A solution of resinous matter, which, laid upon the surface of solid bodies, becomes hard, glossy, impervious to moisture, and gives beauty and durability to them. Under the several heads of Lac, Copal, Mastic, Caoutchouc, and other resins, we have described the process of preparing varnishes from them; we shall therefore in this place take a general, but concise view of the subject. The solvents are either expressed or essential oils, as also alcohol. For a lac-varnish of the first kind, the common painter's varnish is to be united by gently boiling it with some more mastich or colophony, and then diluted again with a little more oil of turpentine. The latter addition promotes both the glossy appearance and drying of the varnish; of this sort is also amber varnish. To make this varnish, half a pound of amber is kept over a gentle fire in a covered iron pot, in the lid of which there is a small hole, till it is observed to become soft, and to be melted together into one mass. As soon as this is perceived, the vessel is taken from off the fire, and suffered to cool a little; when a pound of good painter's varnish is added to it, and the whole suffered to boil up again over the fire, keeping it continually stirring.
After this it is again removed from the fire; and when it is become somewhat cool, a pound of oil of turpentine is to be gradually mixed with it. Should the varnish when it is cool happen to be yet too thick, it may be attenuated with more oil of turpentine. This varnish has always a dark brown colour, because the amber is previously half burned in this operation; but if it be required of a bright colour, amber powder must be dissolved in transparent painter's varnish, in Papin's machine, by a gentle fire.
As an instance of the second sort of lac-varnishes with ethereal oils alone, may be adduced the varnish made with oil of turpentine. For making this, mastich alone is dissolved in oil of turpentine by a very gentle digesting heat, in close glass vessels. This is is the varnish used for the modern transparencies employed as window-blinds, fire-screens, and for other purposess. These are commonly prints, coloured on both sides, and afterwards coated with this varnish on those parts that are intended to be transparent. Sometimes fine thin calico, or Irish linen, is used for this purpose; but it requires to be primed with a solution of isinglass before the colour is laid on. Copal may also be dissolved in genuine Chio turpentine, according to Mr. Sheldrake, by adding it in powder to the turpentine previously melted, and stirring till the whole is fused. See Copal.
A varnish of the consistence of thin turpentine is obtained for aerostatic machines, by the digestion of one part of elastic-gum, or caoutchouc, cut into small pieces, in thirty-two parts of rectified oil of turpentine. Previously to its being used, however, it must be passed through a linen cloth, in order that the undissolved parts may be left behind. See Caoutchouc.
The third sort of lac-varnishes consists in the spirit-varnish. The most solid resins yield the most durable varnishes; but a varnish must never be expected to be harder than the resin naturally is of which it is made. But the most solid resins by themselves produce brittle varnishes; therefore something of a softer substance must always be mixed with them, whereby this brittleness is dimi-nished. For this purpose gum-elemi, turpentine, or balsam of copaiva are employed in proper proportions.
The celebrated " French polish" is effected by a spirit-varnish, treated in a peculiar way*. The following mode of preparing and using it maybe relied upon as genuine, being extracted from that very accurate French work, the Diction-naire Technologique. The varnish is composed of
Gum Sandarach . . . . . . . . . . . . . . . . .
14 oz. &. 2 drachms.
Gum Mastic in drops . . . . . . . .. . . .
7 " 1 "
Shell-lac (the yellower the better) . . . . .
14 " 2 "
Alcohol, of 0.8295. spe. gra . . . . . . . . . .
3 quarts & 1 pint
The resinous gums are to be pounded, and their solution effected by continued agitation, without the aid of heat. When the woods to be varnished are very porous, 7 ounces and 1 drachm of Venice turpentine. In order the better to divide the resins, and to cause them to present a greater surface to the action of the alcohol, they should be mixed with an equal weight of ground glass; the latter preventing the dust of the resin from forming clots, the solution is thus easier made, and in less time. Before applying the varnish, the wood should be made to imbibe a little linseed oil; it must then be rubbed with old flannel, in order to remove the excess of oil; blotting paper may be used for the same purpose, or finely-sifted saw-dust. Afterwards the varnish should be applied, by saturating with it a piece of old soft coarse linen cloth, many times folded into a sort of cushion, and rubbing it softly on the wood, turning the linen from time to time until it appears nearly dry. The linen should be saturated afresh with varnish, and the rubbing be continued in the same manner, until the pores of the wood are completely filled. Care should be taken not to make the linen too wet, nor to rub too hard, especially at the beginning of the operation.
When the varnish sticks, or becomes tacky, a very small drop of olive oil is to be applied with the end of a finger, uniformly all over the cushion. The finishing is effected by pouring a little pure alcohol upon a piece of clean linen, which is lightly rubbed over the varnished wood; and as the linen and the varnish dry, the wood is rubbed more briskly, until it takes a beautiful polish like a looking-glass. Two or three coatings of varnish are sufficient for woods not very porous.
A fine colourless varnish may be obtained, by dissolving eight ounces of gum-sandarach and two ounces of Venice turpentine in thirty-two ounces of alcohol by a gentle heat. Five ounces of shell-lac and one of turpentine, dissolved in thirty-two ounces of alcohol by a very gentle heat, give a harder varnish, but of a reddish cast. To these the solution of copal is undoubtedly preferable in many respects. This is effected by triturating an ounce of powder of gum-copal, which has been well dried by a gentle heat, with a drachm of camphor, and, while these are mixing together, adding by degreesfour ounces of the strongest alcohol, without any digestion. Between this and the gold varnish there is only this difference, that some substances that communicate a yellow tinge are to be added to the latter.
Oil-varnishes are commonly mixed immediately with the colours, but lac or lacquer-varnishes are laid on by themselves upon a burnished coloured ground: when they are intended to be laid upon naked wood, a ground should be first given them of strong size, either alone, or with some earthy colour mixed up with it by levigation. The gold lacquer is simply rubbed over brass, tin, or silver, to give them a gold colour. (See Lacquer.) The coloured resins or gums, such as gamboge, dragon's-blood, etc, are used to colour varnishes.
The essential varnishes consist in a solution of resin in oil of turpentine. The varnish being applied, the essential oil flies off, and leaves the resin. This is used only for paintings.
Before a resin is dissolved in a fixed oil, it is necessary to render the oil drying. For this purpose the oil is boiled with metallic oxides, in which operation the mucilage of the oil combines with the metal, while the oil itself unites with the oxygen of the oxide. To accelerate the drying of this varnish, it is necessary to add oil of turpentine. When resins are dissolved in alcohol, the varnish dries very speedily, and is subject to crack; but this fault is corrected by adding a small quantity of turpentine to the mixture, which renders it brighter, and less brittle when dry.