Yet another site where Anders pops up. This is his website, including a short explanation of his motivation as an experimental archaeologist, a great visual record of his use of cire perdue to cast replicas of items found at Birka, and a few of his articles. He has a lot. Thanks Anders, for recording your findings so clearly.
Article - Scandanavian bronzecasting the the Viking Age and Early Middle Ages
This article is almost identical to another website of Anders', though I like the way this one is found on a links section on the main page of his site.
'The sintered ware is stable and hardy and after being digged-up and washed, a viking age crucible looks the same today; just the way it looked when cooling-off at the workshop's floor a thousand years ago. Even Bronze-Age crucibles haven't changed very much, despite the passing of three thousand years. This gives a perspective on time. It's a strong reminder, of thousand or three thousand years being no more than a snap with your fingers in the universe. '
Anders Soderberg
This article has a very useful map showing the location of 9 Viking Age metalwork sites and quite a few images of extant and recreated in ground furnaces, complete with equipment for bronze casting. He also discusses the difficulties of pouring into clay moulds made from Scandanvian glacial and post glacial clays, which he says contain significant quantities of lime, which produce gas on heating.
Anders says that if you have 5mm walls on your clay mould, it will take around a quarter of an hour to heat it up to 700 degrees C in an unbellowed furnace. He also says that you have around 20 seconds between removing the mould from the fire and significant cooling that will affect the casting process. This is useful because all sources so far agree that the best results for cire perdue casting take place when the clay mould is not only prefired, whilst the clay is removed, but refired/preheated simlutaneously to the heating of the metal in the crucible. This theory is supported by the existence of dual in ground furnaces.
Article - Reconstructing the Trundholm Sun Chariot
This document outlines the process needed to complete a statue of a chariot with wheels and horse. It is outside the period I am looking into for this project but is fascinating, none the less. It is especially interesting to note that he says the horse sculpture is cast around a clay core. I am keen to find out if this means the clay core remains in the object. Wouldn't that mean it would have to float free during casting? Or could it be held in place with plugs that do not show up on the finished item? Perhaps it just means a clay mould was used in the core of the sculpture, not to remain after casting.
Anyway, Anders also discusses the shrinkage rates of bronze and clay on firing. He suggests that it would be usual to expect between 2.8 - 5.6%. More sand in the moulding clay makes a stronger mould but gives a higher % of shrinkage.
Anders says he has grown used to working in beeswax, despite modern claims of it being too sticky. He puts time into polishing the wax model before making the mould, as it reduces the amount of time needed to clean up the cast item.
There is also a detailed explanation of how to apply the clay mould to get a good transfer of the original details.
Article - Scandanavian Iron Age and Early Medieval Ceramic Moulds - Lost Wax or not or both?
Apparently the only sideways proof of the use of the cire perdue method in VA casting is the presence of a bar of wax amongst other casting tools in the burial of
'...a Viking goldsmiths' grave from Mysen in south east Norway. A man was found buried accompanied by crucibles, moulds and a piece of wax (Skjølsvold 1951). An archaeological problem with use of wax in casting, is that most of the wax used would have burnt away when firing the moulds. It disappears rather than being preserved in the soil, except for possibly in a few lucky cases.'
It has been hypothesised that wax and clay can be used in an array of combinations to produce models and moulds, including:
*a metal model (finished item) - pushed into a clay mould - cast from the original clay mould using an open mould
*a metal model - pushed into two open clay moulds that are joined by a fine slip - metal cast into void of clay moulds with natural cracking along the mould join
*a metal model (finished item) - pushed into a clay mould - cast from the original clay mould using an 2 part mould to allow gases to escape during casting
*a metal item - pushed into clay -one sided wax models made from clay mould - wax models used to make more open clay moulds - metal cast into replicated moulds
*a wax model - pushed into a clay mould - wax removed - metal cast into void of clay moulds
*a wax model - pushed into two open clay moulds that are joined by a fine slip - wax removed - metal cast into void of clay moulds with natural cracking along the mould join
This does not even begin to address the question of the elk horn and stone moulds found at the Birka site. There are many options.
A useful tip is the suggestion that a fine layer of oil on the surface of a metal model (finished piece of jewellery used to imprint upon clay to make a mould) can help it be removed more easily. This is relevant to the wax-copy technique.
There seems to some fair points made in the discussion on the advantages of the wax copy method over the direct matrix method - a metal model would be a valuable item of jewellery and it seems that being able to make as many wax copies as possible in a short a time as possible would be a major drawcard. Scenario 1: smith borrows, hires or is lent a piece of jewellery to make copies of on commission. The owner will want it back in their posession as soon as possible. Scenario 2: the smith is working for a very wealthy person who owns many pieces. The smith 'liberates' the item for a half hour to make a mould, making time a valuable commodity. Scenario 3: I am thinking about this on a modern 21st Century view of time use. Our current view of getting things done fast has been accelerated by telecommunications and high speed travel. Perhaps a couple of extra weeks to complete some extra moulds was not a big issue.
There are quite a few more articles to go, including a new journal to look at.
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