The basic mineral pigments in cold water paints are usually chalk (whiting), kaolin (china clay), magnesium silicate or alumina. The chemical examination of a popular brand of cold water paint resulted as follows: Organic matter, 15.5 per cent; calcium carbonate, 15.6 per cent.; insoluble silicates, 40.5 per cent.; alumina, 26.0 per cent.; water of combination, 2.4 per cent. Dissecting this report it will be good logic to say that the mineral matter consisted of an alumina silicate, presumably kaolin or white pipe clay, while the calcium carbonate was introduced in the form of calcium hydrate in order to dissolve the casein, which is found in the analytical report under the caption of organic matter. The combined water belongs partly to the alumina and partly to the calcium hydrate. There was an entire absence of chalk or whiting, which in itself is rather in favor of the paint, because the presence of calcium carbonate in the form of chalk or whiting is not favorable to the wear of the paint in a locality where sulphur gases prevail. There is a difference of opinion among authorities on the subject as to whether whiting or white clay is best for pigment in cold water paint, but as a rule, economy in cost would favor the former, so long as technical objections are not considered. Ordinary white clay is not to be recommended, as there is always more or less risk of the cracking and scaling of the painted surface when such pigment is introduced.
Finally for colored casein or cold water paints the coloring matter requires serious consideration. These pigments must not only be alkali proof, but should also to a great degree be light proof and, besides containing no free acids, must be ground impalpably fine and be bone dry. In addition these colors should have the maximum staining power, because the rule is that the percentage of coloring matter introduced should not be over 30, while the base pigment in a colored paint should not be under 70 parts by weight. That all pigments entering into the paint must be bone dry is important, because if any moisture in the pigment is present, lumps, both large and small, will form in the package and, becoming hard, will not readily dissolve on mixing with water and will give the paint a streaked appearance. The use of colored pigments that are not entirely free from acid, as may be the case with artificial oxide of iron reds, or with chemical pigments that are imperfectly washed, is liable to produce blistering of the paint during application, due to a reaction between the free acid and the alkaline salt that is present with the binder. The colors that will answer in casein paints are rather limited and consist principally of the following: -
For Yellow. - All yellow ochers, raw sienna, chrome yellow, Indian yellow, and some of the latest coal tar derivatives, as lithol fast yellow, chinazol and naphthol yellow S.
For Orange. - Chrome red in several shades for orange chrome, very deep hues, and autol fast orange with naphthol yellow for the lighter shades.
For Brown. - Umber, raw and burnt, burnt ocher, burnt sienna, manganese brown.
For Green. - Oxide of chromium green. Cobalt green, ultramarine green, green earth, also lime greens made from best green earth and colored with Malachite green and brilliant green.
The colors given in the foregoing list will, in proper admixture, produce any tint or shade that may be required in cold water paint, and whenever the color named appears too expensive, it may, if strong enough in staining power, be cheapened by employing more colorless mineral base.
Whenever a solid color (not a tint), is desired, the colorless mineral base only is used, but when a good lively tint is wanted it is of advantage to add zinc oxide or lithopone or even blanc fixe to give solid appearance and sufficient hiding power to the paint, especially where transparent coloring matter is used.
The composition of casein paints does not vary to any great extent. The chief value in this material lies in its proportion of casein, which even in the cheapest commercial brands is not below 10 per cent by weight of the total and has been found in some as high as 20 per cent.
When, however, it goes over this figure, then additions of whiting or china clay are made by the consumer before using it.