Carriage varnish is made much the same as common body varnish, except that to 8 lbs. of gum of second quality to about 2 1/2 gallons of oil, and 5 1/2 gallons of turpentine are used with driers. This varnish is boiled until very stringy, and is used for the wheels and under framework of coaches and other objects not requiring to be polished; it is intermediate in quality between body varnish and the following.
Wainscot varnish consists of 8 lbs. of second quality of gum anime, 3 gallons of clarified oil, 1/4 lb. of litharge, 1/4 lb. of dried sugar of lead, 1/4 lb. of copperas, well boiled until it strings very strong and mixed with 5 1/2 gallons of turpentine. This varnish dries quickly and is principally used for house painting and japanning. When a darker varnish is required as for mahogany a small portion of gold size may be mixed with it.
Pale amber varnish. Fuse 6 lbs. of fine-picked very pale transparent amber, in the gum-pot, and pour in 2 gallons of hot clarified oil. Boil it until it strings very strong. Mix with 4 gallons of turpentine. This will be as fine as body copal, will work very free, and flow well upon any work it is applied to; it dries slowly, but becomes very hard, and is the most durable of all varnishes. It is very excellent to mix in copal varnishes to give them a hard and durable quality. Amber varnish is however but little used on account of its expense.
In making all the above varnishes it should be observed, that the more minutely the gum is fused, the greater the quantity and the stronger the produce. The more regular and longer the boiling of the oil and gum together is continued, the more fluid or free the varnish will extend on whatever it is applied. When the mixture of oil and gum is too suddenly brought to string by too strong a heat, the varnish requires more than its just proportion of turpentine to thin it, whereby its oily and gummy quality is reduced, which renders it less durable; neither will it flow so well in laying on. The greater proportion of oil there is used in varnishes, the less they are liable to crack, because the tougher and softer they are. Increase the proportion of gum in varnishes the thicker the stratum required, and the firmer they will set, and the quicker they will dry.
All body varnishes, or those intended to be polished should have 1 1/2 lbs. of gum to each gallon of varnish when it is strained off and cold. All carriage or wainscot varnishes, or those not intended to be polished should have full 1 lb. of gum to each gallon. But the quantity of gum required to bring it to its proper consistence, depends very much upon the degree of boiling it has undergone; therefore when the gum and oil have lot been strongly boiled the varnish requires less turpentine to thin it, and when boiled stronger than usual a larger proportion of turpentine is required, and if the mixing of the varnish with he turpentine is commenced too soon, and the pot is not sufficiently cool, there may be considerable loss by evaporation. Copal varnishes should be made at least three months before hey are required for use, and the longer they are kept the better they become, but when it is necessary to use the varnishes before they are of sufficient age, they should be left thicker than usual.
In the preparation of spirit and turpentine varnishes, scarcely any apparatus is required; as, generally speaking, the process is almost limited to mixing the resins and solvent together, and agitating the whole until the resin is thoroughly dissolved. Heat is not generally necessary, and although frequently resorted to in order to facilitate the dissolution of the resins, in most instances only a moderate degree of warmth is required, and consequently the preparation of spirit and turpentine varnishes is far more manageable than that of oil varnishes, and entails much less risk of accident.
The resins should be thoroughly free from moisture, and are generally broken into small pieces, in order that they may be dissolved more quickly, and all impurities are carefully picked out; after which the finest and clearest pieces are generally selected and set aside for making small quantities of varnish of a superior quality. Sometimes, with the view of expediting the dissolution of the resins, they are finely powdered before they are added to the solvent; but in this case it is necessary that the agitation should be maintained from the time the resin is added until it is thoroughly dissolved, or otherwise it is liable to agglutinate into one mass that is afterwards very difficult of solution.
In making turpentine varnishes without heat, in quantities of ten or twelve gallons, the resin and turpentine are generally introduced into a large can with a wide mouth, and agitated by stirring with a stout stick; a number of wooden pegs or nails are mostly driven into the stick near the lower end to increase its effect.
Spirit varnishes are generally made in smaller quantities, and to prevent the evaporation of the spirit, the mouth of the vessel is mostly closed and the vessel itself is agitated. In making quantities of four to eight gallons, the resin and solvent are sometimes introduced into a small cask capable of containing about double the quantity, and mounted to revolve on central bearings at the ends. The cask is made to revolve either with continuous motion by a winch-handle, or with an alternating motion, by means of a cord passed around the barrel, and terminating in a cross handle, which the operator pulls to give motion to the barrel in the one direction, and the momentum of which suffices to coil up the cord ready for the following pull, which causes the barrel to revolve in the opposite direction, and so on continually.
Quantities of varnish, not exceeding two or three gallons, are generally agitated in a tin can, rolled backwards and forwards upon a bench covered with an old carpet, or a sack; but whatever method is adopted for the agitation, it should be continued without intermission until the resin is sufficiently dissolved, to prevent the risk of its becoming agglutinated, the time required for which depends upon the solubility of the resin, and the strength of the spirit, but is commonly from three to four hours. The further agitation for the thorough solution of the resin may be either continuous or intermittent, according to convenience, but it should not be abandoned until the solution is perfect; and when it is judged to be complete, the varnish is poured into another vessel for examination, and if any of the resin is not perfectly dissolved, the whole is returned to the vessel for further agitation. When the resin is all dissolved, the varnish is allowed to stand for a few hours, that any impurities may settle to the bottom, and the clear varnish is lastly strained through muslin or lawn into bottles, and allowed to stand for a few days before use.