VARNISH. 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, Manic, CAOUTCHOLIC, and other resins, we have described the process of preparing var nishes 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 pro motes 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 pride 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 staked 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, 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 tuts 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'. varnish, in Papin's machine, by a gentle fire.
As an instance of the second sort of lae-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 beat, in close glass vessels. This is is the varnish used for the modern transparencies employed as window-blinds, fire-screens, and for other purposes. These are commonly prints, coloured on both sides, and afterwards coated with this var nish 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 Cora..
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 email 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 Csourcn000.
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 may be relied upon as genuine, being extracted from that very accurate the Did/me asure Ted:Wog:vie. The varnish is composed of Gum Sandarac!' 14 oz. & 2 drachms.
Gum Mastic in drops 7 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 sentinned 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 $wsent 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 dots, 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 needy 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 tetchy, a very small drop of olive oil is to be ap plied 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 food ; 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.