CASTING AND FINISHING The discovery of copper alloys is lost in the prehistoric ages. Their name was Chalcos with the Greeks and Aes with the Latins. Aes Brundusinum was the name of the alloy used in manufacturing famous Roman mirrors, and possibly the origin of the word "bronze" came from Brindisi (Brundusium), a city on the Adriatic coast of Italy where bronze was manufactured on a large scale. The origin of art bronzes or sculpture in bronze had its birth in the most remote days of human history, but after the invasion of the Roman Empire by the Barbarians, there was a long period of obscurity. It was in Italy during the Renaissance period that the regeneration of the bronzes of art took place, especially by the efforts of Benvenuto Cellini.
Bronzes of art are cast by two different methods: (I) the sand process, (2) the cire perdue, or lost wax process. Judging from the early specimens of bronze castings, undoubtedly the cire perdue method, or something like it, was used long before sand casting.
The moulder gently hammers the damp sand against the plaster pattern, taking care of the undercuts of the model or deep re cesses, by making as many as necessary small pieces of sand in such a way as to be able to release them from the original model by taking them apart without injury either to them or to the model.
After the model is released, the packed sand mould, which bears the impression of the most minute details of the original model, is recomposed and a proper sand core is built inside of it. This core is so cut that it leaves a space between itself and the piece moulds above described.
After this operation the dampness of the sand mould must be carefully eliminated by enclosing it in an oven properly built. When the mould is completely dried, the liquid bronze at about 1,900 degrees Fahrenheit is poured into the iron flasks previously recomposed and clamped together, and it will run through chan nels skilfully cut in the sand, going to fill the empty spaces men tioned above.
The sand is then removed, and after the bronze is cleaned with nitric acid, it will be finished and chiselled by skilful artisans.
At this stage, we will have a perfect replica of the sculpture in wax, and sufficiently hard to permit handling. The artist can
work on it as much as he pleases, obtaining rare results of de tails, which makes this process of casting invaluable. Gates and vents in the shape of wax rods are then properly attached to the wax figure.
Finally, the mould for the metal is formed by blowing or pour ing inside and around the wax a semi-liquid composition, which hardens in a few minutes. This composition of silica, plaster and other chemicals can resist high temperature, and, of course, all the wax inside of it will melt away, leaving a hollow space. This operation is accomplished in large ovens, by baking the moulds over a slow fire. As soon as all the wax is surely melted away, the mould is removed from the oven and packed in foundry earth in a pit provided in the floor. The bronze is then poured from crucibles, and the molten metal will run through the gates (melted away) and fill the space left empty by the wax figure also melted away (lost wax). The figure in bronze is then removed from the silica mould and dipped in acid for a proper cleaning.
With this process, the cast bronzes require very little finishing or chiselling, and the results are far above the sand process.
The so-called "patina" (q.v.) of the finished bronzes is an art in itself, and the different effects of colour are obtained by a large use of different chemicals.
Bronze is an alloy of from 85 to 90% of copper, and from i o to 15% of tin, zinc and other non-ferrous metals. The alloy called United States Standard Bronze is composed of 90% cop per, 7% tin and 3% zinc. This formula is not by any means officially approved by the United States Government, and this name was given by some bronze foundrymen only a few years ago for their own advantage, and strange to say it became an official word. Almost every specification generally written for contracts of art bronzes mentions the United States Standard Bronze as stipulated.
There are hundreds of other formulas of bronze, many of which contain other metals, such as lead, silver, aluminum, etc., which should not be left apart from the specifications of art bronze.
Some formulas of famous art bronzes will show the relative compositions: Some Greek bronzes have: copper 62%, tin 32%, lead 6%. Others have: copper 72%, tin 24%, lead 4.6%, zinc 2%. The famous column VenclOme in Paris has : copper 89%, tin o% (with traces of lead, tin and silver).
The statue of Louis XIV. in Paris has: copper 91%, tin 2%, zinc 6%, lead 1%.
Statue of Moliere also in Paris has: copper 90%, tin 6%, zinc 2.5%, lead 1.5%.
The statue of Frederick the Great in Berlin has: copper 90%, zinc o%.
There should not be any cause of alarm in judging the bronzes of art from the point of view of durability, as there are millions of specimens all over the world in an admirable condition of preservation composed of every conceivable proportion of alloy.
Not long ago some bronzes were discovered belonging to an age precedent to the Incas, with an alloy of 94% copper and 6% tin. (R. BE.)