Technologies Explained | Hot Metal Casting using Cuttlefish Bones for Moulds
Hot Metal Casting using Cuttlefish Bones for Moulds
Published: 29th January 2010
The network members have been experimenting with the use of cuttlefish "bones" to make moulds for pouring hot metal jewellery and votive items.
They offer workshop sessions doing this at some of the events they attend.
This is a YouTube video of Bill Crumbleholme demonstrating metal casting, during a workshop session at the Portland Arts Centre (now sadly closed).
Here are some still images and text describing the technologies.
Laurence watching the metal being heated by blow torch, with cuttlefish moulds ready in background.
The technique is at least as old as Roman times, with several fine examples being held in British museums.
The cuttlefish has a distinctive grain, like tree rings, which can be carved away with simple tools to make fine impressions.
The softer side of two pieces of cuttlefish are rubbed together to flatten them, they are then washed and left to dry. Larger items could be done using three pieces, but that is for another day...
The design is carved into one of the cuttlefish. The other side can be left flat or similarly carved - in which case there must be a system for locating each side correctly relative to each other. The depth of the carving needs to be sufficient to give the casting enough strength.
A "sprue" - the funnel-like entry point for the hot metal is carved at the top. The neck of the sprue must be large enough to allow metal to enter without forming a blockage if it freezes.
The above image shows a partly carved arrow head - not that arrowheads were ever made like this, but the shape is a good one!
When ready the pieces are tied together with wire and a fat coil of clay is pushed round the edges - that stops any metal pouring out of small gaps between the pieces. Two butresses of clay are positioned to hold the mould upright on a flat heat proof surface - a clean metal tin lid works well, so that any spilled metal can be collected and reused.
Various metals can be used, silver works very well, the network has been using "Lead-Free Pewter" a high tin alloy of jewellery grade, which is less precious. That is obtained from Tiranti Metals - see their website
The pewter melts below 300 degrees, an almost domestic cooking temperature! The network uses a bottled gas blow torch (used for pottery firing) and an iron ladle. It takes less than a minute to melt the metal - indeed it must not be overheated.
Several items are cast at the same time from one ladle. It is worth having some extra moulds for any excess metal - which should not be left to cool in the ladle - see about ceramic moulds below.
These snowflakes show how much detail can be acheived, after a bit of practice!
Health and Safety risks are obvious and thick heat proof gloves should be worn. General clothing should be at least fire retardant, a full leather apron looks the part, but a fire fighting suit is probably going too far.
After cooling the moulds are broken open and the cast removed. This should not be rushed - if the metal is still too hot it tends to stick to flesh with a nasty burning smell!
Sometimes the moulds can be reused, by scraping away the burnt cuttlefish surface, the second cast will be a bit bigger! Often the mould burns too much and pieces crack away. Certainly the flat side can be rubbed down and used again - recycling a resource which takes a while to find and prepare.
Cuttlefish are collected from "Dead Man's Cove" at the end of Chesil Beach, preferably after a storm, which throws up a new harvest. Apparently female cuttlefish die after laying their eggs, their flesh is stripped by other fish and crabs and the bouyant bone floats to the surface and is maybe cleaned by seabirds. They wash ashore and are left on the high tide line, in amongst the debris - mostly human plastic rubbish.
Small files and saws or a Dremel drill can be used to cut off the sprue, drill holes for suspension and generally tidy up the edges - fettling as it is known.
The whole process takes about two hours.
The network has also been using fired clay to make moulds, both two peices moulds and open top moulds, where the metal is just poured on top of a flat clay slab into which a shape has been carved or impressed - shells and ammonites work well.
A separate article will be prepared covering the more exciting casting of Bronze - which needs much higher temperatures than pewter and a high level of technology in some ways.
The network welcomes comments from anyone about this article - please leave messages using the comment facility below.
Comments
Bill gave me one of the ammonite pendants shown above - it's lovely.
Ros Fry 02-03-2010
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