Lost Wax Investment Casting
Lost Wax Investment Casting is a lengthy process. It has many stages and requires skill, experience and patience. Part of our mission statement at Dan Gentle Art Foundry is to educate others in this age old system. We hope that in the future we will offer courses in Lost Wax Investment Casting at the farm. Below, we walk you briefly through the history and the steps of the casting process.
The
PROCESS
Step 1
Taking a Negative Mould
A sculpture arrives at the foundry as a solid piece of clay or plaster. The artisan at the foundry needs to recreate the sculpture as a hollow wax form, therefore the first step is to take a negative mould of the item, which will be used to create the wax reproduction.
Step 2
The Wax Reproduction
Most bronze sculptures are hollow therefore the wax reproduction is also hollow.
The sculpture requires an even thickness to ensure a sound casting.
There are a number of reasons why most bronze sculptures are hollow:
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To save on the cost of metal
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To ensure the sculpture isn't too heavy
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The chemical reactions that occur while a metal is hot can lead to poor results if the area is too thick. This is called "porosity" and appears as popped bubbles on the surface of the sculpture. This is highly undesirable.
Step 3
Gating
When it is time to pour the bronze, the metal will require channels to run into the investment and displaced air will require channels to escape. These "channels" are added to the wax reproduction of the sculpture in the form of wax rods. This stage is called "gating". Each sculpture is unique in it's shape, therefore a new gating system needs to be invented by the artisan to ensure the bronze is reaching every point of the sculpture and air is freely escaping. If either is not successful, the bronze sculpture will have gaping holes.
Step 4
Investing
The wax reproduction and gating system is now ready to be invested. Investing is the process of slowly building up layers of plaster, inside and outside the wax, which will be a supporting structure during the bronze pour.
A skilled artisan will know the correct consistency and materials of the plaster mixture and will build the layers patiently to avoid "slippage".
Step 5
Firing the Investment
Now that the wax reproduction is gated and invested, it's time to "Lose the Wax". The investment is placed in a kiln upsidedown, to allow the wax to melt and drain away, leaving space for the bronze to take its place. The firing process also cures and hardens the investment by drawing any moisture out. During the firing stage, the artisan must keep an eye on the moisture content to know when the time is right to remove the investments from the kiln.
A solid investment is imperative during the bronze pouring stage, as a weak investment would break and the bronze would flow through the cracks. This is called a mispour and would be devastating.
Step 6
Melting the Bronze
Bronze is the most common metal that sculptures are cast with. However we also offer aluminium and other mixtures of metals if so desired.
Towards the end of the kiln firing stage, the artisan will commence melting the bronze. The metal MUST be poured when it reaches the maximum temperature, and by that time, the investments must be out of the kiln and buried in pits of sand to further reinforce them. The casting stage requires experience to know when to start melting the bronze, whilst still monitoring the kiln.
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Step 7
Burying the investments
Although the investment is hard and solid, we must still reinforce it by burying it in rammed sand, to further prevent any cracking. The investments are hot as they come out of the kiln and can be very heavy. Sometimes machinery is required to lift the investments out and into the pit. Heat-proof gloves are required at this stage as a must.
Step 8
The Pour
It's the moment we've all been waiting for, and sometimes this could be a month after commencing Step 1, depending how large and fiddly the sculpture is.
The furnace is switched off, silence fills the room and all you can hear is the hydraulic crane and metal pouring. The temperature of bronze at pouring stage is 1400 degrees Celcius. Strict PPE is required to protect all workers near the molten metal.
Step 9
De-investing
Not long after the metal is poured into the investments, it's time to knock the investment off to see how the casting went. This is the moment of truth. All parts of the investment plaster need to be fully removed. Often high pressure water-blasting is the final stage of de-investing.
Step 10
Fettling
Remember the wax rods that were fitted in the gating step? Those are now bronze rods and will need to be removed. They aren't needed anymore. An angle grinder is usually the tool of choice for fettling.
Step 11
Chasing
Our sculpture is cast, however there is still more work to do. Chasing is the process of using hand tools to remove any unwanted bleeds of metal and bring the sculpture back to its original finish.
Step 12
Re-Assembly
If the sculpture was too large to be cast in one piece, it will need to be re-assembled. The artisan is trained in welding to ensure the joins are strong. The welds will also need to be chased to hide them from view, creating a seamless flow from one piece to the next. As well as recreating any texture on the sculpture that was lost during the welding process.
Step 13
Applying a patina
Bronze sculptures will turn a natural green colour if left alone, however some artists request for their work to be "prematurely aged" by applying a green patina. Others have requested blue, white, black and red patinas. There is a wide range of patinas available, please consult us for more information. The patina process usually involves applying chemicals and sometimes heat.
Step 14
Installation
The final stage that we offer as part of our services is an optional installation of the piece. In a public art project this includes building the foundations, working with the site supervisor regarding logistics, timing and installation.
The
HISTORY
Among the very earliest lost wax castings known are small figures of animals mounted on pins and dowelled into the centre of cylinder seals, devices used before the invention of writing to form a characteristic impression on moist clay as an indication of ownership or agreement to a transaction. This seal, carved with figures of cattle, carries a recumbent ram cast in copper and comes from the Uruk period in Mesopotamia, around 3 500 B.C.
Photograph by courtesy of the Ashmolean Museum, University of Oxford
The earliest known written reference to lost wax casting comes from the Babylonian city of Sippar and is dated 1 789 B.C., during the reign of the great King Hammurabi. Written in cuneiform on a clay tablet, this is a receipt for a small quantity of wax issued to a metal worker and is composed in the typically bureaucratic manner of the period.
'Two thirds of a mina of wax to make a bronze key for the temple of Shamash received by the metal worker from the temple treasury
In the presence of Silli-niu-karrak and of the storekeeper, his colleagues
On the nineteeth day of the month of Arabsammu in the year of building the temple wall'
Reference: "The Long History of Lost Wax Casting" by L.B. Hunt