Very small quantities of varnish are generally made in glass bottles, large enough to contain about one-third more than the quantity introduced, and they are shaken up at frequent intervals; but although from the small bulk of the resin it cannot agglutinate into so insoluble a mass as when larger quantities are made, still when the agitation is intermitted, several days are frequently required before the resins are entirely dissolved, as the solution depends more upon the amount of agitation than the length of time the resins are submitted to the action of the solvent.
Sometimes, with the view of preventing the agglutination, and facilitating the dissolution of the resins, coarsely pounded glass is introduced with the resin and solvent; in this case the glass should be thoroughly washed and dried, as mentioned at page 1885, and afterwards sifted, to exclude all the smaller particles, which, from their lightness, would have little effect in preventing the aggregation of the resin, and would be more troublesome to separate from the varnish.
When heat is employed in making spirit varnishes, the lowest temperature should be used that will suffice to dissolve the resins, as otherwise there is risk of losing a considerable portion of the alcohol by evaporation, thereby reducing the strength of the spirit; the varnish is also liable to be made of a darker colour by excess of heat, and those containing shell lac are less clear and hard when made with heat than when made quite cold, as the heated spirit dissolves the greater portion of the wax contained in the shell lac, and which becomes disseminated throughout the mass; but when the solution is made without heat, the principal portion of the wax and other impurities remain undissolved at the bottom.
In making large quantities of spirit varnish with heat, a still and worm are sometimes employed, in order to prevent loss by evaporation; the still is heated by a steam or water-bath, and the resins and solvent are agitated by a stirring-rod passing through a stuffing-box in the head of the still. Quantities of two or three gallons are generally made in a tin can, which is dipped at frequent intervals into hot water, and agitated between every dip by rolling; but in this case it is necessary to loosen the cork every time it is immersed in the hot water, in order to allow the vapour of the spirit to escape, or otherwise the cork would be driven out with great force, and some of the spirit might be thrown on the fire with risk of serious accident. Glass bottles, although convenient from their transparency, should never be employed for making varnish with heat, as they are liable to break from the alternations of temperature. They are, however, often used for making small quantities, and in this case the safer practice is to heat the water only in a moderate degree, and to allow of the continuous escape of the vapour through a small notch cut lengthways in the cork, and which may be closed by the thumb when the bottle is shaken. There is, however, always some little risk of accident in making spirit varnishes near an open fire, when much heat is employed; and a water or sand-bath, placed on the top of a stove so as to be heated only in a moderate degree, will be generally found to afford sufficient warmth, and is, perhaps, the most safe and convenient arrangement for occasional purposes.
Shell lac never requires more than a very moderate warmth to dissolve it, and the solution is frequently made in stone bottles placed near a fire and shaken occasionally. When it is required to be very clear as for metal lacker, it should be passed through filtering paper, before it is bottled.
It need scarcely be observed, that all the utensils employed in making spirit varnishes should be perfectly clean and dry, as the least moisture or even a damp atmosphere is liable to deteriorate the quality of the varnish.
Best white hard spirit varnish to bear polishing is made by adding 2 lbs. of the best picked gum sandarac to 1 gallon of spirit of wine, they are then shaken up without intermission for about four hours, or until the gum is quite dissolved, 18 ounces of Venice turpentine is then moderately warmed, in a water bath, to make it fluid, and poured into the varnish to give it a body. The whole is then well agitated for about one hour, and afterwards strained and put into bottles, which should be kept well corked to prevent the evaporation of the spirit. After standing about a week the varnish is fit for use. This varnish may be made sufficiently pale to be used on white work when the clearest and palest pieces of the gum are carefully selected. When the work does not require to be polished the proportion of Venice turpentine may be reduced one half.
White hard varnish is also made with 3 1/2 lbs. of gum sandarac to 1 gallon of spirit of wine, and when they are dissolved one pint of pale turpentine varnish is added and the whole are well shaken until thoroughly mixed. Another white hard varnish is made with 2 lbs. of gum sandarac, 1 lb. of gum mastic, and 1 gallon of spirit of wine.
White spirit varnish for violins is made with 2 lbs. of mastic to 1 gallon of spirit of wine, and 1 pint of turpentine varnish. This may be made either in the same manner as the white hard varnish, or the ingredients may all be mixed together in a tin can, placed in a warm situation near a fire and shaken occasionally until dissolved.
Brown hard spirit varnish is made in the same manner as white hard varnish, but shell lac is generally used instead of sandarac. Thus a very excellent brown hard spirit varnish that will bear polishing is made with 2 lbs. of shell lac to 1 gallon of spirit of wine, and after they are amalgamated, 18 ounces of Venice turpentine is warmed and added exactly as described for the best white hard varnish. Another very good brown hard spirit varnish consists of 2 lbs. shell lac, 1 lb. of sandarac, and 2 ounces of mastic, dissolved in 1 gallon of spirits of wine. A lighter coloured varnish is made with 2 lbs. of sandarac, llb. of shell lac, and 1 gallon of spirit. After the resins are dissolved 1 pint of turpentine varnish is added and the whole is well mixed by agitation.