(a) When finished wood, papier - mache, composition, or metals are varnished in the usual manner and left to dry in the air, the drying is in most cases imperfect, and the coating more or less uneven. If the surface thus varnished is heated for some time to a temperature of 250° to 300° F. (121° to 149° C), or higher, it is found that the whole of the solvent or vehicle of the gums or resins in the varnish is soon driven off, and the gummy residue becomes liquefied or semi - liquefied, in which state it adapts itself to all inequalities, and if the coating is thick enough presents a uniform glossy surface, which it retains on cooling. This process of drying out and fusion secures a firm contact and adhesion of the gums or resins to the surface of the substance varnished, and greatly increases the density of the coating, which enables it to resist wear and retain its gloss longer. This process of hardening and finishing varnished or lacquered work by the aid of heat constitutes the chief feature of the japan - ner's art. In practice the work to be japanned is first thoroughly cleansed and dried.
If of wood, composition, or other porous material, it is given while warm several coats of wood - filler, or whiting mixed up with a rather thin glue size, and is, when this is hardened, rubbed down smooth with lump pumice. It is then ready for the japan grounds. Metals as a rule require no special preparation, receiving the grounds directly on the clean dry surface. In japanning, wood and similar substances require a much lower degree of heat and usually a longer exposure in the oven than metals, and again a higher temperature may be advantageously employed where the japan is dark than when light - coloured grounds are used: so that a definite knowledge of just how much heat can be safely applied and how long an exposure is required with different substances and different grounds can only be acquired by practical experience. The japanner's oven is usually a room or large box constructed of sheet metal, and heated by stove drums or flues, so that the temperature - which is indicated by a thermometer or pyrometer hung up inside, or With its stem passing through the side - wall midway between the top and bottom of the chamber - can be readily regulated by dampers.
The ovens are also provided with a chimney to carry off the vapours derived from the drying varnish, a small door through which the work can be entered and removed, and wire shelves and hooks for its support in the chamber. The ovens must be kept perfectly free from dust, smoke, and moisture.
Rosin (pale).. ..
Rectified spirit ..
Two or three coats of this are put on the work in a warm dry room. A good black ground is prepared by grinding fine ivory - black with a sufficient quantity of alcoholic shellac varnish on a stone slab with a muller until a perfectly smooth black Tarnish is obtained. If other colours are required, the clear varnish is mixed and ground with the proper quantity of suitable pigments in a similar manner: for red-vermilion or Indian red; green-chrome - green, or prussian blue and chrome-yellow; blue - prussian blue, ultramarine, or indigo; yellow - chrome - yellow, etc But black is the hue commonly required. The following are good common black grounds: -
Balsam of capivi
Oil of turpentine
The asphaltum is melted over a fire, and the balsam, previously heated, is mixed in with it. The mixture is then removed from the fire and mixed with the turpentine.
(2) Moisten good lampblack with oil of turpentine, and grind it very fine with a muller on a stone plate. Then add a sufficient quantity of ordinary copal varnish" and rub well together.
Boiled oil ..................
Oil of turpentine
Melt the asphaltum, stir in the oil, previously heated, then the umber, and when cooling thin down with the oil of turpentine.
(4) An extra fine black is prepared from -
Boiled oil .......
Oil of turpentine
Fuse the gum, rosin, and asphaltum, add the hot oil, stir well together, and when cooling add the turpentine.
From 1 to 6 or more coats of varnish are applied to the work in japanning) each coat being hardened in the oven before the next is put on. The last coat in coloured work is usually of clear varnish without colouring matters, and is in fine work sometimes finished with rotten - stone and chamois - leather. For ordinary work, the gloss developed in the oven under favourable conditions is sufficient. (Scient. Amer.)
(6) Melt 50 lb. Naples asphaltum and 8 lb. dark gum anime, boil for about 2 hours in 12 gal linseed - oil; then melt 12 lb. dark gum - amber and boil it with 2 gal. linseed - oil; add this to the other, and add dryers. Boil for about 2 hours, or until the mass when cooled may be rolled into little pellets. Withdraw the heat, and thin down with 30 gal. turpentine. During the boiling the mass must be constantly stirred to prevent boiling over.
The following are the proportions for black japan; asphaltum, 1,1/2 oz.; boiled linseed - oil, 4 pints; burnt umber, 4 oz. Heat till well mixed, and when cool add turpentine till of a proper consistence.
It is no easy matter for an amateur to japan a tea - tray. In the first place, if the tray is scratched or chipped, burn off the japan; if it is not scratched, cut it down with pumice and water (ground pumice) until it is smooth. Then get some brown japan (Pontypool varnish) - say, 2 gills - and with a camel - hair brush lay it over evenly. See that no hairs get on; if they do, pick off with a knife. Then put in a place free from dust to dry, which will take about 4 days, unless you have an oven. A common oven will do; but take care of dust. When you cannot leave any marks of your fingers, it is hard and ready for rubbing with pumice again, and adding another coat; 3 coats altogether will do. Then pumice, and finish with rotten - stone and water, or putty - powder. Be careful to rub all one way, not across nor roundabout. Then take and rub quickly with the heel of the hand to burnish it. To ornament the same: - If with transfer pictures, varnish the part neatly and very thinly (as it leaves a mark when too much is put on) with copal varnish. Then let it stand for 1/2 hour until it is set or nearly dry. Put on the transfer, press it with the hand take sponge and hot water and dab it lightly; in 3 minutes slide the paper off.