< PROPERTIES OF METALS
WHY YOUR SCULPTURE CAST IS PROBABLY NOT BRONZE
BRONZE - the material most closely associated with sculpture casting, is a word used by the layman and professional alike to describe what is really a wide range of copper based alloys – most of which are not strictly speaking bronzes.
True bronzes are traditionally products derived from the alloying of copper and tin alone. Art founders tend to use alloys which either have additions to the copper base over and above that of tin, or alloys that contain no significant tin content whatsoever. Strictly speaking therefore, few of the cast artworks produced in contemporary (or historical), art foundries could be correctly described as a ' bronzes’ by the above definition. The few art and design works that are produced in a copper/tin material are almost exclusively constructed from wrought stock sections (sheet, bar, angles etc) as fabrications, even this group of alloys usually has a small quantity of phosphorous added (see PHOSPHOR BRONZES).
The majority of sculptures produced in today’s art foundries are cast either in a LEADED GUNMETAL (copper/tin/zinc/lead alloy), or else in a SILICON BRONZE (copper/silicon [plus manganese or zinc]).The silicon bearing alloys contain no significant levels of tin at all, with high zinc content casting grades metallurgically closer to brasses than bronzes.
The use of an art casting alloy which is not a ‘true bronze’ does in fact have a long historical precedent. An excellent example of this can be found in the various castings of pre-colonial Benin – the so called BENIN BRONZES, produced in the West African state of that name during the 10th and 11th centuries. The Benin Bronzes, rich both in imagery and detail, are considered to be amongst the finest and most admired examples of the founder’s art. Research undertaken by the British Museum in 1899 into the metallurgical composition of selected casts from the region revealed that metal samples taken from Benin artifacts contained as little as 0.5% tin, with relatively high proportions of added lead (5.85%), and zinc (14.34%). This combination of metallic elements produces an alloy which would probably be best described today as a LEADED BRASS [ref 1 ].
Contemporary to, if otherwise unrelated to the Benin ‘Bronzes’, are the 10th century works produced throughout Northern India in the alloy known as ASTADHATU. This material was composed of at least seven alloying elements over and above the copper base; including gold, silver, iron, tin, lead, mercury and zinc [ref 2]. Unlike the brass type alloy used by the West African metalsmiths (which itself probably owed more to the limited range of available materials than considered alloying), cosmological beliefs probably influenced the choice of alloying elements in the Astadhatu alloy. On the evedence of the casts produced during this period, both the Indian and African alloys were clearly more that adequate for founding the sculptures and design works of the time.
One historically documented alloy that could be more correctly called a ‘bronze’, is the Corinthian alloy (AES CORINTHIACUM) of classical Greece. Noted in Pliny's ‘Historica Naturalis’ (along with the RHODEAN, DELIAN & AEGINETAN variations), this particular alloy was composed of copper and tin, possibly with minor additions of gold and silver. Itself the subject of legend, having been allegedly (if improbably), formed from the running together of metals during the burning of Corinth (146 BC), CORINTHIAN BRONZE would have made a perfectly serviceable, if potentially expensive foundry alloy - the gold and silver additions were probably intended to enhance the cast material’s ability to take on a high mirror polish and therefore an exceptional lustre [ref 3].
That ‘tradional’ bronzes are little used in modern art foundries diminishes neither a sculpture’s intrinsic value, nor it’s artistic merit. The founder is simply using the best (or at least, best available) alloy for the job in hand. Gunmetals and silicon bronzes, whilst originally developed for engineering applications, are both well suited to the overall demands of modern art foundry practice, offering a median between quality, performance and cost. Sadly, modern versions of Corinthian and other art specific copper alloys are no longer generally available – the expense involved in developing and producing significant quantities of these highly specialised alloys is a luxury beyond the average art founder and sculptor alike.
PRODUCTION METALS & ALLOYS >
|
|
