Having heard of bronze and its extensive usage, especially with sculptures, by both the ordinary person and those most versed in this field, you might think it is an element or metal of itself. Yet, this is the name for a vast spectrum of copper alloys; many of them are not technically ‘bronze’.
‘True-bronze’ is an alloy strictly from the combination of the metallic elements of tin and copper alone. Founders are known to alloy copper with multiple extra metals or other substances that are not tin – if they even add that element in anyways.
Therefore, many casting works produced now and in the past were technically not bronze by the restrictive definition above. The minority of artworks created with solely copper/tin are mainly constructed with wrought sections as fabrications to the exclusion of casting. However, even these alloys may have a tidbit of phosphorus added, and in that case, they are called phosphor bronzes.
an alloy made of copper, zinc, lead, tin or silicon bronze, an alloy made of copper and silicon (occasionally, manganese or zinc) are the two casting materials most used by today’s foundries. The silicon alloy ones contain no significant tin amounts in them, with the higher zinc portion making it lean more metallurgically similar to brass rather than bronze.
Using bronze alloys that are more akin to ‘true brass’ does bear a long and storied historical precedent. For example, in the 10th-11th century, famous Benin bronzes from pre-colonial Benin, West Africa. The state known as Benin at that time had created numerous intricate artifacts richly adorned with imagery with an alloy that had a metallurgical composition that contained as little as 0.5% tin, with comparatively higher proportions of 5.85% lead and 14.34% zinc – the rest being copper. Today’s rough modern equivalent would be leaded brass.
Around the same millennia, 10th-century Northern India produced artworks with the Ashtadhatu, ‘eight metals’, alloy, which had a whopping minimum of seven component elements. Its non-copper building bars were iron, gold, silver, tin, mercury, led, and zinc. It is still used and produced for sculptures, particularly of the high deities of the Hindu pantheon, to this day. Unlike the West African metalworkers, who may have had to create beauty despite scarcity, religious reasons and beliefs were seemingly the primary influence for such an octo-alloy.
Our ancestors did make alloys more akin to ‘true bronze’ as well, one of which is ‘Aes Corinthiacum‘ or the Corinthian alloy of classical Greece. Alongside Aeginetan, Delian, and Rhodean variations, Pliny in his ‘Naturalis Historia’ reports that the alloy was made of copper, tin, and perhaps a minor touch of gold and silver, or maybe none at all. Although the gold and silver drive up the expense, the alloy would be perfectly suitable as a foundry alloy. The artists may have been the two extra elements, purely to make it shinier and have a brilliant and dazzling lustre.
And finally, that the modern foundry does not stick to ‘true bronze’ when making bronze statues has little if any effect on a sculpture’s inherent value, nor the artistic credibility. It simply is using the best available alloys to complete the work at hand. And while gunmetals and silicon bronzes were initially made for engineering purposes, they are more than suited for the modern foundry requirements: offering a balanced trade-off between beauty, quality, and cost.