In general terms, sealing-wax is compounded of resins, tempered and perfumed with proportions of the softer oleo-resins, and variously coloured. It should be glossy, smooth, not brittle, unaffected by the hottest weather, melt without emitting any smoke or nasty smell, have sufficient tenacity not to drop about when melted, and produce a seal of its own lustre and colour. The chief materials used in its composition are shellac and turpentine. The former is sometimes replaced wholly or partially by other resins, such as sandarac, benzoin, mastic, rosin, and pitch; and the latter by balsams of Peru and tolu, and fragrant essential oils. In addition, there are some neutral substances employed to augment the bulk, as gypsum, chalk, magnesium carbonate, zinc white, etc, as well as the colouring matters.
Only bleached shellac is admissible for the finest sealing-wax, though pale samples may do for some light-coloured grades; the ordinary unbleached shellac is fit only for black and brown kinds of sealing-wax. (For bleaching methods, see p. 37.) Of the turpentines "used, Venetian is the best; but it may often be conveniently replaced by a mixture of rosin and oil of turpentine, which possesses an advantage in the facility with which its fluidity can be increased or diminished. The turpentine will usually need to be filtered, which is best effected by heating it in a water-bath at the boiling temperature, 212° F (100° C), and passing it through linen. Other resins, balsams, and essential oils are used only in minor quantities, and should always be selected of good quality.
The most general colour is red. For fine grades cinnabar is necessary; but it so increases the weight of the wax that neutral bodies have to be used with it to counteract this tendency. Cheaper reds are minium, colcothar (Indian red), and bole. Madder lake is now replaced by coal-tar reds, of which the most fiery should be chosen.
Yellows embrace lead chromate (chrome yellow), which is generally used with some neutral white body (as chalk); Cassel or mineral yellow, from fused litharge; and ochre, which is too dull and unpleasant-odoured to be available for any but cheap sorts of sealing-wax.
Blues include Berlin for the darker shades, and ultramarine and mountain blue for the lighter.
The best brown is burnt sienna, though crushed sienna and Cassel brown are used.
Blacks are exclusively represented by carbon in a very fine state of division, as lamp-black, ivory black, soot, etc. This last, which is very much cheaper than the others, may be made equally suitable by judicious treatment, which aims at destroying its brownish tint and unpleasant odour. The treatment consists in careful calcination, which may be conducted in a piece of stove-piping, about 18 in. long, and closed at each end by a tight-fitting cap, one being perforated with a hole somewhat less than a pencil, to allow of the escape of vapours. The pipe is filled to within 1 1/2 or 2 in. of the top, when the covers are attached, and all joints and spaces carefully luted with clay, which may also conveniently form a protective coating to the whole pipe. The latter, when charged, is placed in an open furnace, with the perforated end upmost, and heated to redness. When the contents are thoroughly calcined, the pipe is removed, allowed 24 hours to cool, and emptied. The soot will be found to have acquired a velvety black colour, and to have lost all odour.
Frankfort or " vine " black is prepared by charring vine shoots in similar apparatus to that for calcining soot, washing the ash several times with water to remove alkaline salts, and once with water containing 25 per cent, of hydrochloric acid, taking care to use plenty of clean water after the acidulated water.
White pigments are used as much for making bulk as for imparting colour. Chalk is prepared by washing and drying the powder. Gypsum is used in the form of finest plaster-of-Paris, and the crystalline variety (selenite), powdered and washed, for translucent waxes. Magnesium carbonate is useful for mixing with heavy pigments, to reduce the weight, rather than as a colouring ingredient. Zinc white needs no preparation. Baryta or permanent white is valuable for enamel-like waxes, and may easily be prepared by dissolving barium chloride in rain water and precipitating with sulphuric acid; the precipitate is washed several times with clean water, and dried. Flake white is readily produced as follows: Fuming nitric acid is poured over some bismuth in a glass vessel till the metal is all dissolved. The solution is poured into another vessel containing about 100 times as much rain water, and stirred up. A white powder (nitrate of bismuth) is at once precipitated; this is collected, washed, and dried, and is employed for the best enamel-like white sealing-wax.
It is essential that all the ingredients be dry, and to ensure this they are kept in paper bags on a shelf running round the walls of the stove-room at about 18 in. below the ceiling. The order of adding the ingredients is as follows: The resins and turpentine are first melted together; then the neutral bodies (chalk, &c), if any, are stirred in; next the pigments are added; and the volatile balsams and oils are only introduced at the last moment before "forming." When only one pigment is used, it is simply warmed and stirred into the mass. When a shade is to be produced by a mixture of colours, no neutral bodies are added to the resins, but they are mixed with the colours in a china dish, warmed, and then added to the melted mass. Any required tint is obtained by mixing, and frequent testing.