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11 July 2014

Replicas for the new Staffordshire Hoard gallery – Phase Three

Following on from our last blog the waxes are now ready to be processed through the lost wax casting process. It should be remembered that the Anglo Saxon craftsmen would have used different techniques of hammering and soldering to create these objects. Although some objects in the hoard are cast, they would have used the same theory but different technology (more simple) to cast the objects.

The first step is to assemble the waxes onto a ‘tree’ to allow two to be cast at the same time.

The wax ‘tree’

Image 1. The wax ‘tree’

The next step is to immerse the wax tree into a liquid slurry of investment material (investment is a special form of a type of plaster of Paris) to do this the waxes are inserted into a metal can that holds everything in place while it dries and sets.

The waxes in their can

Image 2. The waxes in their can

The investment slurry is added

Image 3. The investment slurry is added

Once dry the cans are placed in a high temperature oven where the investment material effectively bakes and sets very hard and the wax melts away leaving a cavity in the investment in the shape of the artefact into which the molten metal can be poured.

The metal is melted in a specialist furnace using an induction coil to generate the heat required, the Anglo Saxons had charcoal! The hot can is then placed in a centrifugal casting machine to force the molten metal into the investment material.

The casting machine

Image 4. The casting machine

The molten metal just before casting

Image 5. The molten metal just before casting

The can is then removed from the machine and allowed to cool, the caster judges the temperature of the metal by the colour of the ‘button’ that can be seen glowing in the image below and only when it has cooled to just the right temperature (another skill the Anglo Saxons must have had) is the can plunged into cold water to shatter the investment material and release the castings.

The ‘hot button’ on the can

Image 6. The ‘hot button’ on the can

The castings appear for the first time

Image 7. The castings appear for the first time

The next step requires the removal of the investment residues using high pressure jets of water and we get to see the detail of our casting.

The cast tree

Image 8. The cast tree

All that remains for us to do is remove the artefacts from the tree with a small hacksaw and they are ready for the next stages in the reproduction process which will be covered in some detail in the next blog.

Removed from the tree ready for the next process

Image 9: Removed from the tree ready for the next process

Frank Cooper,
JIIC manager