Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/13



To understand this subject intelligently it will be necessary to possess some slight knowledge of chemistry and of the materials used by chemists, but any intelligent person can, by careful reading of these descriptions, test substances qualitatively without the aid of expensive apparatus or external assistance of any kind. For quantitative analysis a delicate chemical balance will, of course, be required. Take paints first. White lead is now very seldom sold in the dry state, but samples are occasionally met with. Its covering power being superior to that of any other known paint, it is very largely used, and it is frequently adulterated with substances of inferior quality. The most common adulterants are sulphate of baryta and chalk.

Sulphate of Baryta. Treat a small quantity with dilute nitric acid, and heat on the sandbath. If any insoluble remains, it is either sulphate of baryta or insoluble silicates. Filter, take a portion of the insoluble on a piece of clean platinum wire moistened with hydrochloric acid, and test at the blowpipe. If the flame be colored green, the precipitate is sulphate of baryta. By moistening the wire in hydrochloric acid the green color is reproduced many times.

Insoluble Silicates. If no green color appears, the insoluble is a silicate. This may be proved by forming a bead with microcosmic salt on a loop at the end of the platinum wire, and taking some of the precipitate on this bead, fusing it again in the blowpipe flame. If small infusible particles whirl around within the bead while in the flame, the presence of silicates in the precipitate may be inferred.