For many years back it has been the aim of those interested in paint materials to devise a comparatively cheap coating for the exterior as well as the interior of buildings that did not have the defects of ordinary whitewash, nor those of kalsomine, something that could be prepared by the consumer on the spot without the aid of heat and without the necessity to apply the paint in the hot state.
That the curd of skimmed milk in combination with quick lime has great cementing properties has been known to the initiated for centuries and many old joiners still use this mixture in preference to glue for joining woodwork together. Many old formulas for water paints give skimmed milk as one of the chief ingredients, but so far as wearing quality of the coating is concerned, the recipes are not worth the paper they are printed on.
This, however, is not due to the presence of the milk as a binder, but to the form in which the milk is introduced.
The casein of cows' milk is composed of 53 to 54 per cent of carbon, 7 per cent hydrogen, 16 per cent nitrogen, 22.5 per cent oxygen, 8 per cent sulphur and 85 per cent phosphorus. It comes into commerce as a flocculent white powder that is nearly insoluble in water, but nearly soluble in hot alcohol. By the addition of soda, borax, quick lime, water glass, etc., in fact, with any alkali, it becomes wholly soluble in water.
The difference between casein and other albumens lies in the fact that casein contains a small percentage of phosphorus, as above noted.
While casein interests us only for its utility as a binding medium for paint, there are numerous uses for it in other lines of trade, such as imitations of articles usually made of celluloid, ivory, etc., for forming plastic figures, apparatus, also for glazes of confections and the like, where it does not collide with the pure food laws.
It is difficult to produce casein that is absolutely free from fat and calcium phosphate. By the use of a mineral acid, especially acetic acid, casein is precipitated out of milk or out of casein salts. This is important, when the casein is to be used as the binding medium in cold water paint. When solutions of casein are heated by the addition of caustic lime the casein is precipitated in insoluble form, but the simplest method is to precipitate it by the addition of a mineral salt.
However, in the manufacture of cold water paint, the casein must be soluble in the usual way and in order to make the paint most resistant to moisture and water the addition of formaldehyde is resorted to, which also acts as a disinfectant, wherefore such paint is especially adapted for the interior of hospitals, schools, sanatoriums, etc., but great care must be taken in adding the formaldehyde, otherwise the casein may be thrown out of the paint, thus leaving it without binder. The addition of formaldehyde is protected by letters patent and there are any number of other patents on cold water paints and on casein, though none of them vary considerably, excepting as to the method and means of precipitation, while all of them agree that casein is produced from skimmed milk.
To relate the various processes would be carrying coal to Newcastle, so we will confine ourselves to the description of a few of the more interesting.
U. S. Patent No. 745,097 and German Patent No. 135,745 are almost identical in the method of producing a casein that is practically free from fat, at least, it answers all practical purposes in that respect. It has been ascertained by chemical analysis that skimmed milk usually contains 2 to 3 per cent fat, so that casein made from the untreated milk would contain 6 to 8 per cent fat. The patent claims are that a casein free from fat can be obtained by mixing the skimmed milk with alkalies and separating the fat by passing the mixture through a centrifugal apparatus, after which the casein is precipitated in the usual manner by the treatment with acid. In order to separate the fat from the milk more readily, the milk and alkali mixture is warmed before passing it into the centrifugal machine. The precipitate is collected, washed, pressed and dried or used, as the case may be, in paste form after washing and pressing.
Another patent describes a process in which sulphurous acid is used for precipitating casein from the milk. The milk is brought to a temperature of from 130 to 160 degrees Fahrenheit and, while being agitated in a covered tank with stirring device, the acid is run in. It is claimed that, by this method, the precipitation takes place more promptly and more completely than by any other.
Bichamp says that a pure casein, free from ash, may be obtained by precipitating skimmed milk in the cold way with acetic acid. The precipitate is washed several times and ammonium carbonate added, until alkaline reaction takes place, when acetic acid is again used to neutralize.
Before we consider the manufacture of cold water paints from casein it may prove of interest to some of our readers to learn the various and manifold uses casein is serving in the industrial world.
In the manufacture of certain foodstuffs, in calico printing, in soapmaking, in paper manufacture and others too numerous to mention, in fact, where animal glue was formerly indispensable it is now being used almost exclusively. We must not pass by without considering the great value of casein in cements for porcelain, chinaware, earthenware, glass and stone.
W. A. Hall claims that a fire resisting cement can be produced with a mixture of casein, phosphate of soda and sodium sulphite, to which is added some pulverized air slaked lime. A well known cement putty is made from cement in dry powder, brickdust or ground quartz, mixed with casein. A small portion of air-slaked lime will improve it. It can also be made from the curd of skimmed milk and sifted air slaked lime with the same dry ingredients, omitting the casein.
For mending fractures in porcelain, glass, etc., the best cement is made by dissolving casein in silicate of soda (water glass). A preparation from casein, that is at the same time antiseptic as well as water resisting, is made by using borax in dissolving the casein. This is used for sizing fabrics, which are placed in the solution and after removing therefrom and while still wet, are given a coating of a weak solution of tannic acid, which renders the casein insoluble in water.
For paper and paste boards, etc., a good size can be made by dissolving casein in a solution of borax and the addition of liquid ammonia. Must be done with the aid of heat.