For white-lead paint, the best pure white-lead is chosen, kept secure from the air. It possesses good covering power, but blackens in contact with air containing sulphuretted hydrogen, and is injurious to those using it. Coloured lead paints consist of a basis of white-lead with a certain quantity of colouring pigment, separately ground in oil, and added to the 2 last coats. "When the white-lead is bought dry, it must be ground up with raw linseed-oil by means of a stone muller on a marble slab. The thick paste thus produced is thinned and softened by adding a little oil and turps and working well with a palette knife. The colouring pigments are added at this stage, and the consistence is rendered creamy by adding more oil and turps; the whole is finally passed through a canvas strainer. Just before use, it is thinned down to a working consistence by adding more oil and turps, and the driers are then introduced.