In order that the paint may be sufficiently ground in one operation, roller mills are made with several pairs of rollers, one above the other, the lower rollers being somewhat nearer together than the upper. The paint is fed on to the top pair of rollers, and, after going through these, passes to the next, and finally, after going between all the pairs of rollers, collects in a receiver below in a finished condition. By the use of such mills the paints can be ground sufficiently fine in one operation, if a proper number of rollers is used. When only two or three pairs are used and the paint is to be very finely ground, it must be passed through the whole mill two or three times.
CHAPTER LXX.
THE EXAMINATION OF PIGMENTS.
When an accurate examination of a pigment is required, the only course is to conduct an exact chemical analysis, which can only be done by an expert chemist in a well-equipped laboratory. But often it is necessary in the course of trade to decide rapidly the nature of a pigment or to detect the adulteration of a dear pigment with a cheaper, and for this purpose it is important to have a method of examination which can be conducted without much apparatus, and which requires no great chemical knowledge. It is quite possible to test the majority of pigments in a simple manner. Few reagents are necessary. For an examination of the mineral pigments the following are generally sufficient: hydrochloric, nitric and sulphuric acids, caustic soda solution and ammonium sulphide.
The examination of a pigment containing organic compounds is somewhat more difficult, especially when it is necessary to ascertain the nature of the colouring matter. In this case additional reagents are required—stannous chloride, alum solution, etc.
Since the present work is intended to meet the requirements of the practical man, the behaviour of the different pigments towards ordinary reagents is given in tabular form, the pigments of the same colour being taken together. Since the colouring matters of organic origin, as they occur in the lakes, require a rather more detailed examination, the testing of pigments composed of inorganic materials only is first given, afterwards the properties of the organic colouring matters, so far as is necessary, will be added.
Mineral Pigments.—If the substance to be examined is in the form of a dry powder, it may at once be tested with the reagents mentioned, but if it is a paint or water colour, the oil or gum must be first removed, otherwise it would not be possible to recognise with certainty the action of the reagents.
From water colours, which are ground with gum Arabic or tragacanth solution, it is easy to separate the pigment from the binding medium. The colour is allowed to stand in a tall narrow beaker with a somewhat large quantity of water. After some time the lumps become soft, they are then repeatedly stirred up, the pigment allowed to deposit, and the water, which now contains the binding medium, poured off.
The removal of oil from a paint is somewhat more difficult. A quantity of the paint is placed in a flask with a mixture of equal parts of strong alcohol and ether, or with benzine. The flask is lightly corked, and, after frequent shaking, allowed to stand. The liquids mentioned are good solvents for oils. After a day or two the pigment will generally have deposited at the bottom of the flask. The solution is then poured off, the residue mixed with a small quantity of the solvent and transferred to a filter, where another small quantity of solvent is poured over it. The solvent is allowed to drain off, and the residue is dried. A powder without coherence should be left; this is the pigment free from oil. It can be treated like an originally dry colour.
The pigments are most conveniently examined in test tubes. If these are not at hand the reactions may be carried out on a plate of glass lying on white paper. The powder is placed on the glass and the reagent dropped on to it from a glass rod dipped in the liquid.
Examination with the Blowpipe.—From the behaviour of pigments at high temperatures important conclusions may be drawn as to their nature. For this purpose small porcelain crucibles are used; broken pieces of porcelain or an iron spoon may also be employed. An ordinary spirit lamp is generally sufficient as a source of heat. In some cases a higher temperature is required, which is obtained by means of the blowpipe.