Calcining
Colour earths are sometimes calcined at a high temperature in order to modify their structure and shade, the operation being accompanied, in some cases, by the destruction of organic admixtures and the expulsion of volatile constituents.
An important feature of calcining is that it improves the covering power of many colours, especially heavy spar and certain ferric oxide pigments. This alteration is probably due to the heat causing the finest particles to cohere, and also to the expulsion of chemically-combined water, etc.
The change of shade, which is often dependent on the degree and duration of the heating, is probably also connected with cohesion; but in many instances it is attributable to chemical modifications produced by the treatment; ferric hydroxide, for example, losing its water of hydration when heated and becoming transformed into ferric oxide.
The details of the calcination process vary with the nature of the material, and will therefore be described, together with the appliances used, when we deal with the colours which require to be put through this treatment.