Lost Wax Casting
Lost wax casting is one of the most common methods for producing a large number of copies of an original piece of jewellery. Charms, pendants, earrings, rings and chain links are some of the many items that can be reproduced using this technique.
The earliest examples of casting using the lost wax method date back to around 3000bc. The technique has been refined as more technology (such as vacuum pumps) has become available, but the principals are as they were so many years ago.
The master, (or original) is made by hand as any other piece of jewellery would be, often in sterling silver. It is completely finished with great care taken to ensure it is as perfect as it can be, after all every copy can only be as good as the original.
When the jeweller is satisfied with the finish of the master he makes a rubber mould of it.
A mould frame holds thin sheets of soft rubber and the piece to be copied. This frame is a box with a removable lid. The bottom is lined with rubber sheets and the piece to be copied is placed in the middle. Rubber pieces are then placed around the item leaving the smallest of gaps. When the sides have been built up to the level of the top of the original, more layers of rubber are placed on top until the frame is slightly overfilled. The lid is secured applying some pressure to the rubber inside and it is then heated in a kiln until the rubber melts into a single block surrounding the piece of jewellery inside. It is then removed from the kiln and allowed to cool.
The next step is to carefully cut the rubber mould open on three sides, leaving a hinge of rubber on the fourth side. Alternatively, the sides are all cut through in a pattern that allows the resulting two halves to be easily realigned accurately.
The piece being copied is then removed from the block leaving a space in the rubber exactly the same size and shape of the original. A channel is cut from one edge of the mould to the gap left by the original. This allows the mould to be filled with liquid wax.
The next step is to create a wax copy of the original item. The rubber mould is held together in a frame and molten wax is poured into the channel cut earlier. It is left to cool and then the mould is opened and the wax copy along with the wax filling the channel (the sprue) is removed. The rubber mould can then be filled with wax again and again to make as many copies as is required. These rubber moulds can produce thousands of wax copies over their lifetimes. Usually many pieces are cast together, so as many wax copies are created as are required. These are then joined to a wax base using hot glue, or where the items are small, they may be joined to a wax ‘tree’ so many more can be cast together. The bottom of the tree is then attached to a wax base.
The base holding the wax copies is placed inside a flask that will be filled with the investment material (similar to plaster of Paris). The investment must be evenly mixed and free of bubbles to ensure a good result. The air bubbles are normally removed by placing the liquid investment into a vacuum chamber. The investment is then poured into the flask, covering the wax copies and left to set hard.
The now solid block of investment is heated in a kiln to around 700 degrees celsius for some hours until the wax has melted/vapourised from within the investment. Hence the name ‘lost wax’. This leaves an empty mould that will be filled with molten silver or gold to produce the final copies of the original piece of jewellery.
The most common method for filling the mould with molten metal is by using centrifugal force, this ensures the metal completely fills the empty chambers and eliminates air bubbles. It is called ‘spin casting’ or ‘centrifugal casting’. The flasks are removed from the centrifuge and plunged into cold water, this process is called ‘quenching’.
The next step is to remove the pieces from the base and sprues. The sprue is cut away from the item being cast and is sanded smooth. Careful placement of the sprue in the mould helps to make this process easier, less time consuming and in an area that is inconspicuous.
Finally the finished copy is polished, often in a device called a ‘tumbler’. A tumbler is a container similar to a barrel, with protrusions around the inner walls to ‘agitate’ its contents thoroughly. It contains thousands of tiny stainless steel pins in a solution of water with a little detergent to which the piece to be polished is added.
Left over pieces of the metal (gold or silver) in the form of the base and sprues are collected and reused in the next casting.