Summary
This article looks at tools found in a workshop that was active around 2500 BC in Sumer, in the same area as the modern suburbs of Baghdad. This article, written by Christopher Davey, was an extract from the Bulletin of the Institute of Archaeology, University of London, 20, 1983. This article has some excellent images, including extant pieces and ancient images of the smithing process. It was found at www.aiarch.org.au on November 17th, 2012.
Metal workshop
The workshop in question belonged to a copper smith who also worked in alloys i.e. bronze. This is evident from the traces of copper, tin, iron and nickel that were found inside a used crucible. Several crucibles, moulds and bellows were found, constructed from a variety of clays.
Crucibles
The crucibles found at the Tell edh-Dhiba'i workshop were shaped like a curved funnel with a large opening at the top and a tear drop shaped opening on the side of the bottom, unlike the 'open crucibles of the Levant, Anatolia and Iran (Tylecote, 1976: 20)'. This shows that, including the open crucible common to the region and similar to the open crucible used at Birka in the 9th and 10th centuries, the curved crucible used in ancient Egypt and Mesopotamia, the lidded Migration Era crucible, and the bowl form with a hole in the side found in Keos, Byblos and Sinai (Dunand: 1958; Petrie: 1906), there were numerous functional designs for crucibles that have been used through out history and around the world.
The author describes the fabrication of the crucibles thus:
'The fabric of the crucibles was fairly coarse containing silica and having holes left by straw which was consumed during firing. It was estimated that the vessels were fired to around 800 degrees Celcius and the exterior of the unused crucible seems to indicate that towards the end of the firing they were subjected to oxidising conditions
These crucibles were 'subjected to repeated applications of high temperatures' (p. 6), leaving the portion that was closest to the heat 'white and friable.'
"A total of five crucibles and one fragment of a crucible were found at Tell edh Dhiba'i. Their condition varies from good in the case of the unused crucible 8 to very fragile for those at the end of their operational life.' (p.2)
This suggests that is was normal to use crucibles more than once before they surcame to structural deterioration due to heat exposure, but also that crucibles were not used for so long that it was unusual to have back ups ready to go in the workshop.
Another factor that indicates that crucibles were reasonably disposable was the use of a mould to create their shape. If a crucible lasted for a year, it may be unusual for a workshop to have a specific mould on hand to create new crucibles. The presence of a moulds and indication of mould use in shaping indicate that crucibles were a high-turnover smithing tool.
'The crucibles are very similar size and shape and in fact crucibles 7 and 4 are almost identical as are crucible 8 and fragment 5. The remaining two crucibles were slightly larger than the others. The uniformity would suggest that the crucibles were made by hand with the aid of a concave mould which formed the basic curve.' (p.2)
This also indicates that crucibles were made on site, and a crucible mould could be considered a plausible part of a metal smith's tool kit.
Moulds
Also amongst the finds were some open moulds used for casting ingots (either bar or ring, as was popular a the time). After taking cultural and time differences into account, a form of a mould for recasting metal for trade may be considered a plausible tool for a metal smith. The site at Tell edh-Dhiba-i is distinct because it belonged to a smith who 'did not use stone moulds common to the Levant and Northern Mesopotamia (Davey 1977).'
Another kind of mould may be of even more relevence to building the concept of a metal smith's shop in Birka.
'Object 19 was identified as a cire perdue mould... The mould was made from two distinctly different types of clay; the inner cylinder surrounding the pin of fine yellow clay and an outer shell of coarser buff-coloured clay. The bottom of the mould has been broken although it appears not to the extent neccesary to extract the pin. The mould was therefore unused. The unbroken end of the mould is funnel-shaped, indicating that it was placed in or by the furnace in an upright position with the funnel end uppermost.' (p.10)
The article says that cire perdue has been used since atleast 4000BC (de Jesus, 1980; Hunt 1980; Lucas, p.10). The moulds appear to be rare finds prior to Roman times, though the author states the method was widely used in a variety of cultures including Roman, Indian, Sumerian, Bablyonian and Egyptian.
Cire perdue method
'A fine slurry was initially applied to a wax model and after it had dried a second coat of fine clay was added. Finally, a third layer of coarser clay was added to strengthen and protect the mould' (p.10).
The author states that this method has been used in India since ancient times right through to today.
Sand casting
Among the finds was a clay model of an axe head, used to impress a shape into sand or clay, with molten metal being cast into the cavity. The model has a hole for a rod to aid removal from the moulding material, set at a 45 degree angle.
'It is generally believed that sand casting commonly used in foundries today was a technique developed in the 18th century AD (Tylecote 1962: 110). Theophilus, however, described the use of clay as a casting medium in the 11 th century AD (Hendry 1847: 313, Agricola, 1950: 405, 536).
The Biblical description of the manufacture of the bronze objects for the temple of Soloman states that it was executed by a metalworker from Tyre who cast them 'in the compactness of the soil' (1 Kings 7: 46). This is generally translated as 'in the clay ground' (Revised version) although the Jerusalum Bible actually refers to 'sand casting'. The Bedouin today...have a tradition of using loose earth for casting (Weir, 1976: 67) and there is no reason why metal workers during the Old Babylonian period could not also manufacture a waterless, sand-clay mixture into suitable material for casting.' (p.12)
Furnace
The article suggest that the furnace would have been 'sunk into the ground so as to provide maximum insulation' (p.13) and also suggests that multiple crucibles may have been heated at once (p.14) - the Egyptian Old Kingdom symbol for copper is very similar to two curved crucible sitting back to back (p.7). The reliefs found in the Egyptian tomb of Mereruka, Saqqara from the Fifth Dynasty and in the Mastaba of Wep-em-nefert at Giza show:
...'that melting is performed in two crucibles placed back to back and with six men using blowpipes aimed at the charcoal placed in front of each crucible. After the metal was melted, the crucible was taken to the mould, held with the assistance of two stones or lumps of clay and then a blockage in the mouth of the crucible was removed to allow the metal to discharge into the mould.' (p.6)
These furnaces are shown in the ground. The Egyptian furnaces are shown being assisted by mouth belows. Pot belows were found at Tell edh Dibha'i, just 2, hand formed. The author suggests they could be operated by foot. They were made from 'medium fine clay' heated to 500 degrees Celcius, measuring '0.45m in diameter and 0.16m high' (p.2). I need to look into these further, though they have an obvious lip around the top, which would allow a bladder or cover to be secured. A one and a half inch pipe sticks out for two inches at the base of one side, with a small channel, 2/3rds of an inch in diameter running, presumably into the vacant belly of the pot. The whole pot has a very flat base, giving it a sturdy appearence.
The share of casting technology and knowledge
Many parallels are drawn between ancient Egyptian and Mesopotamian (where the dig site was located) metal working techniques. These techniques are not shared with some of their near trading partners {Anatolia, Iran and the coast of Levant}, however, even where one culture traded metals with another, or was in a position of control over the other region for a significant period of time (p. 14). The author postulates that this was because of the differences in fuel availability. This is an example of how neighbouring cultures can continue on parallel but distinct technique paths, without the 'information bleed' that we often assume as recreationists. Just because a neighbouring, trading culture had a technique, does not mean the metalsmiths of a region picked up.
Relevance to metal smiths in 10th Century Birka
I believe that where I cannot find sufficient information to pin down a technique, it would be plausible to draw from these ancient techniques, since many of these tools and methods are still in use to this day, in a wide variety of locations and cultures around the world. I can document the presence of a table top furnace from Baghdad in Scandanavia several centuries before Birka. It may well have been used or admired by several generations of Norsemen until it was laid where it was found in modern times. Since there was trade with the peoples of the Near and Far East, I think it is plausible to draw on techniques that were used amongst these cultures, where I am unable to discover the exact method used to create the pieces in a 10th century Birka metalsmiths workshop.
No comments:
Post a Comment