French polishing. The finest of shellac finishes is French polish. It is a thin, clear, permanent finish, but the process takes time and patience. It is not much used in practical work, because of the time expense, but is often employed in school shops, because only a few materials are necessary, it dries quickly, and gives a beautiful finish. The polished surface is obtained by adding successive thin coats according to the following process:
(1) Preparation. The surface of the wood must be perfectly smooth and even, sandpapered in the direction of the grain, stained, if desired, filled, rubbed smooth and quite dry. (2) Apply two or three thin coats of shellac. After each coat when dry, rub with No. 00 oiled sandpaper or No. 00 steel wool. Wipe thoroly. (3) Make three pads, about the size of a walnut, of clean, white, cotton waste, enclosed in some fine old or washed cloth with no sizing or lint,—one pad for shellac, one for oil, and one for alcohol. Fill one pad with shellac of the consistency of milk, enough in the pad so that when squeezed hard it will ooze out. The common mistake is to put too much shellac into the pad. Rub with circular motion, as indicated in Fig. 304, never letting the pad stop on the surface. (4) Sprinkle a very little finely powdered pumicestone and put a little oil on the surface of the wood here and there with the tip of a finger. Rub with second pad until surface is dull. Wipe clean. Repeat (3) and (4) several times. Some use raw linseed oil to prevent sticking. Others use three or four cloth coverings on the shellac pad, removing the outer one as it dries. A simpler way is to keep the shellac in pad, 1, thin by moistening with a little alcohol. (5) Spiriting off (Follows process 4.) Dampen pad, 3, with very little alcohol and wipe quickly in the direction of the grain. This should remove the circular marks. Too much alcohol in this third pad will "burn" a dull spot. The rubbers are said to improve with use, and may be preserved in closely stoppered jars to prevent evaporation. The different kinds of pads should be kept separate. Or the cotton waste may be thrown away, and the cloths washed in strong borax water. In the process just described, shellac alone, dissolved in alcohol, is used. The shellac may be used with other ingredients: for example, 1 pint grain alcohol, ¼ oz. gum copal, ¼ oz. gum arabic, 1 oz. shellac. Strain through muslin.
Fig. 304. Direction of the Pad in French Polishing.
Another recipe for finishing. Use 4 drams grain alcohol, 2 drams orange shellac, 5 drams tincture of benzoin, 1 teaspoonful of olive oil. Dissolve and strain. Apply with pad in direction of grain.
Oil or Copal Varnishes. The old Cremona varnish once used for violins is supposed to have had amber (Greek, electron) as its base. It was a fossilized coniferous resin found on the shore of the Baltic Sea. The art of making it is said to be lost, probably because of the difficulty and danger of melting it, for this can be done only in oil on account of the danger of ignition. Hence its use has been abandoned.
Perhaps the most beautiful of all varnishes is lacquer, much used in China and Japan. It is made from the juice of the lacquer tree, (Rhus vernicifera) which is tapped during the summer months. The juice is strained and evaporated and then mixed with various substances, such as oil, fine clay, body pigment, and metallic dust, according to the ware for which it is intended. The manufacturing secrets are carefully guarded. The application of it is very difficult, the sap of young trees being used for first coats, and of old trees for the finishing coats. It must be dried in a damp, close atmosphere. For the best work ten or twelve coats are elaborately rubbed down and polished. Even the presence of it is very poisonous to some people and all workers in it are more or less affected.
The solvent or vehicle of the modern copal varnishes consists principally of linseed oil with some turpentine. Their base is Copal, a fossil, resinous substance of vegetable origin. The gums of which they are made have been chemically altered by long exposure in the earth. Other gums, as mastic, dammar, sandarac, and even resin are sometimes mixed with copal to cheapen the product or to cause more rapid drying. Copal is a generic name given originally to all fossil resins. Copals, as they are called, come from New Zealand, Mozambique, Zanzibar, West Africa, Brazil, and the Philippines. The best of the Copals is said to be the Kauri gum, originally exuded from the Kauri pine tree of New Zealand. The tree is still existent and produces a soft, spongy sap, but the resin used in varnish is dug up from a few feet under ground in regions where there are now no trees. A commercially important copal and one noted for its hardness is the Zanzibar or East African Copal. It is found imbedded in the earth at a depth not greater than four feet over a wide belt of the mainland coast of Zanzibar, on tracts where not a single tree now grows. It occurs in lumps from the size of small pebbles to pieces weighing four or five pounds. The supply is said to be practically inexhaustible.
As to the manufacture of the Copal varnishes: first of all, a high grade oil is boiled at a high temperature, with different materials to oxidize it; for instance, red lead or oxide of manganese. The heat throws off the oxygen from the red lead or manganese. The oxygen is absorbed by the linseed oil, which is then put away to settle and age. When a batch of varnish is made, the gums are melted in a large kettle and then the requisite amount of oil is added and these carefully boiled together. This is removed from the fire and cooled down to a point, where turpentine can be added without volatilizing. These are thoroly mixed and then filtered under pressure and tanked and aged. The different grades of varnish depend upon the treatment of the oil, the proportion of oil and turpentine, the qualities of the gums, the aging, etc. Some by rubbing give a very high polish, some give a dull waxy finish, some are for out-of-door use, as Spar varnish and carriage varnish, some are for floors, some for furniture, some are high priced, some are cheap.
Process of Varnishing. The preliminary processes are the same as those for applying shellac, i. e., the surface of the wood must be perfectly even and smooth, and the staining, filling, and drying complete. Quick drying varnishes, like shellac, are applied, with but little on the brush. The heavy, high lustre varnishes, on the other hand, are applied with the brush full so that the varnish may even drip off the work. Then proceed as follows: Wipe off from the work the extra varnish with the brush and clean the brush on the edge of the cup. Repeat till the varnish is flowed over the work evenly. Be particularly careful, in that respect, of edges and corners. Set to dry in a dustless place. When dry and hard repeat the process from three to six times. Each coat must dry thoroly before the next coat is applied.