Casting Guide

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Scaling Up

Often, an artist will produce a small object (maquette) that requires enlarging (scaling up) before the casting process can begin.

Usually this necessitates the construction of a skeleton (armature), taking measurements from the original and enlarging them proportionately, around which a copy of the original piece will be sculpted in consultation with the artist.

Mould Making

A mould enables wax replicas to be made of the original sculpture and makes possible the casting of multiple bronze (editions) of the same object.

Moulds are composed of two parts:
A coating of rubber, which captures in negative the form and surface texture of the original sculpture (even the fine imprints of fingerprints) and an outer coating of plaster (‘the ‘jacket’), to support the flexible rubber.

The number of sections in which a mould is made is dependent upon the size and form of each individual sculpture.

After the mould is removed the original model is then retained for reference only and is no longer used in the casting process.

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Painting and Pouring the Wax

Each section of the mould is carefully painted with hot wax to ensure a good reproduction of the texture of the original.

The mould is then closed up and filled with cooler wax.

This wax is then poured out, leaving a thin skin on the interior of the mould, approximately 4mm thick.

When the wax has cooled sufficiently, the mould is removed revealing a hollow wax replica of the original sculpture.

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Attaching the Sprues

A system of wax tubes (sprues) is then attached to the wax pattern to create channels through which the bronze will eventually flow.

Sprues are attached by heating the ends with a hot tool and affixing to the wax pattern at regular intervals.

The sprues rise up to a single point above the wax pattern, to which a funnel shaped wax cup is attached.

It is through this point that the bronze will be poured.

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Investing

The investment is built up using a ceramic slurry and stucco (fused silica).

The investment (ceramic shell) is built up in layers around the wax pattern and system by pouring over the pattern or by dipping the pattern into the slurry. Then the stucco is applied to the still ‘wet’ wax pattern to thicken up each layer and to allow a ‘key’ for the next layer to allow for cross bonding.

The wax pattern is now placed into a drying cabinet in which the temperature and humidity are carefully controlled. Drying time varies on the size of each individual sculpture.

Once dry, this process is repeated again and again up to twelve times or until the required thickness is achieved (this also varies on the size and shape of the job).

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Dewaxing

Prior to the investment being placed in the kiln, the wax cup is melted to aid dewaxing. The investment is then placed upside down in a kiln and heated at 725°C for approximately 1 hour.

During this process the wax melts and pours out through the cup. It is collected at the base of the kiln and can then be re-used.

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Inspecting and Firing

At this stage a careful inspection is made of the investment to make sure no cracks have appeared in the shell.

Any cracks are sealed over with a ceramic putty.

After the investment passes inspection it is fired in the kiln at approx 950°C for one hour to strengthen the shell to the point where it can withstand the heat and pressure of molten bronze.

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Casting

Once the investments are cool enough to be handled (around 300 - 400°C) they are placed in a bed of sand to support them.

Meanwhile, the bronze is being melted in a furnace.

Once the metal reaches 1200°C it is ready to be poured into the ceramic shell investment. This is done by decanting the metal into a pre heated small pot - a crucible - and then poured into the investment at a reasonable speed.

The investments, now filled with bronze, are left to cool until the following day.

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Fettling

The investments are removed from the sand and the ceramic shell is removed from the bronze cast by gentle tapping on the cup or the spruing system, or by grit blasting the cast which disperses the ceramic shell whilst leaving the bronze intact.

The spruing system and the cups are cut from the casts and then re used.

Welding and Chasing

Many sculptures are cast in several sections, especially large scale pieces.

It is the job of the metalworker (chaser) to weld together these pieces to form a whole whilst removing any evidence of a join. He will do this by TIG welding all the sections together with wire matching the parent metal. Any excess weld can then be ground down carefully, first with an angle grinder then with a variety of tools ranging from air powered tungsten burrs to small chisels and matting tools.

The metalworker skilfully blends the joint into the surrounding surface matching the texture exactly. In the case of smooth or polished pieces, joins will be linished (ground away) with abrasive pads of ever finer grit until the surface is smooth enough to be polished.

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Patinating

A number of different surface finishes can be applied to the finished bronze. The most common being chemical patination. This is the application of a variety of chemicals either cold, or hot ( using a blow torch) which react with the copper in the casting to create many different surface colours. This process mimics the natural effects of weathering but at a greatly accelerated rate.

Colours can range from black/brown/green to white or even blue, or any combination of colours. A good patina will work in sympathy with the sculpture enhancing the form and surface contributing to the overall feel of the piece.

Alternatively, a piece may be polished or lacquered or perhaps even painted, according to the wishes of the artist.

Wax Sealing and Fixing

A patinated sculpture will need to be waxed to seal the surface and to fix the colour.

The wax may be buffed to achieve a high polish as required.