Reformulation of fine translucent porcelain
A low-clay version of fine translucent china was designed to have desired properties and acceptable behaviour during manufacture. Low-clay contents of 10 to ISw/o were employed to reduce the deterious effects of preferential clay particle alignment and its adverse effect on colour. For example, alignment of clay particles in cast wares causes anisotropic drying and firing shrinkages and these cause distortion. Replacing clay with a pre fired body with low Fe and Ti contents, as in the present case, allowed a very white material to be produced. The prefired body was made with calcium carbonate, aluminium trihydroxide, quartz and a small fluxing addition of an hydrated magnesIUm carbonate. Desification of a whiteware is enhanced by increasing volume fraction of the viscous liquid and reducing by its viscosity. Both of these also enhance sagging. Consequently, compositional change cannot on its own lead to the favourable combination of high density, required to give translucency, with little sagging, that will allow wares to be fired without significant distortion. It was found by trail and error that use of finer particles reduced the sagging occurring in the densification heat treatment and enhanced densification. This finding allowed the body to be designed so that it densified without sagging excessively. After establishing this important result, an iterative approach was employed to produce a ware that was very white, translucent, had the required thermal expansion coefficient for glaze fitting, shrank acceptably during the first firing and did not sag during the second firing that stimulated glost firing. The finding on sagging was applied to an anorthite/mullite porcelain body that was under development. This body was also made with a low-clay content and the same prefired body. This whitware has potential to replace bone china and hard porcelain for use in the servere service conditions of hotels and restaurants. It has a higher fracture toughness than hard porcelain but has the same scratch resistant glaze and is more resistant to thermal shock. The sagging of the anorthite/mullite porcelain was substantially reduced while the body was densified. This was achieved by using a combination of finer particles and a reduction in the liquid-phase content that developed during firing.