Physical properties of cold water paint should be tested for spreading and covering power, for free working under the brush and drying property, for resistance to moisture and water, atmospheric conditions, heat and behavior when mixed with coloring matter and last, but not least, the quantity of water required to reduce a given weight of cold water paint in the dry or paste form to brushing consistency. These tests are not as important to the user or consumer of cold water paint as they are to the manufacturer, because the trade expects that he or his agents know all about the various requirements, otherwise they will not have the confidence of the purchaser, nor will they be enabled to meet claims of disappointed consumers with any degree of certainty or belief in their explanation, should by any chance the material not come up to their claims or fulfill the expectations of the user or his patrons. It goes without saying that cold water paints will not meet any and all conditions of surface, otherwise very little oil or varnish paints would be sold and used. To test spreading and covering power of cold water paint it is only necessary to weigh out a certain quantity, say one pound of the dry powder, when sold in that form, place in a clean pot, adding enough cold water and stir until a medium paste, free from lumps, is formed. This is allowed to stand, say about 30 minutes, although it is beneficial to stir it several times during that period. Then the mass is well stirred with sufficient water, until it has assumed the consistency of an oil-paint ready for the brush, when the quantity of water used is determined by weight. A good cold water paint with casein binder should require one and one-half pints (1 1/2 pounds) of water for every pound of dry paint. When the cold water paint is sold in paste form, it is only necessary to weigh out a certain quantity of the paste, reducing it with cold water and when of the right consistency, determine, by reweighing, how much water has been required. To ascertain spreading and covering power, a given quantity of the paint is applied on a plastered wall or suitable wooden surface in the same manner as it is done in practice. One pound of good cold water paint, ready for application with the brush, should cover at least 2 1/2 to 3 square yards, unless the surface be very rough and porous.
The working property of the paint is best tested on different surfaces, on smooth and rough plaster, on smooth and rough wood, also on brick and stone and with suitable brushes in a normal temperature, not below 60 degrees Fahrenheit. The drying of the paint of course, depends on the porosity of the surface, as it will set much more rapidly on absorbent than on close, non-porous material. To test cold water paint for resistance to atmospheric influences, it is best to apply several coats on cement, as well as on lime mortar plaster, with exposures to the south as well as to the north and it is well to have this done in winter and in summer also. The observation should be conducted at least for two months in summer and three or four months in winter. To try its resistance against water the painting tests should be flushed with water from time to time.
The trial of the paint for resistance to heat is best tested by coating wall spaces near radiators or heaters, stoves, ranges, etc., making frequent observations as to the appearance of cracks, blisters or scales. Of course, the surface, where such tests are made, must be in perfect condition. If then cracking, blistering or scaling is not apparent, the paint has sufficient heat resistance. The test can also be made by coating sheet iron Nos. 18 or 20, and submitting same to a temperature of 212 to 240 degrees Fahrenheit in drying ovens for a few days. When after the iron has cooled the paint does not show cracks or blisters or does not scale, it is sufficiently heat resistant. As to miscibility and behavior with color cold water paints in colors or tints can be prepared only with limeproof or alkali proof colors, and the base used for colored paints must necessarily have a greater portion of casein binder in its make-up than is used for the ordinary paint that is neither a white nor a colored paint of great body. The process of manufacture of casein cold water paint is on the whole rather simple. The chief requisite is a good mixing and sifting apparatus, which will turn out appreciable quantities of dry paint in a day's time. As has been stated, it is necessary for the solution of casein to add an alkaline salt. For economic reasons it is best to use for this purpose calcium hydrate (hydrated lime) in powder form. This is at present prepared on a large scale in special machinery, where the output warrants such. But for preparing it on a smaller scale, it is only necessary to spread fresh burnt lime out on a clean floor, where it is sprinkled with water from a hose with fine spray nozzle. In a few minutes the lime falls into powder, while emitting carbonic acid. When the reaction is over and the hydrate sufficiently cooled, it is sifted in a cylindrical sifting machine for the protection of the workmen. If the calcium hydrate is to be transported it is packed in paper-lined barrels to keep it from contact with the air, which would make it unfit for use in dissolving casein on account of its avidity for moisture. The relation of lime and water in the hydrate is usually 10 parts by weight of water to 33 1/3 parts lime, and the composition of casein cold water paints is alkali soluble casein, calcium hydrate or another alkaline salt and a mineral pigment, varying according to the idea of the maker, also white and colored pigments.
It is not absolutely necessary to use alkali soluble casein, excepting as noted above for economical and practical reasons in the manufacture of cold water paint. When water soluble casein is used the material contains the necessary mediums or additions for the purpose. Such additions are usually borax or bicarbonate of soda, very seldom calcium hydrate.