Copyright © 1998 by Ian Currie
Once the following example is understood, we can use the Calculation Page to work out our corner glaze recipes.
The calculation applies only to the Base Glaze. Any colourants or opacifiers will be the same in all 35 glazes. In this example we will use 1% the colourant cobalt oxide. More on this at the bottom of the page.
Firstly we look at the chosen recipe and remove all clay ingredients (e.g. kaolin, ball clay) and all pure silica ingredients (quartz, flint, silica) from the recipe. Here is an example:
18% Feldspar
29% Whiting
28% Kaolin
25% Silica
+1% Cobalt carbonate
First delete the kaolin and silica from the recipe. What's left is Glaze No. 31. We can input these figures into the Calculation Page to produce all 35 recipes in the set, and also the batch recipes for the 4 Corner Glazes used for Volumetric Blending.
All 35 glazes have the same set of fluxes as the original, but alumina and silica is varying from glaze to glaze. So the original recipe should exist somewhere, more or less, in the set. In this particular example, the original recipe had 28% of kaolin and 25% of silica. This puts it roughly halfway between the diagonally adjacent glazes 4 and 8 on the grid. We can see this by refering to the kaolin and silica percentages for Glazes 4 and 8 in the Recipe Table.
Where colouring oxides and opacifiers are included in the original recipe, we usually use the same percentage in each glaze, so that they are a constant percentage over the whole set. (If using volumetric blending to prepare the glazes, adding say 1% of a colouring oxide to each of the Corner Glazes will ensure that all glazes will contain 1%.) In some situations however, some colouring oxides act as a flux, and if they are used as a major ingredient, we incorporate them in the "flux ingredients", and in this case, their percentage varies over the set. For example manganese dioxide can be used in high percentages, up to 100%, for gold colours or other metallic effects and in such a case it should be treated as a flux, which in fact it is. But if we were just adding a small percentage as a "colourant" then we add the same percentage to each of the glazes.