Urushi is the milky secretion of Rhus vernicifera, and is the material for the well-known Japanese lacquer varnish. The tree is cultivated in many parts of the country, throughout almost all latitudes, e.g., at Dewa, Aizu, Hiroshima, and in many places about Tokio; the best urushi, however, is obtained at Yoshino. The tree is very similar in aspect to the ordinary wax-tree, and attains the height of 9-12 ft. Trees about 15 years old yield the largest amount of the juice. Two sorts of the juice are generally obtained from a tree, and by different processes. They are distinguished as ordinary " ki-urushi" and " seshime-urushi."
Ki-urushi (or raw lacquer) is the better of the two, and is collected best in June by making shallow cuttings in the stem of the tree, when it exudes as drops from between the outer and inner barks. A single tree yields on an average about 2 1/2 grammes of this kind of juice. Branches and twigs of the tree, some of which are usually cut down each year, when steeped in water for some months and afterwards warmed in the fire, give out an inferior kind of juice; this is seshime-urushi, which is used as. under varnish after being mixed with some drying oil.
The juice is never sent to market in the form in which it comes from the tree, but is usually mixed with more or less of what is called "mokuyiki" (literally wood-juice), e.g., what is ordinarily called Yoshino. Urushi consists of 60 per cent, of the genuine juice with 40 per cent, of mokuyiki, while the inferior quality contains as much as 70 per cent, of the latter substance. Further, in the hands of varnish makers, some quantity of linseed oil is generally added to the already mixed juice, which, if excess is avoided, does not much impair the drying power of urushi.
Different colours are imparted to urushi by the addition of body pigments, such as lamp-black, vermilion, indigo, orpiment, etc.; thus red lacquer is prepared with 20 parts of linseed oil, 70 parts of urushi juice, and about 10 parts of vermilion, etc. Such is a rough yet general account of the extraction and preparation of urushi juice for varnish-making. The pure and unaltered urushi is a thick greyish fluid of dextrinous consistence, which under the microscope is found to consist of minute globules, some of darker, the others of lighter colour, mixed with small particles of opaque brownish matter, the whole being held mixed in the form of intimate emulsion. It has a characteristic sweetish odour, and specific gravity 1-0020 (20° C); some specimens, such as that obtained from Hachioji, contained a good deal of bark dust and other impurities, which raise its specific gravity as high as 1.038. If the juice be exposed to moist air in a thin layer at about 20° C, it rapidly darkens in colour and dries up to a lustrous translucent varnish.
It contains a small quantity of volatile poison, which acts terribly on some persons, producing a very disagreeable itching.
A peculiar acid, which I now call urushic acid, is the main constituent of the original juice, as well as of the portion soluble in alcohol. The juice also contains a very small quantity of a volatile poisonous body, which also passes into alcoholic solution, being almost completely driven out during the drying of the acid at 105°-110° C. It is a pasty substance of somewhat dark colour, having the characteristic smell of the original juice, readily soluble in benzene, ether, carbon bisulphide, less easily in fusel oil and petroleum of high-boiling point, completely insoluble in water. Its specific gravity taken at 23° C. is 0-9851; it remains unchanged at 160° C, and above 200° C. decomposes slowly with carbonisation. Exposed to the air, it neither dries up, nor shows any sign of change as the original juice does, and in other respects it is a very stable body. From the alcoholic solution of the acid many metallic salts can be produced, most of which are slightly soluble in alcohol, but almost insoluble in water.
Gum is another normal constituent of urushi, and forms 3-8 per cent, of the original juice.
As gum is insoluble in alcohol, it is conveniently separated by treating that portion, of the original juice insoluble in alcohol with boiling water, filtering, and finally evaporating the aqueous solution of gum over the water-bath till the weight of the substance remains constant. In this way a friable light-coloured substance is obtained, tasteless and inodorous; this is the anhydrous gum.
A mixture of gum and urushic acid (and with water) in the proportion in which they exist in the juice, does not undergo any change whatever, even when exposed to the condition most favourable for the drying of the lacquer. Moreover, part of the gum can be extracted in an unchanged state from the once perfectly dried lacquer; and snce it exists in the original juice in the form of aqueous solution, it probably serves to keep the constituents of the juice in a state of uniform distribution and intimate emulsion. It may also act as a binding material, and assist the adhering power of the lacquer when laid upon any surface.
The results, so far arrived at, may be summed up in the following statement: -
Urushi juice (lacquer) consists essent tially of four substances, viz., urushic acid, gum, water, and a peculiar dias-tatic matter; and the phenomenon of its drying is due to the oxidation of urushic acid, C14H1802, into oxyurushic acid, C14H1803 which takes place by the aid of diastase in the presence of oxygen and moisture. (H. Yoshida.)
This is simply the process of laying a coating of varnish, and afterwards drying by artificial heat. This second operation, the baking, is the essential part in japanning. The art was originated in Japan, whence we have derived the name. Many examples of japanning on papier-mache may be seen at fancy repositories where various ornamental nicknacks imported from Japan are on sale. In making this ware the Japanese employ a lacquer which exudes from an. indigenous tree. Successive coats are laid on, each one being thoroughly dried in the sun before the application of another. Thus a thick hard coating is made, which may be smoothed and polished by abrasive materials, though the natural lustre suffices for general requirements. Gilding and other ornamentation is then made to adhere by means of boiled oil. The whole is finally finished by a coat of clear varnish. The above is a rough sketch of the art as practised by the originators, but we have to deal with modern japanning, and confine our observations to its application to metal.