It was known and in common use in Italy in the 17th and probably the I6th century, under the name of aqua di rasa or di raggia, and was used for thinning varnishes.
Coal-naphtha (pp. 644-5) consists of hydrocarbons, whose general type-formula is 0„ 112„ 6. The lowest term of the series, 0811, (benzol), boiling when pure at about 82° (149° F.), is of very important nse as a solvent for various purposes, and of more limited use in certain varnishes required to dry instantly.
The next 3 members—toluol (p. 648), xylol (p. 648), and cumol, which last boils at 166° (363° F.), are very useful solvents for sorne of the varnishes of Div. IV., which are required to dry hard in less time than those made with turps.
Alcohols (pp. 192-214).—Vinic or ethyl alcohol (C2H20), in the form of "methylated spirit," is now the chief menstruum used for all spirit-varnishes (Div. IV., Class B) and mixed-vehicle varnishes (Div. IV., Class 0). The pure (" clean ") spirit, in spite of the very heavy duty paid upon it, is sometimes used in special cases, where a varnish is required to leave no smell whatever behind, as the methylated spirit always leaves a faint smell, which lasts for weeks and is very perceptible where the varnished surface is extensive, or enclosed. The methylated spirit of com merce is very pure, and generally strong enough for tbe commoner spirit-varnishes containing only lac, sandarach, and the pine-resins; but for those containing the copals, kauri, &c., it should be rendered as nearly absolute as possible. The best method of doing this is to shake it up with about one-tenth of its weight of salt of tartar (carbonate of potash) which has been dried at a very low red beat, then letting it lie in contact with the salt for a few days. If the salt of tartar does not remain quite dry and powdery in the spirit, the process must be repeated with fresh salt of tartar, until it ceases to absorb any more water. The spirit may then be used as it is, or distilled to remove any small impurity it may have taken up from the carbonate of potash.
No spirit should ever be used in Class C, Div. IV., which has not been thus drit 13ottles or tins containing " dried " alcohol should be kept very tightly closed, or the spirit will rapidly weaken by absorbing water from the atmosphere ; and the same may be said of all spirit-, or mixed-vehicle varnishes.
Methyl-alcohol, pyroxylie spirit, or wood-spirit (CH40), exists in large quantity in rectified " wood-spirit " in mixture with various other bodies, such as aldehyde, acetate of methyl, &c., and especially acetone (p. 39), a body which rather assists than otherwise the solvent power of the methyl-alcohol. Its boiling-point is lower, about 66° (150° F.), instead of 78°.4 (173° F.) and its solvent capabilities are quite equal if not superior to those of common alcohol, especially when it oontnins acetone (as is usually the ease in the commercial article), by which also its volatility is somewhat increased. Its use is now almost confined to mixing with ordinary alcohol to make it unfit for drinking.
Propyl-aleohol (C4H80) is at present too dear for use in varnishes, otherwise it would have certain advantages over common alcohol, in its greater solvent power, its higher boiling-point, 96° (205° F.) Find consequent slower drying enabling it to be more easily laid on with the brush. It is also lese liable to " chill " in a damp atmosphere whilst being laid on. Ether, carbon bisulphide (p. 601), acetone (p. 39), chloroform, and acetic ether (p. 39), have limited use as varnish-solvents for a few exceptional purposes, especially when the varnish has to be "floated " on to the work, as in most photographic varnishee. They have very great solvent powers over resins, but the great volatility and inflammability of the first two render their use and storage very dangerous.
Besides the before-mentioned simple solvents, a considerable number of mixtd or compound solvents prepared from two or more of them (one being common alcohol), are used in the preparation of varnishes with resins which are not completely soluble in any simple solvent.
.71ety/teners.—These should, if po,sible, be of the same nature as the solid constituents of the varnishes they are intended to render more flexible, so that, on the evnporatiou of the solvent, they mny make a homogeneous mixture with the solid residue, without any tendency to separate, which would render the varnish cloudy or opaque when dry. For water-varnishes, may be used clnrified honey or other unerystallizable sug,ar, " over-boiled " glue (which will not gelatinize on cooling), and especially glycerine, which, being by far the most deliquescent, should be very sparingly used, or the glazes toughened with it will certainly get sticky in damp weather. In spirit-varnishes, a great many substances are used in the trade, which though very effective for a short time after the varnish is applied, are certain to lose their flexibility after a time,—such are the turpentines, and other oleo resins, of which the most generally used is Venice turpentine, the slowest drier of them all, but still leaving the varnish brittle after a year or so, or even in a few months in fL warm climate. Far superior to Venice turpentiue, but rather dearer, is copaiba balsam (p. 1639), which, by reason of the large percentage (50-60). and high boiling-point, about 250° (482° F.), of its volatile oil, retains its characters for a much longer period. The best of all substances, however, for toughening spirit-varnishes is castor-oil (p.1380), which, being colourless, never drying, and being very soluble in alcohol, would be universally used, were it not for a tendency which it is supposed by the trade (erroneously we think) to have, of separating from the dried varnish aud rising to the surface as a greasy filtn. Linseed- and poppy-oils (pp. 1393,1409), are also sometimes used in spirit-varnishes, and those made with mixed solvents, in which these oils are more soluble. Manilla elemi (p. 1649) is also used, bnt soon loses its virtues after the drying of the varnish. Its resin, however, is very hard land tough, and has, like benzoin, the property of giving gre,at lustre to varnish.
Essential-oil-varnishes (Class A, Div. IV.) are often brittle, and require toughening. This is usually done by the addition of a small quantity of a drying oil, which, if it exceed 20 per cent. of the resin dissolved, should be previously boiled with driers. Camphor (pp. 571-8) is else used, for though a dry solid, it has the property of making varnishes flexible and tough. It is, however, supposed to have the fault of evaporating out of the varnish in time, leaving it porous aud withont lustre. It ought never to exceed 7-8 per cent. of the resin.
Driers.—These are substances added to, or boiled with, the drying, oils, to increase their power of absorbing the oxygen of the air, and therefore make them dry much more quickly. The only ones whose reputation has survived to the present day, are the oxides and other compounds of lead and manganese.
The principal function exercised by a " drier " is that of acting as a " carrier " of the atmo spheric oxygen to the molecules of oil in its immediate neighbourhood, and this action should be, so to speak, regenerative and continuous ; e,ach molecule of the drier, after giving up some of the oxygen it contains, and thus becoming reduced to a lower degree of oxygenation, should have the power of immediately retaking the lost oxygen on exposure to the air, nnd re-forming the higher oxide, ready to give up a fresh quantity of oxygen to the air, and so ad infinitum. Manganese oxide possesses this power in a very high degree, the hydrated monoxide, which, at the moment of its precipitation, is perfectly white, changes colour in a few minutes on exposure to the air, rapidly becoming brown. It is in this state that it acts most powerfully as a drier, and, when used in quantities of less than 1 per cent., makes a very pale drying oil. Lead oxide, in quantities exceeding 5 parts in 1000, gives a very dark colour to oils when heated with them to a high temperature, as in the common " boiled oil."