Hardwood lacker is made like the brown hard varnish with 2 lbs. of shell lac to 1 gallon of spirit of wine, but without turpentine. Another hardwood lacker is made with 1 lb. of seed lac and 1 lb. of white rosin dissolved in 1 gallon of spirits of wine.
French polish is made in a great variety of ways, but the simplest, and probably the best, consists of 1 1/2 lbs of shell lac dissolved in 1 gallon of spirits of wine without heat. Copal, sandarac, mastic, and gum arabic, are frequently used in making French polish, partly with the view of making the polish of a lighter colour, and partly to please the fancy of the polisher, and the proportions of the different gums are varied almost infinitely, but with little advantage. A polish that is by some considered to be very good is made with 12 ounces of shell lac, 6 ounces of gum arabic, and 3 ounces of copal to 1 gallon of spirits of wine. When a dark coloured polish is required, 1/2 lb. of Benzoin is sometimes added to 1 lb. of shell lac dissolved in 1 gallon of spirits, or 4 ounces of guaiacum are added to 1 1/2 lbs. of shell lac; at other times the polish is coloured to the required tint with dragon's blood.
The shell lac alone makes the hardest and most durable polish, and it is a frequent practice to make the polish rather thicker in the first instance than it is required for use, as it maybe readily thinned by the addition of spirit. But if it should be made too thin originally, it would require to be thickened by dissolving a further portion of shell lac. With the view of avoiding any risk of the polish being made too thin in the first instance, the proportion of shell lac is frequently made 2 lbs. to the gallon of spirit. Other resins are sometimes added, with the view of making the polish tougher. Thus sometimes the polish is made with 1 1/2 lbs. of shell lac, 4 ounces of seed lac, 4 ounces of sandarac, and 2 ounces of mastic to the gallon of spirit; at other times the proportions are 2 lbs. of shell lac and 4 ounces of Thus to the gallon of spirit.
When a lighter coloured lac varnish, or polish, is required than can be made with the palest ordinary shell lac, the bleached lac, sold under the name of white lac, may be employed with advantage. The varnish made with the white lac is at first almost colourless, but becomes darker by exposure to the light.
Various modes have been adopted for bleaching lac varnish. In 1827 the Society of Arts rewarded Mr. G. Field and Mr. H. Luning for their methods of effecting this object, which are described in Vol. XLV. of the Society's Transactions. Mr. Field's process is as follows: "Six ounces of shell lac, coarsely pounded, are to be dissolved by gentle heat in a pint of spirits of wine; to this is to be added a bleaching liquor, made by dissolving purified carbonate of potash in water, and then impregnating it with chlorine gas till the silica precipitates, and the solution becomes slightly coloured. Of the above bleaching liquor add one or two ounces to the spirituous solution of lac, and stir the whole well together; effervescence takes place, and when this ceases, add more of the bleaching liquor, and thus proceed till the colour of the mixture has become pale. A second bleaching liquid is now to be added, made by diluting muriatic acid with thrice its weight of water, and dropping into it pulverized red lead, till the last added portions do not become white. Of this acid bleaching liquor, small quantities at a time are to be added to the half-bleached lac solution, allowing the effervescence, which takes place on each addition, to cease before a fresh portion is poured in. This is to be continued till the lac, now white, separates from the liquor. The supernatant fluid is now to be poured away, and the lac is to be well washed in repeated waters, and finally wrung as dry as possible in a cloth."
Mr. Luning's process is as follows: "Dissolve 5 ounces of shell lac in a quart of rectified spirits of wine; boil for a few minutes with 10 ounces of well-burnt and recently-heated animal charcoal, when a small quantity of the solution should be drawn off and filtered; if not colourless, a little more charcoal must be added. When all colour is removed press the liquor through silk, as linen absorbs more varnish, and afterwards filter it through fine blotting paper."
Dr. Harems process, published in the "Franklin Journal," and reprinted in the "Technological Repository," Vol. I, 1827, is as follows: "Dissolve in an iron kettle one part of pearlash in about eight parts of water, add one part of shell or seed lac, and heat the whole to ebullition. 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 colour deepens by washing and consolidation; dissolved in alcohol, lac bleached by the process above mentioned yields a varnish which is as free from colour as any copal varnish."
A nearly colourless varnish may also be made by dissolving the lac as in Dr. Hare's process; bleaching it with a filtered solution of chloride of lime, and afterwards dissolving the lime from the precipitate, by the addition of muriatic acid. The precipitate is then to be well washed in several waters, dried, and dissolved in alcohol, which takes up the more soluble portion, forming a very pale but rather thin varnish, to which a small quantity of mastic may be added.
Attempts are frequently made to combine copal with all the spirit varnishes, in order to give them greater toughness and durability, and although copal cannot be entirely dissolved even in pure alcohol, still a moderate portion will be taken up by strong spirit of wine when a temperature of about 120° is employed with frequent agitation of the varnish. In this manner a light coloured varnish may be made with £ lb. of shell lac, | lb. of copal to 1 gallon of spirits of wine containing about 95 per cent, of alcohol. The copal should be powdered quite fine, and may either be added to the shell lac and spirit at the commencement, in which case the shell lac should also be powdered, or the shell lac may be first dissolved, and the powdered copal added; but in either case it is only the more soluble portion of the copal that is taken up, and the remainder settles to the bottom in a viscid mass, from which the varnish may be decanted and strained for use. Copal may be added in the same manner to the white hard varnishes, and it is sometimes recommended to fuse the copal and drop it into water before attempting to dissolve it in spirit, but the advantage of adding copal to spirit varnishes is very questionable.