Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/14

6 Chalk. The presence of chalk can only be ascertained after separating the lead. This is best done by adding ammonia solution to the nitric acid solution until alkaline, then sulphate of ammonia in excess, and boiling for five minutes. Filter off the black precipitate which is formed, and to the filtrate, first tested with an additional drop or two of sulphate of ammonia to insure the removal of the whole of the lead, add ammoniac oxalate. If a white precipitate appear, it is calcic oxalate. Test a portion of this precipitate at the blowpipe. A brick-red color imparted to the flame verifies the presence of chalk.

White Lead. The presence of lead should be ascertained. This is indicated by the black precipitate given with sulphate of ammonia. It may be best ascertained by boiling the nitric acid solution to expel the free nitric acid, adding dilute sulphuric acid to the clear solution, dissolving the white precipitate of sulphate of lead thus formed in ammoniac acetate, and adding potassic chromate to this solution. A heavy yellow precipitate (chromate of lead) forms when lead is present.

Sulphate of Baryta and Silicates. Take 20 grains of the sample, treat with dilute nitric acid as before. The quantity taken for analysis may be weighed in a watch-glass or a small basin, but should be transferred to a beaker, and the glass or basin washed with distilled water before the acid is added. If this precaution be not taken, and the acid added directly in the watch-glass or basin, to be washed into the beaker afterwards, the violent effervescence which takes place on the addition of the acid will occasion considerable loss by spurting. If, after heating with nitric acid, an insoluble remains, a few crystals of chlorate of potash may be added to the boiling liquid to insure the solution of all soluble substances. The boiling is continued for a few minutes, then cold water is added, and the whole passed through a filter. The insoluble on the filter is washed with hot water until the water leaving the filter is no longer