Elimination of discolouration in reformulated bone china bodies

Abstract

In order to reduce the thermal expansion coefficient (TEC) to make it fit high scratch resistance glazes, which have lower TEC values, bone china body is reformulated. This is done by reducing the amount of bone ash, which is responsible for the formation of a crystalline phase of beta-tricalcium phosphate (beta-TCP) with a high TEC value and adding prefired anorthite materials, which has a lower TEC value, in the body. Fired samples of reformulated bone china show a bluish-green tint due to iron impurities. When about 5 wt.% of ZnO was added to the reformulated bone china bodies this bluish-green tint was removed and the degree of the whiteness of reformulated body was again become similar to commercial bone chinas. The cause and elimination of discolouration of reformulated bone china have been investigated. ZnO containing reformulated bone china bodies develop zinc-spinel (gahnite, ZnAl2O4) crystals, which are the reason for the increasing whiteness of samples by incorporating iron ions into the spinel structure. (c) 2004 Elsevier Ltd. All rights reserved.