White lead improves by keeping. It should not be exposed to the air, or it will turn grey (see p. 407). Old white lead of good quality goes further and lasts better than if it is used when fresh; moreover, the paint made with fresh lead has a tendency to become yellow.
Fresh white lead often has a yellowish tinge, caused by the presence of iron.
Uses, Advantages, and Disadvantages. - Of all the bases for paints white lead is the most commonly used, and for surfaces of wood it affords in most cases the best protection, being dense, of good body, and permanent. It has the disadvantage, however, of blackening when exposed to sulphur acids, and of being injurious to those who handle it.
"The testing of the quality of white lead is a very simple operation, as it is only necessary in the case of dry white lead to digest it with nitric acid, in which it dissolves readily on boiling. When ground with oil, the oil should be burnt off, and the residue treated with nitric acid; or
"The ground white lead with the oil may be boiled for some little time with strong nitric acid, which destroys the oil, and dissolves the lead on the addition of water.
"The sulphate of baryta being insoluble in acid remains behind, and can be collected on a filter, washed with hot distilled water, and weighed."