Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Patent
1996-05-08
1998-05-05
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
501 20, 501 26, 501 32, 501 65, 501 67, 428428, 428701, C03C 1004, C03C 804
Patent
active
057473951
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
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 requirements 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.sub.2 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.sub.2 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.
SUMMARY OF THE INVENTION
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.degree. to 900.degree. 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 conc
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Mangat Harbans Kaur
Smith Peter John
Bell Mark L.
Cookson Matthey Ceramics & Materials Limited
Troilo Louis M.
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