This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The next process it that of bronzing. The colour known as "bronze" is that which a piece of that metal would take through the natural process of atmospheric oxidation, if it were exposed to a dry atmosphere at an even temperature. But the manufacturer, not being able to wait for the slow action of nature, calls chemistry to his aid, and by different processes produces on the surface of the piece a metallic oxide of copper, which, according to taste or fashion, varies from black to' red, which are the 2 extreme colours of copper oxide. The discovery of old bronzes, buried for centuries in damp earth, and covered with verdigris, suggested the colour known as vert antique, which is easily produced on new metal by the action of acetic or sulphuric acid. In the 15th century, the Florentine artisans produced a beautiful colour on their bronzes by smoking them over a fire of greasy rags and straw. This colour, which is very like that of mahogany, is still known as Florentine or smoked bronze. Bronze can also be plated with gold and silver, nickel and platinum, like every other metal.
On this subject, Gornaud says that the manufacturer of art bronzes begins by giving the style and general proportions to the artist, who is his first and most important assistant. The artist takes the clay, the model, the style, and arranges it into its varied forms; soon the architecture is designed, the figures become detached, the ornaments harmonize, and the idea embodied in the outline becomes clear. The manufacturer, before giving his model to the founder, should indicate with a pencil the parts which ought to be thickest, lest some be found too light, without, however, altering the form • he should also mark the parts to be cut in the mould to facilitate putting together. Care must be taken to rub with hard modelling wax all the projecting parts which serve to join the pieces, so that the turner may not want matter. He must carefully verify all the pieces separately, and cover with wax the angles and ends of the leaves - in a word the weak parts. Generally the model is cast in half-red bronze, in the following proportions (the body of it is harder, and less easy to work) : -
Copper ...... .. .. 91-60 per cent.
Zinc............ 5.33 "
Tin.......... .. 1.70 "
Lead............ 1.37 "
Objects destined to be gilded require a little more zinc than those of plain bronze. The models just described serve to make the moulds in moulding sand, the moulds being afterwards baked in a stove heated to 572° F. (300° C). They are fastened horizontally with binding screws, in order to run in the bronze; the temperature, when cast, varies from 2732° to 3272° F. (1500° to 1800° C).
The Japanese word corresponding to the English "bronze" is karakane, which means "Chinese metal"; whereas the brass alloys are called shin-chu. The spelter used for the latter is imported. The industry of bronze-casting is of very ancient origin; at first foreign metal, imported either from China or Corea, must have been used, as Japanese copper has only been produced since the beginning of the 8th century; by that time, however, the industry of bronze-casting had already reached a certain state of perfection. This is shown by the fact that the priest Giyoki, who lived about this time, proposed the erection of a monster bronze statue of Buddha, which was carried into effect. There were formerly 3 of these statues in Japan, each about 50 ft. in height. Other specimens of large bronze-castings are the famous bells of Nara, Kiyoto, Nikko, Shiba in Tokio,and others, which have an average height of 15 ft. and are more than 10 ft. in diameter. Statues of all sizes, bells, vases, water-basins, candlesticks, incense-burners, lanterns, etc., have been manufactured in large quantities for temples and their approaches.
Portrait-statues, like the monuments erected in foreign countries to honour the memory of celebrated men, have never been made in Japan. As articles for household uses, may be mentioned fire-pots, water-pots, flower-vases and basins in which miniature gardens are made, perfume-burners, pencil-cases, small water-pots of fanciful shapes for writing-boxes, paper-weights, and small figures representing divinities. These bronze-casting3 are either made in the simple and severe style of the old celebrated Chinese bronzes, or else are specimens of the peculiar character of Japanese art, which chooses it subjects from natural life, either combining them with lively scenes showing a great deal of humour, together with the most minute copying of nature, or else using them to produce some artistical effect. The bronze is cast in clay moulds formed upon models made of a mixture of wax and resin, which is melted out from the finished mould previous to pouring the metal in. The artist who makes the model generally does the casting himself, and in most cases the workshops consist only of the master's family and 2 or 3 assistants. The melting furnaces are of exceedingly small dimensions, and generally made of an iron kettle lined with clay.
After casting, the pattern is carefully corrected and worked out by chiselling, but the best bronze-casters prepare the model, the mould, and the alloy in such a way as to produce castings which need no further correcting or finishing. In some cases also the whole pattern is produced merely with the chisel working upon a smooth surface; this, for instance, is frequently done in the provinces of Kaga and Yechiu, which are very important centres of the bronze industry. The bronzing of the pieces is done in many different ways, each manufacturer having his own particular process, which he modifies according to the composition of the alloy and the colour he wishes to produce. The chemicals used for this purpose are very few in number, and limited to vinegar, copper sulphate, and verdigris as the principal substances; other materials, used less frequently, consist of iron sulphate, red oxide of iron, and lacquer. It may be added, as a peculiarity, that an infusion of Eryanthus tinctorius is also made use of in the bronzing process.
The ornamentation of bronze castings is not only produced by relief patterns moulded or chiselled, but also by inlaying the objects with gold, silver, or with a different alloy. This kind of workmanship is called zogan, and is principally carried on in the provinces of Kaga and Yechiu. The process by which the inlaid work is effected differs according to the nature of the material on which it is produced. Sometimes the design is hollowed out to a certain depth with a graver or chisel, and the ornamenting metal, silver, gold, etc., generally in the shape of threads, is laid into the hollow spaces and hammered over, should the alloy be soft enough; the edges of these grooves are first slightly driven up, so that when the silver or gold has been laid in, they can be easily hammered down again, so as to prevent the inlaid metal from getting loose. Or else the surface is merely covered in the required places with a narrow network of lines by means of filing, and the thin gold or silver leaf fastened on to this rough surface by hammering.
This last process is the one used mostly for inlaid iron-work. It is also said that the design is often produced by a process very similar to that of the so-called niello; only instead of the black sulphuretted silver and copper, a more easily fusible alloy is used. Inlaid work of the above kind is principally made in Kaga and Yechiu, at Kanasawa and Takaoko, where the alloy used for the bronze-casting is mostly composed of copper, tin, zinc, and lead. In addition to the castings, the repousse' work should be mentioned, by which mostly small metallic ornaments for swords, tobacco-pouches, etc, and also larger pieces, such as tea-pots, scent-burners, vases, etc, are produced; the inlaying of this kind of ware is sometimes of extraordinary delicacy and beauty. The dark-blue colour shown by a great number of smaller pieces is that of the shakudo, composed of copper, and 3 and 4 per cent. of gold. Finally, attention should be called to the so-called moku-me, a word which might be rendered by " veins of the wood." The metal-work designated by this name presents a sort of damask pattern composed of variously-coloured metals, chiefly white silver, red copper, and a dark-blue alloy.
Pieces of this very difficult sort of workmanship are produced by overlaying and soldering together a certain number of plates of the said metals or alloys, by hammering, kneading, resoldering, filling up the hollow spaces with new metal, and repeating these operations many times; finally, when stretched out into a thin sheet, this composition shows the aforesaid pattern all composed of veins of the different metals that have been made use of.