Technique: Delft Clay Casting

A studio jeweller is just what it says on the tin - a person who makes everything in their studio start to finish. As they usually make one-off pieces or maybe a very limited number, they often use ancient processes to create their designs.

Delft Clay casting is one such technique. It allows the maker to create the original model in an inexpensive material, such as a cheaper metal or even carved from wax. The jewellery can then be made at the bench, without the investment in complicated and costly machinery. The single-use mould gives very accurate results and the maker is in full control of the process, timing and costs.

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Nicole van der Wolf recently had the perfect opportunity to use this technique - her customer wanted a ring made in 14k green gold. Green gold is still popular in the Middle East and Asia, but not common in Ireland. Luckily Nicole has some experience making her own alloys, so she was well able to take on this project.

Because of the shape of the ring and the metal the best way to make the ring was to cast it in one piece. Delft-Clay casting was the perfect solution for this commission. It’s a bit of fun so we thought we’d explain the process:

Step 1 - Preparing the clay.
The clay is a mix of very fine sand and oil and it packs together quite tightly in its packaging. To get the best results you need to break it up into fine pieces before compacting it into the mold. A steel rule, a quick chopping action and a little patience is all it takes.

Step 2 - Preparing the lower mould.
The simple aluminium ring-mould comes in two parts. Place the lower ring with the lip down on a flat surface and pack in the chopped clay. Level the surface and it is time to insert the model. This model was made in silver and sent to the customer to make sure he was happy with the design and size of the ring.

Step 3 - Powder the mould.
With the model in place, apply a fine layer of mica or talcum powder to keep the two halves from sticking - this is important as you’ll have to open the mould to remove the model later.

Step 4 - Create the upper mould.
Position the upper ring of the mould - aligning the reference marks on the outside of the mould. Pack in more clay.

Step 5 - Open the mould and remove the model.
This needs to be done gently and carefully as any disturbance of the surrounding clay will affect the final result so it is important to go easy with this step.

Step 6 - Cut the pouring hole and air vents into the clay.
The molten metal starts to set in a matter of split seconds, so it is important that it moves as quickly as possible to completely fill the mould. Even the air in the mould can stop it from moving quickly enough, so we cut as many air vents as possible to allow the air to escape out ahead of the molten metal. While cutting the vents and the pouring hole it is important not to disturb the imprint of the model and take care you leave no loose grains of sand as these will affect the end result.

Step 7 -Assemble the mould and pour in the molten metal.
This is where those reference marks on the outside come in handy - it is what you use to accurately line up the two halves of the mould.

Next, the metal is brought up to temperatures around 1400C. If it is not hot enough it will not pour well (it will look a bit like a “slushy” and will just back up in the pour hole). The metal won’t fill the mould properly and you will have to start all over again. If it is too hot it will bubble and form a very rough surface which will need more cleanup afterwards. Because this is a manual process the temperature is gauged by eye. You can’t be sure that the pour has worked properly until you open the mould, so this stage is pretty exciting!

After a successful cast, there is still a bit of work to do to get to the final piece. This little "spider" needs a bit of surface cleanup but is actually perfect! Here it is ready for engraving and of course a photo of the end result: 14k green gold, made in Ireland, and hand engraved with an Irish harp - 100% precisely as he wanted it.

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