Scientific American 37, 27.5.1854
Spirit and turpentine varnishes are prepared by mixing the resins and the solvent together, and agitating the whole with a stick having a number of pegs or nails driven in near the lower end until the solution is complete. The resins should be dry, and in small pieces, with the impurities picked out; the finest and clearest pieces of the guru are net aside for superior varnishes. Turpentine varnishes are made in quantities of 10 or 12 gallons; spirit varnishes from 4 to 8 gallons. In making the latter, the ingredients are sometimes put into a cask of 8 or 16 gallon's capacity, and mounted no as to revolve upon bearings at the ends. An alternating motion is given to the barrel bypassing round it a cord terminating in a cross handle. When the operator pulls this cord towards him the barrel rotates and winds the cord up in the other direction so no to be ready for a second pull, which, in like manner, winds the cord in the opposite direction, and so on. Agitation must be kept up, or the resin will agglutivarnate. After 3 or 4 hours, or when the solution is complete, the varnish is left for a few hours to deposit solid impurities, and is their strained through muslin or lawn into bottles. Coarsely pounded glass is sometimes added to prevent the agglutination of the resin. When heat is employed in making spirit varnishes, the source of heat should be a water or a sand bath, and a still and worm may be used to prevent loss by evaporation, the resins and solvent in the still being kept in motion by a stirrer passing through a stuffing box in the head. Shellac contains a little wax, which is apt to get diffused through the varnish when heat is applied. The inflammable nature of the ingredients will of course suggest the necessity for caution iu making spirit varnishes. The utensils employed must be quite clean and dry.
Best white hard spirit varnish, such as will bear polishing, is made by adding 2 lbs. of the best picked gumsandarach to 1 gallon of alcohol and agitating for 4 hours, until the solution is complete. 18 ozs. of Venice turpentine, (or 9 ozs. if the work is not to be polished,) are to be moderately heated in a waterbath until quite fluid, and added to the varnish to give it body. Agitate for an hour, strain and put into bottles, which must be kept well corked. After remaining undisturbed for a week the varnish is fit for use. If the clearest and palest pieces of gum be selected, this varnish will be pale enough for white work.
White Hard Varnish.
(No. 1.) 3½ lbs. of gum sandarach to 1 gallon of spirits of wine, and when the solution is complete add 1 pint of pale turpentine varnish, and shake the whole well together.
(No. 2.) 2 lbs. of gum sandarach, 1 lb. of gum mastic, and 1 gallon of alcohol. White spirit varnish for violins. — 2 lbs. of mastic to 1 gallon of spirits of wine and 1 pint of turpentine varnish.
Brown hard spirit varnish is similar to white hard varnish, only shelllac is used instead of sandarach. Dissolve 2 lbs. of shelllac in 1 gallon of spirits of wine, and then add 18 ors. of Venice turpentine, warmed. This varnish will bear polishing. Or, 2 lbs. of shelllac, 1 lb of sandarach, and 2 (rm. of mastic dissolved in 1 gallon of spirits of wine. A lighter color is produced with 2 lbs. of sandarach, 1 lb. of shelllac, and 1 gallon of spirit. When the solution is complete add a pint of turpentine varnish, and agitate the whole well together. If a pale lac varnish be required, white or bleached lac may be used. "Dissolve 5 ozs. of shelllac in a quart of rectified spirits of vine; boil for a few minutes with 10 ozs. of wellburnt and recently heated animal charcoal, when a small quantity of the solution should be drawn off and filtered; if not oolorles, a little more charcoal must be added. When all color is removed, press the liquor through silk, as linen absorbs more varnish, and afterwards filter it through fine blottingpaper."
Dr. Hare has published a method of bleaching lac: — "Dissolve in an iron kettle 1 part of pearl-ash in about 8 parts of water, add one part of shell or seed-lac, and heat the whole to ebulition. When the lac is dissolved, cool the solution and impregnate it with chlorine gas till the lac is all precipitated. The precipitate is white, but the color deepens by washing and consolidation; dissolved in alcohol, lac bleached by this process yields a varnish which is as free from color as any copal varnish." The application of the chlorine must be made by a person acquainted with chetnistry. Hence chloride of lime is safer as a bleaching agent, the lime being afterwards dissolved out front the precipitate by the addition of muriatic acid. The precipitate is to be washed several times, dried and dissolved in alcohol with the addition of a little mastic. This varnish is very pale, and rather thin.
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