Bubble control in Ceramic Glazes, Part 2

As part of an international co-operative research project within the GROWTH program (FP5 - 5th Framework Programme) involving porcelain and tile manufacturers, raw materials suppliers and ceramic institutes, investigations were conducted into bubble control in ceramic glazes. The project included a literature inventory of relevant mechanisms, parameters and cause-effect relations. A short overview sums up the present state of the knowledge of bubble generation and growth as well as related defects, primary and secondary causes of bubble defects and the relevant influencing parameters. The conclusions regarding bubble defects are discussed, indicating the different views on issues still under discussion, for instance bubble movement and the origin of bubble content. A classification matrix for bubble defects and a relation matrix for causes and influencing parameters were drawn up. The inventory and the matrices were the basis for the creation of a database on glaze defects. The concept of the database and its use in ceramic production are described. A combination of test methods was developed to define relevant glaze characteristics and specify the glaze in relation to its bubble growth behaviour. Parameters such as glaze porosity, sealing temperature and viscosity were taken into account. A numerical model developed to describe the mechanism of bubble generation and bubble growth was successfully validated by laboratory testing. General conclusions are presented. On this basis the methods of characterising and subsequent improvement posibilities for different ceramic glaze types were defined and tested on laboratory scale and in production situations. Tests with additives intended to improve glazes provided a basis for the definition of which additives are suitable for which glazes. The results of pre-pilot and pilot tests of improvement possibilities were used to devise a general glaze improvement routine based on classification and characterisation by the methods set out here. The results and the improvement routine using the inventory, the database and the numerical model are presented, as is a technological concept for glazes with controlled bubble growth. Possible future applications of the project results are discussed.