We also run short duration bronze casting training courses from the foundry, see website www.bronzecasting.org.uk for more details
Above picture shows John McKenna skimming bronze during a pour, see below for more pictures.
A4A Studio Bronze Foundry
John has always worked in bronze and has a fascination with the lost wax casting process throughout his 20 year career as a sculptor.
The John McKenna Studio bronze foundry was set up to cast A4A Art for Architecture and McKenna's artwork in bronze and other artists work.
The Studio foundry using a tilting furnace can pour approx 120kg of bronze each melt. John uses both the ceramic shell technique and the investment lost wax process. The process is basically outlined below.
Above: A small gas fired lift out furnace for melting smaller amounts of metal upto 30kg of bronze. The burner enters tangentially at the base of the furnace and the flame spirals up around the crucible. The crucible can just be seen at a dark red heat inside the furnace being taken to high temperature before charging with bronze.
Above: John McKenna's Studio bronze foundry commissioned an oil fired tilting furnace, which can be manouvered using their fork lift truck or overhead crane. In the operation above a fine steady stream of molten bronze is being poured by John McKenna into the casting cup of a ceamic shell mould. The worm gear on the tilting mechanism allows very small increases in tilt angle so not too much bronze is poured at once. Peter Scott Tudhope is ready to skim the surface of the molten bronze just in case there is any dross floating on the surface. Another assistant operates the forklift.
Ceramic Shell process quick outline. The process of ceramic shell casting John uses involves using a lost wax process. It starts by making flexible rubber moulds of a sculpture form, in these pictures a pile of oversized modelled clay books. The flexible rubber mould is supported in a GRP jacket. Molten wax is then carefully painted and slushed into the detail of the rubber mould resulting in a highly accurate faithful positive wax copy of the sculpture, in a shell-like form. The wall thickness of the wax will vary slightly but a uniform thickness is sought.
The 'new' wax shell version of the sculpture is assembled to check all is as the original sculpture intended. The wax book pile in the left picture is part of the Sir Alexander Stone monument.
The wax sections are seperarated into castable pieces, a series of wax sprues are then wax welded to each section. These sprues, 'runners and risers' are where molten bronze will flow through into the sculpture at a later stage.
The sprued wax sculpture sections are repeatedly dipped into a refractory ceramic slurry and coated with a fused silica sand, drying carefully between coats until a 'ceramic shell' is built up in layers around the sculpture section. This ceramic shell case mould is created from refractory materials which will withstand the high temperature of molten metal
The positeive wax inside the ceramic shell case mould now has to be melted away, hence 'cire perdue' or lost wax process. This void where the wax was will become the bronze casting, the molten bronze replacing the 'lost' wax.
The wax is lost from the ceramic shell mould by firing it in a lost wax burnout kiln. See below left. When operating, this kiln very quickly melts the wax out of its ceramic shell case, some of the wax is reclaimed and the remainder is burnt away at a very high temperature. The ceramic shell case is fired to a high temperature almost like a piece of china. Pictue below shows the process in action where you can just see a shell inside the kiln.
Once the ceramic shell mould is fired it is ready for bronze to be poured into it. Bigger shells are buried in sand to support the mould during bronze pouring. On the right is a picture of a ceramic shell cast - one of the books from the wax pile above, after pouring. It is filled with bronze and is hot and heavy.
The shell is carefully chipped away from the bronze casting to reveal the book in bronze with its sprue system intact. The sprue runners and risers are cut away from the casting and the piece is sandblasted clean. This book happens to be the one at the very base of the wax book pile.
When all the wax sculpture parts have gone through this process the sculpture is assembled ready for patination colouring and final waxing.
Below is the finished casting of the wax book pile at John McKenna's foundry studio, prior to patination.