This increase in these network forming oxides increases hardness and chemical durability. Conventionally such characteristics are only found in porcelain glazes which are fired at higher peak temperatures of between 1200°C and 1450°C. Conventional non-porcelain tableware bodies are for instance generally too soft for use at these higher firing temperatures. These glazes also are calculated to have low thermal expansions which will allow their use on low- expansion bodies. Typically for glazes firable at temperatures up to 1150°C, the SiO content can be up to 64%. In the present invention the particular mixture of components, and especially the increase in Li 2 O and the reduction relative to conventional compositions of the total Na 2 O and K 2 O proportion, allows this higher SiO 2 content. The combination of CaO and Li 2 O provide a stronger fluxing effect than K 2 O and Na 2 O. Whilst Li 2 O is acknowledged as a very powerful low temperature flux, it has generally been avoided in such glazes due to its adverse effects on chemical durability. In the invention the B 2 O 3 content is lower than is conventional for such glazes. This lower B 2 O 3 content is found to have an advantageous effect in combination with the higher Li 2 O content. Also, as boron tends to be volatile, it is advantageous to reduce the proportion thereof, particularly for use in gas kilns. A problem with glazes, and a problem which is particularly encountered with for example porcelain glazes, is devitrification, i.e. crystallisation. It has been found advantageous in the present invention to make a complex system involving a large number of compounds to render the glaze less prone to devitrification. Calcium is generally required for high temperature fluxing. Including calcium is also found to be advantageous to enable fritting of the glaze material. Otherwise, fritting would not generally be possible as the viscosity of the material would usually be too high with such a high SiO 2 and Al 2 O 3 content. There are thus described glazes and glaze formulations which provide for considerable advantages in providing glazes firable at firing temperatures up to 1150°C but with the improved durability of higher temperature firing glazes, and also with relatively low coefficients of expansion. The glazes may be partially or wholly fritted or made from one or more suitable frits to achieve the desired composition and maturing characteristics. The frits and raw materials are mixed or milled together with a suspending agent, usually water, binders and stains as required. The resulting glaze is applied by conventional methods to a desired substrate and fired to achieve a good finish. The invention also provides colours which can be formed from pigments in combination with materials having compositions as per the above glazes. These materials may be fritted. Such colours can be used to decorate ceramic and glass items. It is to be realised that a wide variety of formulations may be made according to the present invention, and additional materials may be included to further enhance glaze performance. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims: Claims: 1. A lead free glaze, the glaze being firable at a peak firing temperature of less than 1150°C, the glaze comprising a borosilicate with greater than 3% Li O, and a total Na 2 O and K 2 O content of between 0.3 and 8%. A glaze according to claim 1 wherein the glaze comprises less than 5% Li 2 O. 3. A glaze according to claims 1 or 2 wherein the total content of Na 2 O and K 2 O is less than 5%. 4. A glaze according to claim 3 wherein the total content of Na 2 O and K O is less than 2%. 5. A glaze according to any of the preceding claims wherein the glaze comprises between 55 and 85% SiO . 6. A glaze according to any of the preceding claims wherein the glaze comprises up to 15% Al 2 O 3 . 7. A glaze according to any of the preceding claims wherein the ratio SiO 2 : Al 2 O 3 is between 7:1 and 13:1. 8. A glaze according to claim 7 wherein the ratio SiO 2 : Al 2 O 3 is between 8:1 and 10.5:1. 9. A glaze according to any of the preceding claims wherein the glaze comprises less than 8% B 2 O 3 11. A glaze according to claim 10 wherein the glaze comprises between 5 and 8% CaO. 12. A glaze according to any of the preceding claims wherein the glaze comprises MgO. 13. A glaze according to claim 12 wherein the glaze comprises upto 5% MgO. 14. A glaze according to any of the preceding claims wherein the glaze comprises BaO. 15. A glaze according to claim 14 wherein the glaze comprises upto 5% BaO. 16. A glaze according to any of the preceding claims wherein the glaze comprises SrO. 17. A glaze according to claim 16 wherein the glaze comprises upto 5% SrO. 18. A glaze according to any of the preceding claims wherein the glaze comprises ZnO. 19. A glaze according to claim 18 wherein the glaze comprises upto 8% ZnO. 20. A glaze according to any of the preceding claims wherein the glaze comprises Bi 2 O 3 . 21. A glaze according to claim 20 wherein the glaze comprises upto 5% Bi 2 O 3 22. A glaze according to any of the preceding claims wherein the glaze comprises V 2 O 5 23. A glaze according to claim 22 wherein the glaze comprises upto 2% V O 24. A glaze according to any of the preceding claims wherein the glaze comprises WO. . 25. A glaze according to claim 24 wherein the glaze comprises upto 2% WO 5 26. A glaze according to any of the preceding claims wherein the glaze comprises MoO. . 27. A glaze according to claim 26 wherein the glaze comprises upto 2% MoO, s 28. A glaze according to any of the preceding claims wherein the glaze comprises ZrO 2 . 29. A glaze according to claim 28 wherein the glaze comprises upto 8% ZrO 30. A glaze according to any of the preceding claims wherein the glaze comprises TiO 2 . 31. A glaze according to claim 30 wherein the glaze comprises upto 2% TiO. 32. A glaze according to any of the preceding claims wherein the glaze comprises La 2 O . 33. A glaze according claim 32 wherein the glaze comprises upto 2% La 2 O 3 34. A glaze according any of the preceding claims wherein the glaze comprises CeO . 35. A glaze according to claim 34 wherein the glaze comprises upto 2% CeO 36. A glaze according any of the preceding claims wherein the glaze comprises GeO . 37. A glaze according to claim 36 wherein the glaze comprises upto 2% GeO 2 38. A glaze according to any of the preceding claims wherein the glaze comprises SnO . 39. A glaze according to claim 38 wherein the glaze comprises upto 8% SnO 2 40. A glaze according to any of the preceding claims wherein the glaze comprises partially fritted components to provide the required composition. 41. A glaze according to any of claims 1 to 39 wherein the glaze comprises wholly fritted components to provide the required composition. 42. A glaze according to any of the preceding claims wherein the glaze is made by mixing together constituent components with a suspending agent. 43. A glaze according to any of claims 1 to 41 wherein the glaze is made by milling together constituent components with a suspending agent. 44. A glaze according to claims 42 or 43 wherein the suspending agent is water. 45. A glaze according to any of claims 42 to 44 wherein the suspending agent comprises binders. 46. A glaze according to any of claims 42 to 45 wherein the suspending agent comprises stains. 47. A glaze according to any of the preceding claims wherein the glaze is firable at a peak firing temperature of between 1050 and 1150°C. 48. A firable colour, the colour comprising a mixture of a pigment, and a material with a composition the same as a glaze according to any of the preceding claims. 49. A firable colour according to claim 48 wherein the material is fritted. 50. A lead free glaze substantially as hereinbefore described. 51. A firable colour substantially as hereinbefore described. 52. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.

Inventors: CAMPBELL IAN FRANCIS, DUKES DAVID JOHN, CAMPBELL, IAN, FRANCIS, DUKES, DAVID, JOHN
Applicants: BRITISH CERAMIC RES LTD, CAMPBELL IAN FRANCIS, DUKES DAVID JOHN, BRITISH CERAMIC RESEARCH LIMITED, CAMPBELL, IAN, FRANCIS, DUKES, DAVID, JOHN
Application year: 1998
Main classification: C03C8
IPC codes: C03C8