VARNISH. Lac varnishes, or lacquers, consist of different resins in a state of so lution, of which the most common are, mastich, sandarach, lac, benzoin, copal, amber, and asphaltum. The menstrua 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 di luted again with a little more oil of tur pentine. The latter addition promotes both the glossy appearance and drying of the varnish.
Of this sort is the 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 and suf fered 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 var nish, when it is cool, happen to be yet too thick, it may be attenuated with more oil of turpentine. 'Phis varnish has al ways 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 dig. solvedin 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, mas tich alone is dissolved in oil of turpentine by a very gentle digesting heat, in close glass vessels. This 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 afterward 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 he primed with a solution of isinglass, before the colour is laid on.
Copal may be dissolved in genuine Chio turpentine, according to i‘lr. Shel drake, by adding it in powder to the tur pentine previously melted, and stirring till the whole is fused. Oil of turpentine may then be added, to dilute it sufficient ly. Or the copal in powder may be put into a long-necked matrass with twelve parts of oil of turpentine, and digested several days on a sand-heat, frequently shaking it. This may be diluted with one fourth or one fifth of alcohol. Metallic vessels, or instruments, covered with two or three coats of this, and dried in an oven each time, may be washed with boiling water, or even exposed to a still greater heat, without injury to the var nish.
A varnish of the consistence of thin turpentine is obtained for aerostatic ma chines, by the digestion of one part of elastic gum, or caoutchouc, cut into small pieces, in thirty-two parts of recti fied 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.
The third sort of lac-varnishes consists in the spirit-varnish. The most solid re sins yield the most durable varnishes ; hut a varnish must never be expected to lie harder than the resin naturally is of which it is made. Hence, it is the height of absurdity to suppose that there are any incombustible varnishes, since there is no such thing as an incombustible re sin. But the most solid resins by them selves produce brittle varnishes : there hire something of a softer substance must always be mixed with them, whereby this brittleness is diminished. For this purpose, gum elemi, turpentine, or bal ,am of capaiva, are employed in proper proportions. For the solution of these bodies the strongest alcohol ought to be used, which may very properly indeed be distilled over alkali, but must not have stood upon alkali. The utmost simplicity in composition, with respect to the num ber of the ingredients in a formula, is the result of the greatest skill in the art ; hence it is no wonder, that the greatest part of the formulas and recipes that we meet with are composed without any principle at all.