Technique
 

 

Technique (A brief description of the lost wax casting process)

  The lost wax process is a traditional method of casting bronze sculpture which goes back to something like 5000 years ago. It is still used today in the casting of bronze sculpture because of the ability to achieve very fine detail and deep undercuts although more modern equipment and methods are used nowadays.

  Mould: once the clay original is finished you are then ready to take a negative mould from it using a flexible rubber type compound this is either painted onto the sculptured surface or poured between the clay sculpture and a plaster jacket. The mould should really be made in at least two parts or more depending on the size or complication of the sculpture. You now have a rubber negative mould supported by a plaster jacket this mould is going to make a hollow wax positive of the original sculpture.
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  Waxing up:When the rubber/plaster negative mould is finished you are then ready to start applying melted wax to the surface of the rubber mould using a paint brush. After several coats of wax the mould is assembled and cool wax is then poured inside to completely fill the mould. It is left a few seconds to increase the section thickness and then the rest is poured back into the melting pot, you now have a hollow wax positive. When the wax has solidified the cavity is then completely filled with plaster and grog, this is called the "Core" and is done to eventually achieve a hollow bronze casting approximately 1/4" inch section thickness.
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  Retouching wax: The sculpture now in wax with core inside is ready for the artist to tidy up all the seam lines from the waxing process and to sculpt the fine detail before being finally signed and dated. Nails are now hammered half -way in through the wax into the core all over the sculpture, these nails keep the core central when the wax is melted out in the kiln.
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  Runners & risers: Wax runners are welded to the bottom of a pouring cup using a hot knife and go to various parts of the sculpture, when melted out these wax runners will leave a cavity for the molten bronze to flow to the sculpture. Risers are then attached to the top of the pouring cup the -se will act as vents to release hot gases and air from the cavity of the mould when the bronze is poured. The wax sculpture with runners and risers is now ready for plaster and grog (investment) to be applied to make the outside mould.
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  Investment mould: An initial layer is applied to the wax sculpture until the thickness is about 1/2"-3/4" by dipping or painting. A steel or plastic sheet is then wrapped around the sculpture and sealed at the bottom, investment material is then poured in until it reaches the top of the wax pouring cup. The mould is now ready for burning out in the kiln prior to casting.
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  Burning out & casting: The mould is placed in the kiln and slowly taken up to temperature over 48 hours depending on the size of mould, this will drive all moisture out of the investment and melt out the wax leaving a cavity for the molten bronze ( hence the lost wax process). The mould is taken out of the kiln and put into a pit or steel ring packed around with sand to stop it from splitting when casting. Bronze is then melted to approximately 1200 degrees centigrade depending on the grade and then steadily poured into the mould. When everything has cooled down the mould is split open and the investment is carefully chipped away to reveal the bronze sculpture with it"s runner and riser system. The sculpture is now ready to have it"s runners and risers taken off and the surface of the sculpture fettled/chased a process of working the sculptured surface to the desired finish.
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The above process is consuming taking many weeks by skilled craftsmen.

 
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