The sp. gr. of linseed-oil is, in some cases, of value in estimating its quality ; but, as the varia tions are slight, it would be difficult to detect them in so thick a liquid by means of an ordinary hydrometer. A simple method of' obtaining an approximate result is to procure a sample of oil of known good quality, and to colour it with an aniline dye. A drop of this tinted oil will, when placed in the oil to be tested, indicate, by its sinking or swimming, the relative density of the liquid under examination. Freshly-extracted linseed-oil is unfit for making paint. It contains water and organic impurities, respecting the composition of which little is known, and which are generally termed " mucilage." By storing the oil in tanks for a long time, the water and the greater part of the impurities are precipitated, forming at the bottom of the ciatern a pasty mass Imown as "foots." To accelemto the purification of the oil, and to remove at least a portion of the colouring matter, various methods aro in use. The action of sulphurie acid upon linseed-oil is not so favourable as npon otbcr oils. It is, however, sometimes employed, in the proportion of 2 parts of a mixture of equal volumes of commercial sulphurie acid and water to 100 parts of oil. The dilate acid is poured gradually into the oil, and the mixture is violently agitat,ed for several hours. It is then run into tanks, and allowed to settle. A concentrated solution of chloride of zinc has been substituted for sulphuric acid in the proportion of about per cent. of the weight of the oil. When the reaction is complete, stestm or warm water is admitted into the liquid, in order to Glorify it. Oil treated in this way loses a considerable proportion of the colouring matter which it originally contained. When the oil is to be used for white paint, it is sometimes bleached by exposing it to the action of light. On a large scale, this is done by placing it in shallow troughs, lined with lead and covered with glass. The lead itself appeara to have some influence upon the bleaching of tho oil, for the decoloratiou is not so mpid if the troughs be lined with zinc. For small quantities, a shallow tray of white porcelain or earthenware, similar to those in use for photographic purposes, gives very good results, the white surface increasing the photo-chemical action. It is not quits clear whether the presence of water accelerates the bleaching of oil by this method; soma manu facturers consider its presence necessary, others omit it. Various salts are added to the water, the one most in use being copperas. (See also Oils and Fatty Substances, p. 1461.) However the oil may have been prepared, it will, if kept for a long time, deposit a sediment. At first, this contains mucilage ; but the sediment from old oil consists chiefly of the products of decomposition of the oil itself. The presence of oxygen is not necessary for this decomposition ; but it is increased by the action of light. Raw linseed-oil dries more slowly than boiled ; but the resulting film is moro brilliant and durable. Raw and boiled oil are therefore usually mixed in proportions varying according to the time which can be allowed for the paint to dry, or to the properties required of tho film. For the ordinary kinds of paint, equal parts of boiled and raw oils are customary. Linseed oil heated to a temperature of 176e-204i° (350°-400° F.) dries much more rapidly than in its raw etate. The maximum of drying power is, however, obtained by the addition of certain metallic oxides, which not only part with sorne of their own oxygen to the oil, but also act as carriers between the atmospherio oxygen and the heated liquid. This heating of the oil with oxides is known as boiling, although the liquid is not volatilized vrithout decomposition, aa is the case with water. At about 260° (500° F.), bubbles begin to rise in the oil, producing acrid, white fumes on coming into contact with the air. The gas thus given off consists chiefly of vapour of acroleln mingled with carbonic oxide. There is no advantage in heating the oil to a higher temperature than 176i° (350° F.). Accurate experiments have shown that the drying properties of tha oil ara not increased by heating it beyond this point, while its colour is considerably darkened. For the finer qualities of boiled oils, it is essential that the mw oil should have been stored for soma time, so that it may be free from mucilage. This mucilage is the chief source of the dark colour of some boiled oila ; when heated, it forms a brown substance, which is soluble in the oil itaalf, and extremely difficult to remove. The oxides usually added to the oil during boiling are litharge or rcd-lead, the former being preferred on account of its lower price. About 2-5 per cent. by weight of the oxides or driera is gradually stirred into the oil after it has been slowly raised to a temperature of about 149° (300°F.). The stirring should be continued until the litharga is dissolved, or it would cake on the bottom of the pan, and cause the oil to burn. Litharge may even bo reduced to a cake of metallic lead when the fire is brisk. Some pans are furnished with atirrera and gearing by which the latter can be worked, either by hand or steam. The material of which the pans are made is either wrought- or cast-iron. Copper pans are sometimes used with the object of improving the colour of the oil. Little ia known respecting the chemical reactions which take place during the boiling of oil. Even when the air is excluded during the process, the drying properties are greatly increased, and, if boiled long enough, the oil is converted into a solid substance. The loss of weight which ensues is dependent npon the temperature, and the time during which the operation continues. It is lees when the air is freely admitted than if the pan is covered with a hood. The vapours given off by the oil are of an extremely irritating character, and should be destroyed by passing them through a furnace. As their mixture with air in certain proportions is explosive, this furnace should be situated at some distance, and the gases be conducted into it by means of an earthenware pipe. (See also Oila and Fatty Subatanoes, p. 1449.) T. Holmes' apparatus for grinding pigments is shown in Figs. 1099 and 1100. The granite roller A revolves against a feed-roller B, travelling in the opposite direction, and at a lower speed, by which meana, A feeds itself with the material to be ground. The roller A also works against a concave granite block D, to which is communicated a slow reciproeating motion in a direction parallel with the axis of the roller, thus assisting the grinding and equalizing the wear. A " doctor " cleans the surface of A as the pigment accumulates upon it. Brinjes and Goodwin's Machine is shown in Figs. 1101, 1102, and 1103. The oil and pigments having been measured or weighed, are placed in the trough h. This is provided with stirrers, similar to those in a pug mill, which are driven by means of the pulley /, m being a loose pulley ; by shifting the strap on to this, the machine can be stopped at once. When the oil has been thoroughly incorporated with the pigment, the mixture is allowed to run through the spout g on the roller a. Working . against a, is a second roller b, and this in its turn bears upon a third roller c. In order to prevent the grooving of the faces of the rollers, which always takes place when they revolve in the same plane, there is an arrangement by which a slight lateral motion is communicated to b, in addition to the rotary motion. A pin fixed upon the rig-id bracket h works in the grooved cam which is keyed on the shaft of the roller b. The grinding power of the machine is considerably increased by this modification. The rollers are worked from the pulley d ; the loose pulley e receives the strap when a pause in the working of the machine becomes necessary. The details of the con struction of the grinding-machine are given in Fig. 1103. The rollers a b c are constructed of granite or porcelain ; for fine grinding, the latter substance is preferable. They are adjusted by
means of the screws g h. These are furnished with spiral springs, so that should a nail or other hard substance get between the rollers, these can rise in their bearings, letting the nail fall down at the back. The " dootor " or scraper f removes the paint from the surface of the roller c ; a are also provided with smaller scrapers, which remove any paint that may cake upon their surfaces. Where extreme fineness is requisite, the paint is again passed through the machine, and this operation is sometimes repeated several times.
In working these or any other form of grinding-rollers, great care mnst be taken to clean them thoroughly immediately after use. If tbe 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 waking parts become clogged with solidified oil, a strong solution of cauetle soda or potash will remove it. By means of the same solutiona, porcelain rollers may be kept quite white, even if used for mixing coloured paints. Al though the colour of most pig ments is improved by grinding them finely in oil, yet there are some which suffer in intensity when their size of grain is re duced. Chrome red, for in stance, owes its deep colour to the crystals of which it is composed, and when these are reduced to extremely fine frag ments, the colour is consider ably modified.
When paint is not intended for immediate use, it is packed in metallic kegs. The construction of these, as made by B. Noakes & Co., is shown in Fig. 1104. For exportation to hot climates, the rim of the lid is sornetirnce soldered down, a practice which effectually prevents access of atmo spheric oxygen. Whito-lead paint is frequently packed in wooden kege ; these prevent the dis coloration sometimes caused by the metal of 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 the atmospheric oxygen, and prevent any further solidification of the oil. The paint may be atill better protected hy 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 eoat 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 ahown that such a priming not only dries quickly itself, but also accelerates the drying of the next coat. The latter action must he attributed to the oxygen contaiued in the red-lead, only a small portion of which is absorbed by the oil with which it is mixed. The drying of paint is to a great extent dependent upon the temperature. At a 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 49° (120° F.), the same paint will become solid in a few hours. The drying of paint being a process of oxidation, and not evaporation, it is essential that a good supply of fresh air should be provided. When a film of fresb paint is placed with a certain quantity of 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 means of turpentine, the whole of this liquid evaporates on exposure to the air. There must, therefore, be a plentiful access of air, both to remove the vapours formed, and to afford a fresh supply of active oxygen. The presence of moisture in the air la rather beneficial than injurious at this atage. Especially in the case of painta mixed with varnish, moiat air appeara to counteract the tendency to crack or shrink. Under the erroneous impression that the drying of paint is a apeciea of evaporation, open fires are sometimes kept up in freahly-painted rooms. It ia only when the temperature is very low, that any benefit can result from thia practice ; as a rule, it rather retards than haatens the solidification of the oil, which cannot take place rapidly in an atmosphere laden with carbonic acid. Tlae firat coat of paint ahould be thoroughly dry before the aecond ia applied. Acrylic acid is formed during the oxidation of linseed-oil, and unless this be allowed to evaporate, it may aubsequently liberate carbonic acid from the white-lead present in moat painta, and give riae to bliaters. Sometimes a second priming-coat is given ; but usually the aecond coat applied containa the pigment. Ilia, as soon as dry, is again covered by another coat, and aubsequently by two or more finiahing-coats, according to the nature of the work. Before the firat coat is applied to wood, all holea ohould be filled up. The filling usually employed is ordinary putty. This, however, sometimes consists of whiting ground up with oil foots of a non-drying character. When the films of paint are dry, the oil from the putty exudes to the surface, causing a atain. The best filling for ordinary purposes is whiting ground to a paste with boiled linseed-oil. For finer work, and for filling cracks, red-lead mixed with the same vehicle may be employed. There is no advantage in laying ou the paint too thickly. A thick film takes longer to dry thoroughly than two thin filina of the same aggregate thickness. Paint is thinned down or diluted with linseed-oil or turpentine. The latter liquid, when used in exceaa, causes the paint to dry with a dull surface, and haa an injurious effect upon its stability. Sometimes the last coat of paint is mixed with varnish, in order to give it greater brilliancy. In thia case, apecial care must be taken that the previous coata have thoroughly solidified, or cracko in the final coat may ouhaequently appear. The same remark applies when the surface of the paint is varnislaed. The turpentine with which the varnish is mixed has a powerful action upon the oil contained in the paint, if the latter is not thoroughly oxidized. The exterior of the paint ia tlaua softened, and the varnish ia enabled to shrink and crack, eapecially in warm weather.
The method of applying paint by meana of hrushea is too well k-nown to need description ; but a few worda as to the proper treatment of the brushes may not be superfluoua. The bristles are frequently fastened together by means of glue or size, which is not perceptibly acted upon by oil, and if brought into contact with thia liquid alone, there would be no complaints of loose hairs coming out and spoiling the work. It ia a common practice to leave the brushes in a paint pot, in which the paint is covered with water to keep it from drying. The bruahee are certainly kept soft and pliant in this way ; but at the same time the glue is aoftened, and the briatles come out as soon as the brush is used. After use, brushes ahould be cleaned, and placed in linaeed-oil until again required, when they will be found in good condition. Treated in this way, they will wear so much better that the little additional trouble entailed is amply repaid.
W. F. R.