Oils and turpentine should be pure and free from dust and other substances.

To assist the process of drying paints, driers are used. Those most in use are sugar of lead, litharge, and white copperas. When ground and mixed with paint, they assist the process of drying very much. Where it does not affect the color, red lead may be used as a drier.

Boiled linseed oil with litharge, one gallon of the former with one-fourth pound of the latter reduced to a powder, makes an excellent drier. It should here be remarked that driers have a tendency to injure the colors, and hence should not be used in finishing coats.