Red and Pink Colors

Let us look at some typical ceramic color stains. Red stains can be obtained with iron oxide, chromium-alumina combination, copper compound, or cadmium sele-nide. The red iron ore hematite is iron oxide (FejOj, di-iron trioxide). Iron oxide is perhaps one of the oldest red coloring materials for ceramics. The tint of the red color produced by iron oxide depends on how it is produced particularly critical are the temperature of calcination (to be explained below) and the grain size. Larger grain sizes reduce the brightness of red color. 

The iron oxide is typically produced by calcining iron sulfate, FeSO -THjO. Heating a material without melting (fusion) is called calcination. The chemical reaction that takes place upon heating up to 650°C is 2FeSO 7H,0 Fe,v 

When you boil chrome yeUow (lead chromate, PbCrO ) in a dilute caustic soda (sodium hydroxide, NaOH) solution, the color changes to bright red (called coral red ). This is due to the formation of basic lead chromate PbCrO Pb(OH)j. 

This pigment can be used as stain on the glaze, but can be applied only to art and studio pottery, as the toxic lead content is too high. 

Mostly for this reason (toxicity of lead), coral red has been superseded by cadmium selenide (CdSe). Both cadmium and selenium themselves are chemically toxic (see Chap. 15), but their combination, cadmium selenide, is extremely insoluble and resistant to chemical attack, and hence would not be made to a form that can become toxic. In reality, this stain is a mixture of cadmium sulfide (CdS) and cadmium selenide. With a higher proportion of selenide, the color is bright red, while with an increasing proportion of cadmium sulfide color becomes orange. 

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