In conformity to these rules, a fine co lourless varnish may be obtained, by dis solving 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 undoubt edly preferable in many respects. This is effected by triturating an ounce of pow der of gum copal, which has been well dried by a gentle heat, with a drachm of eamphor, and, while these are mixing to gether, adding by degrees four ounces of the strongest alcohol, without any diges tion.
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. The most ancient description of two sorts of it, one of which was prepared with oil, and the other with alcohol, is to be found in "Alexhis Pedemontanus Dei Secreti," Lucca, of which the first edition was published in the year 1557. But it is better prepared, and more durable, when made after the following prescription : Take two ounces of shell lac, of arnatto and turmeric of each one ounce, and thir ty grains of fine dragon's blood, and make an extract with twenty ounces of alcohol in a gentle heat.
Oil varnishes are commonly mixed im mediately with the colours, but lac or lac quer 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.
Pere d'Incarville has informed us, that the tree which affords the varnish of Chi na is called tsi•chou by the Chinese. This tree is propagated by offsets. When the cultivator is desirous of planting it, lie takes a branch, which he wraps up in a mass of earth, by meats of flax. Care is taken to moisten this earth ; the branch pushes out roots, and is then pruned and transplanted. This tree grows to the size of a man's leg.
The varnish is drawn in spring. If it be a cultivated tree, it affords three ga therings. It is extracted by indiums made in the spring; and when the var nish, which is received in shells, does not flow, several hog's bristles, moistened with water or saliva, are introduced into the wound, and cause it to run. When the tree is exhausted, the upper part of it is wrapped in straw, which is set on fire, and causes the varnish to precipi tate to the bottom of the tree, where it flows out of perforations made for that purpose.
Those who collect the varnish set out before day-break, and place their shells beneath the apertures. The shells are not left longer than three hours in their place, because the heat of the sun would evaporate the varnish.
The varnish emits a smell, which the workmen are very careful to avoid res piring. It produces an effect whieh they call the bud of the varnish, When the varnish issues from the tree it resembles pitch. By exposure to the
air, it gradually becomes coloured, and is, at last, of a beautiful black.
The juice which flows from incisions made in the trunk and branches of the rhos toxicodendron possesses the same properties. It is a white milky fluid, which becomes black and thick by the contact of the air.
To make the varnish bright, it is eva porated by the sun ; and a body is given to it with hog's gall and sulphate ofiron.
The Chinese use the oil of tea, which they render drier by boiling it with orpi ment, realgar, and arsenic.
To varnish any substance, consists in applying upon its surface a covering of such a nature, as shall defend it from the influence of the air, and give it a shining appearance.
A coat of varnish ought, therefore, to possess the following properties : 1. It must exclude the action of the air ; be cause wood and metals are varnished to defend them from decay and rust. 2. It must resist water ; for otherwise the ef; feet of the varnish could not be perma nent. S. It ought not to alter such co lours as are intended to be preserved by this means.
It is necessary, therefore, that a var nish should be easily extended or spread over the surface, without leaving pores or cavities ; that it should not crack nor scale ; and that it should resist water. Now resins are the only bodies that pos sess these properties.
Resins, consequently, must be used as the bases of varnish. The question which of course presents itself must be, then, how to dispose them for this use ; and for this purpose they must be dissolved, as minutely divided as possible, and com Lined in such a manner, that the imper fections of those which might be dis posed to scale may be corrected by o thers.
Resins may be dissolved by three agents : L By fixed oil. 2. By volatile oil. 3. By alcohol. And accordingly we have three kinds of varnish : the fat or oily varnish, essential varnish, and spirit var nish.
Before a resin is dissolved in a fixed oil, it is necessary to render the oil dry ing. 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.
The essential varnishes consist of a so lution 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.
When resins are dissolved in alcohol, the varnish dries very speedily, and is subject to crack ; but this fault is correct ed by adding a small quantity of turpen tine to the mixture, which renders it brighter, and less brittle when dry.
The coloured resins, or gums, such as gamboge, dragon's blood, &c. are used to colour varnishes.
To give lustre to the varnish after it is laid on, it is rubbed with pounded pumice stone and water ; which being dried with a cloth, the work is afterward rubbed with an oiled rag and tripoli. The surface is, last of all, cleaned with soft linen cloths, cleared of all greasiness with powder of starch, and rubbed bright with the palm of the hand.