The recipe contains 6% red iron oxide. The chemistry is high Al2O3 (from 45 feldspar and 20 kaolin) and low SiO2 (the recipe has zero silica, it calculates to a 4:1 Al2O3:SiO2 ratio). The remainder is 20% calcium carbonate, a little talc and calcium phosphate. The reduced iron is glossing what would otherwise be a very matte surface. Reducing the iron percentage to 4% produces a yellow mustard color (we thus named this "Red Mustard"). Iron-red glazes in oxidation often require double this percentage of iron oxide.