National Museums Liverpool Blog - More on the Sefton Park bronzes

More on the progress of this reconstruction project from Kathy Wedge (here's the background to the project in case you've missed previous instalments), plus progress snaps on Flickr.

Wax model at foundry showing sprues and top of head cut off

The finished clay sculpting has now been approved by Liverpool City Council, but that is only part of the story. We now have to produce the actual foundry cast bronze reliefs from these clay sculptures.

The clay panels which as previously mentioned are very heavy were loaded onto a van using a hoist and fork lift truck and taken to a fine art foundry.  The panels are being cast using the ‘lost wax’ method of casting. This involves a thin layer of silicone rubber being painted over the clay models which defines the detail of the sculpting, further thicker layers of silicone are then applied until an accurate mould is produced. But of course  silicone rubber is not rigid, so a plaster jacket to hold the rubber stable and reduce the risk of distortion also has to be made.

Once this mould and jacket have been produced, wax is painted into the mould to form a wax model. The mould is taken off leaving a hollow wax sculpture which is cleaned and the detail checked for imperfections. The hollow wax sculpture is cut into sections for ease of casting, particularly where there are extending features, and wax sprues are added to the sections of the wax model, that will act as air vents when the wax melts at the casting stage.

The pieces of the hollow wax model are the dipped several times into a ceramic mixture which form ceramic shell around the sections which are strong enough to withstand the high pressures and thermal shock of the molten metal used in the casting.

The molten bronze is poured into the shell via a cupped sprue or air vent and as it is poured the wax melts running out of the air vents leaving the bronze casting encased in the ceramic shell.

The ceramic shells are broken off the bronze castings and the sections are welded together very carefully, and polished so that the joins cannot be seen. That is the end of the actual casting process but not the end of the story. More about the rest of the process in our next post.