In working any form of grinding-rollers, great care must be taken to clean them thoroughly immediately after use. If the paint be allowed to dry upon the surface of the rollers, it is difficult of removal, and interferes with the perfect action of the machine. Should the working parts become clogged with solidified oil, a strong solution of caustic soda or potash will remove it. By means of the same solutions, porcelain rollers may be kept quite white, even if used for mixing coloured paints. Although the colour of most pigments is improved by grinding them finely in oil, there are some which suffer in intensity when their size of grain is reduced. Chrome red, for instance, owes its deep colour to the crystals of which it is composed, and when these are reduced to extremely fine fragments, the colour is considerably modified.
When paint is not intended for immediate use, it is packed in metallic kegs. For exportation to hot climates, the rim of the lid is soldered down, a practice which effectually prevents access of atmospheric oxygen. White-lead paint is frequently packed in wooden kegs; these prevent the discoloration sometimes caused by iron kegs. When paint is mixed ready for use, it will, if exposed to the air, become covered with a skin, which soon attains sufficient thickness to exclude atmospheric oxygen, and prevent any further solidification of the oil. The paint may be still better protected by pouring water over it, or it may be placed in air-tight cans. If it has been allowed to stand for some time, it must be well stirred before using, as the pigments have a tendency not only to separate from the oil, but also to settle down according to their specific gravity.
Of whatever nature the surface may be to which the paint is to be applied, great care must be taken that it is perfectly dry. Wood especially, even when apparently dry, may on a damp day contain as much as 20 per cent, of moisture. A film of paint applied to the surface of wood in this condition prevents the moisture from escaping, and it remains enclosed until a warm sun or artificial heat converts it into vapour, which raises the paint and causes blisters. Moisture enclosed between two coats of paint has the same effect. Paint rarely blisters when applied to wood from which old paint has been burnt off; this is probably due to the drying of the' wood during the operation of burning.
The first coat of paint applied to any surface is termed the " priming-coat." It usually consists of red-lead and boiled and raw linseed-oil. Experience has shown that such a priming not only dries quickly itself, but also accelerates the drying of the next coat. The latter action must be attributed to the oxygen contained in the red-lead, only a small portion of which is absorbed by the oil with which it is mixed. Kail, of Heidelberg, prepares a substitute for boiled oil by mixing 10 parts whipped blood, just as it is furnished from the slaughter-houses, with 1 part of air-slaked lime sifted into it through a fine sieve. The two are well mixed, and left standing for 24 hours. The dirty portion that collects on top is taken off, and the solid portion is broken loose from the lime at the bottom; the latter is stirred up with water, left to settle, and the water poured off after the lime has settled. The clear liquid is well mixed up with the solid substance before mentioned. This mass is left standing for 10 or 12 days, after which a solution of potash permanganate is added, which decolorizes it and prevents putrefaction.
Finally the mixture is stirred up, diluted with more water to give it the consistence of very thin size, filtered, a few drops of oil of lavender added, and the preparation preserved in closed vessels. It is said to keep a long time without change. A single coat of this liquid will suffice to prepare wood or paper, as well as lime or hard plaster walls, for painting with oil colours. This substance is cheaper than linseed-oil, and closes the pores of the surface so perfectly that it takes much less paint to cover it than when primed with oil.
The drying of paint is to a great extent dependent upon the temperature. Below the freezing-point of water, paint will remain wet for weeks, even when mixed with a considerable proportion of driers; while, if exposed to a heat of 120° F. (49° C), the same paint will become solid in a few hours. The drying of paint being a process of oxidation and not evajtoration, it is essential that a good supply of fresh air should be provided. When a film of fresh paint is placed with air in a closed vessel, it does not absorb the whole of the oxygen present; but after a time the drying process is arrested, and the remaining oxygen appears to have become inert. Considerable quantities of volatile vapours are given off during the drying of paint; these are due to the decomposition of the oil. When the paint has been thinned down by turpentine, the whole of this liquid evaporates on exposure to the air. There must, therefore, be a plentiful access of air, to remove the vapours formed, and afford a fresh supply of active oxygen. The presence of moisture in the air is rather beneficial than injurious at this stage. Especially in the case of paints mixed with varnish, moist air appears to counteract the tendency to crack or shrink.
Under the erroneous impression that the drying of paint is a species of evaporation, open fifes are sometimes kept up in freshly-painted rooms. It is only when the temperature is very low that any benefit can result from this practice: as a rule, it father retards than hastens the solidification of the oil, which cannot take place rapidly in an atmosphere laden with carbonic acid. The first coat of paint should be thoroughly .dry before the second is applied. Acrylic acid is formed during the oxidation of linseed-oil, and unless this be allowed to evaporate, it may subsequently liberate carbonic acid from the white-lead present in most paints, and give rise to blisters. Sometimes a second priming-coat is given; but usually the second coat applied contains the pigment. This, as soon as dry, is again covered by another coat, and subsequently by two or more finishing-coats, according to the nature of the work.