A medium is the liquid vehicle with which the pigments are ground ; and the same medium is usually employed in applying the pigments and to obtain, by their use, the different degrees and gradations of color.
In mineral colors, various oils are used for this purpose. A thick or fat oil, which is usually a heavy oil derived from turpentine, is very generally used to amalgamate and hold the color. But with this medium alone, it would be impossible to paint, as it is thick, sticky, and quite unmanageable until thinned. Now, since this thick oi is the residuum from the evaporated spirits of turpentine, spirits of turpentine is used to dilute it to the proper consistency for painting.. This my be freely added in any desired proportions.
An expert in the use of mineral colors seldom has occasion to deviate from these two mediums in ordinary decorations. Spirits of turpentine dries quickly and a certain amount of skill is required to use it successfully. The less skillful the painter,
