VARNISH, a thick, viscous, shining liquor, used by painters, gilders, and various other artificers, to give a gloss and lustre to their works; as also to defend them from the weather, dust, &c.
There are several kinds of varnishes in use, and many receipts have been published for the preparation of them; but very few of these can be depended on. The following are extracted from Dr Lewis's Commerce of Arts, and seem to be well authenticated.
1. Gold-coloured Varnish, or Lacquer.—Silver, coated with a transparent gold-coloured varnish, is made to resemble gold so exactly, as wholly to supply the place of gold in some of the works called gilt. The basis of the varnish, or what gives adhesiveness and glossiness to the colouring matter, is a solution of lac made in spirit of wine.
Stick-lac is only to be used for varnishes: what is called shell-lac, or the grains formed into plates by melting them in boiling water, does not answer so well. The spirit must be highly rectified, or freed as much as possible from any admixture of phlegm or water. * Some feed-lac, reduced into fine powder, is then added to it, in the proportion of about three ounces to a pint: the vessel being set in a moderate warmth for 24 hours and frequently shaken, a part of the lac dissolves; and the spirit, now tinged of a reddish brown colour, is strained off from the undissolved part, and set by for a day or two to settle. The digestion should be performed in a wide-mouthed vessel, covered so as to prevent the exhalation of the spirit: the undissolved lac softens into a viscous mass, so as scarce to be got out through a narrow aperture.
In different portions of the foregoing solution, poured off clear after the straining and settling, some gamboge and annotto are dissolved separately; the gamboge communicates a high yellow colour, and the annotto a deep reddish yellow. The solution of the gamboge is mixed with about half its quantity of that of the annotto, and trial made of the mixture on some silver leaf: if the colour inclines too much to the yellow or the red, more of the one or the other liquor is added till the true golden colour is obtained. There are sundry other materials, from a due mixture of which a like colour may be produced; such as turmeric, saffron, dragon's-blood, &c.
The silver leaf being fixed on the subject, in the same manner as gold leaf, by the interposition of proper glutinous matters, the varnish is spread upon the piece with a brush or pencil. The first coat being dry, the piece is again and again washed over with the varnish till the colour appears sufficiently deep. What is called gilt leather, and many picture-frames, have no other than this counterfeit gilding. Washing them with a little rectified spirit of wine affords a proof of this; the spirit dissolving the varnish, and leaving the silver leaf of its own whiteness. For plain frames, thick tin-foil may be used instead of silver. The tin-leaf, fixed on the
the piece with glue, is to be burnished, then polished with emery and a fine linen cloth, and afterwards with putty applied in the same manner: being then lacquered over with the varnish five or six times, it looks very nearly like burnished gold. The same varnish, made with a less proportion of the colouring materials, is applied also on works of brass; both for heightening the colour of the metal to a resemblance with that of gold, and for preserving it from being tarnished or corroded by the air.
2. Spirit Varnish. Black varnish, for japanning on wood or leather, is prepared by mixing lamp-black or ivory-black with a proper quantity of a strong solution of gum lac in spirit of wine, such as that above-described. The lamp-black is commonly preferred to the ivory-black, on account of its uniting better with the fluid, and working smoother. The thicker part of the varnish, which settles at the bottom, is used with the lamp-black for the first coatings, and the mixture applied at different times, in a hot room, one layer after another is dry, till a full body of colour is obtained: after which the piece is washed over in the same manner, several times, with the finer part of the varnish, just tinged with the black, so as to make a coating of sufficient thickness to bear polishing with tripoli.
3. Amber Varnishes for Papier Maché, &c. Papier maché is made of cuttings of white or brown paper, boiled in water, and beaten in a mortar, till they are reduced into a kind of paste; and then boiled with solution of gum-arabic or of size, to give tenacity to the paste, which is afterwards formed into different toys, &c. by pressing it into oiled moulds. When dry, it is done over with a mixture of size and lamp-black, and afterwards varnished. The black varnish for these toys is prepared as follows. Some colophony, or turpentine boiled down till it becomes black and friable, is melted in a glazed earthen vessel, and thrice as much amber in fine powder sprinkled in by degrees, with the addition of a little spirit or oil of turpentine now and then: when the amber is melted, sprinkle in the same quantity of sarcocolla, continuing to stir them, and to add more spirit of turpentine, till the whole becomes fluid; then strain out the clear through a coarse hairbag, pressing it gently between hot boards. This varnish, mixed with ivory-black in fine powder, is applied, in a hot room, on the dried paper paste; which is then set in a gently heated oven, next day in a hotter oven, and the third day in a very hot one, and let stand each time till the oven is grown cold. The paste thus varnished is hard, durable, glossy, and bears liquors hot or cold.
A more simple amber-varnish, of great use for many purposes, and said to be the basis of the fine varnishes which we see on coaches, &c. is prepared, by gently melting the amber in a crucible till it becomes black; then reducing it into a powder, which looks brown; and boiling the powder in linseed oil, or in a mixture of linseed oil and oil of turpentine. Drying oil is commonly made choice of by the workmen; but it seems more eligible here to take the oil unprepared, that the boiling requisite for giving it the drying quality may be employed at the same time in making it act upon the amber.
By the previous melting of the amber, its nature is
changed, and part of its oily and saline matter expelled, as happens in the common distillation of it. When the distillation is not far protracted, the caput mortuum, or shining black mass which remains in the retort, answers as well as the amber melted on purpose. Hence some of our chemists, instead of urging the distillation to the utmost, by which the amber would be reduced to a mere coal, find it more advantageous to discontinue the process when the thinner oil and greater part of the salt have arisen, that the remaining mass may be in great measure soluble in oils, so as to supply the common demand of the varnish-makers.
It has generally been thought, that amber will not at all dissolve in oils till it has thus suffered a degree of decomposition by fire. Hoffmann relates an experiment in his Observationes Physico-chemice, which discovers the solubility of this concrete in its natural state. Powdered amber, with twice its quantity of oil-olive, was put in a wide-mouthed glass, and a digestor, or strong copper vessel, being filled about one-third with water, the glass was placed in it, the cover of the digestor screwed down tight, and a moderate fire continued an hour or more: when cold, the amber was found dissolved into a gelatinous transparent mass.
In Dr Stockar's very curious Specimen inaugurale de succino, printed at Leyden in 1760, there are sundry more important experiments on this subject, made by himself and Mr Ziegler of Winterthur. They found, that by continuing a simmering heat 12 hours, and confining the vapour as much as stone-ware vessels would bear without bursting (the danger of which was avoided by making a small notch in the cork-stoppers) powdered amber dissolved perfectly in expressed oils, in turpentine, and in balsam of copaiba: a strong copper vessel, with a cover screwed on it, seems most eligible, and for the greater security, a valve may be made in the cover, kept down by a spring that shall give way before the confined vapour is of sufficient force to be in any danger of bursting the vessel. Though such a heat as converts part of the oil into strong elastic vapours, and the forcible compression of the vapour, are expedient for hastening the dissolution, they do not appear to be essentially necessary; for by digestion for a week in close-stopped glass-vessels, in which the compression could not be very great, solutions equally perfect were obtained.
The solution in rape-feed oil, and in oil of almonds, was of a fine yellowish colour; in linseed-oil, gold-coloured; in oil of poppy-seeds, yellowish red; in oil olive, of a beautiful red; in oil of nuts, deeper coloured; and in oil of bays, of a purple red. It is observable that this last oil, which of itself, in the greatest common heat of the atmosphere, proves of a thick butyraceous consistency, continued fluid when the amber was dissolved in it. The solutions made with turpentine, and with balsam of copaiba, were of a deep red colour, and on cooling hardened into a brittle mass of the same colour. All the solutions mingled perfectly with spirit of turpentine. Those made with the oils of linseed, bays, poppy-seeds, and nuts, and with balsam of copaiba and turpentine, being diluted with four times their quantity of spirit of turpentine, formed hard, tenacious, glossy varnishes, which dried sufficiently quick, and appeared greatly preferable to those made in the common manner from melted amber.
Varnish. 4. Varnish for Metals. Iron snuff-boxes, mourning-buckles, &c. are coloured black, by making them considerably hot, and applying on them in this state a thick mixture of lamp black, with a certain varnish called gold size. There is a gold size used for gilding, or fixing gold leaf on wood, &c. † The size here meant is a composition of a different kind, consisting of drying oil, turpentine, and the pigment called Naples yellow; which last ingredient is used for giving a high gold colour to the mixture, to fit it for some of the other purposes for which it is employed.
† See Gilding gold leaf on wood, &c. 5.
The workmen are said frequently to employ for this purpose a mixture of lamp-black with the scummings, &c. of different oil-paints: the mixture is applied with a pencil, and the piece afterwards baked in an oven, with a heat somewhat greater than that used for the papier mache. Naples yellow, a superfluous ingredient in the black varnish, is the basis of the dark brown which we see on some iron snuff-boxes, this pigment changing to a brown in baking with the varnish.
Laying on of VARNISHES. 1. If you varnish wood, let your wood be very smooth, close-grained, free from grease, and rubbed with ruffles. 2. Lay on your colours as smooth as possible; and if the varnish has any blisters in it, take them off by a polish with ruffles. 3. While you are varnishing, keep your work warm, but not too hot. 4. In laying on your varnish, begin in the middle, and stroke the brush to the outside; then to another extreme part, and so on till all be covered; for if you begin at the edges, the brush will leave blots there, and make the work unequal. 5. In fine works, use the finest tripoli in polishing: do not polish it at one time only; but after the first time, let it dry for two or three days, and polish it again for the last time. 6. In the first polishing you must use a good deal of tripoli, but in the next a very little will serve: when you have done, wash off your tripoli with a sponge and water: dry the varnish with a dry linen-rag; and clear the work, if a white ground, with oil and whiting; or if black, with oil and lamp-black.