Under white lead and linseed oil we have given tests for the purity of these important substances. To make tests, it is always well to have samples on hand, if possible, of known purity for comparison. In testing colors for tinting; strength, tone and transparency are to be noted. First, determine the fineness of grinding which will influence the staining power to a marked degree. Then add equal parts of weighed samples of the colors to ten or twenty times by weight of white zinc or white lead in oil; use the same pigment for both samples, because the same colors have very different tinting power in different whites. Spread the reduced pigments on glass, side by side. Further reductions can be made; the extreme is usually fifty to one. Transparency and brilliancy can be judged by spreading thin layers of the samples side by side on glass, and judging by the eye.

The covering power of bulk paints, such as white lead, is a most important consideration as also is the density or opacity of the coat. These features depend upon the pigment used, its fineness, the amount of oil and turpentine or the proportions of each used in thinning and the manner of applying the paint. In comparing two white leads or other pigments ready for use, it is important that the same amount and kind of thinners be added and similar tools and methods be used to spread the paint to be compared.

The spreading power of a given quantity of paint or varnish depends, of course, largely on the kind and condition of the surface to be covered. Cement and other absorbent surfaces require much more paint than non-absorbent surfaces like the metals. A gallon of paint ready for use will cover from 350 to 500 square feet of planed boards, depending upon whether it be the priming or succeeding coats. The latter covering the greater surface. On metallic surfaces, from 600 to 800 square feet will be covered by the same amount.

Varnish of good body will cover somewhat more surface than paint, particularly when the work has been filled; 600 square feet to the gallon is a good average. Stains and other liquids of thin body will cover from 600 to 800 square feet to the gallon, except on very absorbent surfaces. A pound of floor wax should cover, under ordinary conditions, at least 1,000 square feet of surface.