|Monthly Tech-Tip |
In response to a query about the suitability of a clay-feldspar material for use in our porcelain bodies this company asked for our "index requirements". What does that mean? They want to know what chemistry I want (they credit all pertinent properties to the chemistry). However, I must consider physical properties like the plasticity, drying performance, green strength, soluble salts (to which chemistry is unrelated); and fired properties like shrinkage, deformation and density (to which chemistry is often imprecisely related). I need to put a sample into a porcelain and compare test results to our existing products. Yes, Fe2O3 and TiO2 percentages do relate to whiteness and translucency. But only when reliable and measured over time at the same lab. A lower iron percentage on the data sheet of a kaolin from one company certainly does not mean it will produce a whiter porcelain than a kaolin from another. Nor does a higher flux content mean it will vitrify a porcelain better (it depends what minerals are delivering the fluxes and what other minerals are present).
In ceramics, glazes and bodies have a chemistry, a mineralogy and a physical presence. All of these need to be understood to adjust and fix issues.