Difference between revisions of "Bronze Working"
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After iron, bronze was probably the commonest metal used by the Anglo-Saxons and Vikings. Bronze is a mixture of copper and a small percentage of tin (and sometimes a small amount of lead). Any alloy, or mixture, of copper and tin is called bronze. Many bronze alloys also contain small amounts of other materials.
The simplest bronze is copper mixed with small amounts of tin. Tin increases hardness, making bronze more resistant to wear than is copper. Bronzes with 10 percent or more tin are harder, stronger, and more resistant to corrosion than brass, which is a copper and zinc alloy. Brass was also used in the period, and is often confused with Bronze and vice versa if no actual analysis is done on the 'Copper alloy' as it is called in most reports of finds.
Leaded bronze has lead, usually small amounts, mixed in to act as a lubricant. Bell bronze is another variation and is very hard and provides a special tone to bells that no other alloy can match.
As a material it was used for making a wide variety of objects but was especially common for jewellery such as brooches, buckles, belt ends, dress pins and rings.
Making bronze items was a difficult and complex craft carried out by specialists at the sites where the ore was dug. Once the copper ore was dug out of the ground the copper had to be separated from the waste material. This was done by smelting the ore in a furnace with sand and charcoal. When the temperature inside the furnace reached about 1100°C (by pumping with hand bellows) the copper melted and flowed to the bottom where it was drawn out and cast into ingots.
When the craftsman had his copper ingots there were several ways he could make the finished casting.
Sometimes, if he wanted to produce a lot of similar items he would make a model of the item in wood or lead alloy and make a clay mould from this, or make an antler mould by carving directly into the antler. From these moulds he could then cast waxes to use as the masters for the bronze casting.
If he wanted to make a one off casting he made a model of the object he wanted to cast out of wood or beeswax. If he used wood he would press the wood into clay to make the shape he wanted. Once the clay had been fired and the wood had burned away he could use it as a mould. If he was using wax he would wrap the wax model in clay (leaving a spout through which he could later pour the molten metal) and dry the clay by firing or leaving it somewhere warm and dry. This heating would melt the wax and allow it to be poured off or voided, leaving a hollow mould.
Having made the mould, the smith took enough copper to make the object and melted it in a clay crucible. To turn it to bronze he added about 10% tin (and as mentioned above occasionally some lead, to make the molten metal flow better) to the molten copper. He then poured this into the mould to achieve the object he wanted. If there was still any wax in the mould the hot bronze would evaporate it off. When the bronze had cooled the mould was broken open and the cast object was taken out. If the object had not cast properly it could be re-melted and used for a later casting. If it was judged to be good it was cleaned up, polished, and used.
Moulds could also be made by carving out of stone, usually soapstone or slate and occasionally old Roman tiles. Some of these stone moulds were quite detailed, often in two halves, others were much cruder one part moulds. These one piece stone moulds were often used for ingot moulds.
Sometimes items were cast as blanks, usually in a clay or stone mould, although an iron mould has been found in York. These blanks would then be cleaned up and be decorated by engraving or punching.
Objects were also made out of bronze wire or by cutting sheets of bronze to the right shape and stamping designs into the surface with iron tools.
Bronze was even used to cover iron objects. This was done by coating the object with tallow and then applying bronze foil over this. If the item was hollow it was then filled with charcoal. The object was then covered in clay and placed in a fire. Bellows were used to raise the temperature of the fire so that the bronze melted and coated the surface of the object. This made items more decorative and prevented them from rusting.
Fine bronze (and sometimes gold and silver) sheet were sometimes embossed with a bronze die. These sheets could then be attached to other items for decoration. The best known examples of these are probably the helmet plates from Sutton Hoo in Suffolk and Valsgarde in Sweden. These sheets were sometimes tinned or silvered and were also often used to support elaborate filigree work. Bronze wire was also used to decorate iron objects by cutting a channel in the iron with an engraving tool and then hammering the wire into the channel creating fine contrasting lines in the work.
Original article by Ben Levick, 1993
Revised by Roland Williamson, 2000