Casting at Shetland Jewellery
Hi, my name is Tenneka, I’m here to tell you a bit about our casting process here at the workshop. When I started with the company, I was taken on as a jeweller and a lost wax caster, and have been fascinated with the process ever since.
Casting involves a lot of steps (so bear with me), and although it may seem like it takes a lot of time, it helps to significantly shorten our production time while maintaining consistency.
Before we can get casting, we start from an idea or a drawing, which is the first step and arguably the most difficult bit (especially for our team of jewellers, as we are all so creative and have too many ideas!)
Once we have selected our design, we can start making our master pattern, which is essentially the original and first piece of every design. We make this by cutting, shaping and layering silver to make our drawings come to life … and we take great care to make sure it’s perfect, as every piece we make after is a direct copy. To finish our master’s, we add on what we call a sprue, which acts as a feeder for the metal to flow into our pieces when we cast, but it later gets cut off.
After our master is made, we can start to make a mould from it. To do this, we take layers of rubber, 7 to be exact, and melt them down around our master in a large heat press called a Vulcan press. After around an hour in the press, our moulds are ready to be cut open. We carefully take a scalpel and start cutting our master out of the mould, being very careful not to cut into the design. Once the master is free from the mould, we melt a hole in the bottom which connects to our sprue. We also add a date and product code to the opposite side. This final step is crucial, or we would get very confused as we have a lot of moulds….
Our moulds then get injected with a hot wax, which is allowed to cool inside. Once removed, we have a perfect replica of the original piece that we previously created in silver. The mould allows us to create a lot of waxes in a short space of time and each is a perfect replica, making our process a lot quicker and more uniform.
Setting up for a cast is the next step of our process, and to do this, we take our ready-made waxes and organise them into “cans” of similar pieces or thicknesses. We then weigh each can of wax to calculate how much metal we will need to cast it. We do this by weighing our waxes and converting each weight to its corresponding metal weight (this is different depending on which metal we are using).
When everything has been calculated, we can then set up our waxes on their rubber bases. Each base is numbered, and this helps us to keep track of what is inside and also what weight of metal is allocated to each base/can number. We have to be careful when setting up our waxes that nothing is touching, or they will be fused together once we cast them – sometimes this can be a bit of a challenge if we have a lot of pieces to set up.
Once the fiddly bit of setting up our waxes is all finished, we must make another mould. This one is for our metal to flow into - we are nearing the end, I promise! To do this we take our bases with the waxes on and cover them with a metal can. We then mix up what we call investment, which is a really fine powder we mix with water and pour around our waxes and vacuum to remove any air pockets and bubbles. Investment is quite like plaster of paris, but a much finer powder, and the temperatures must be a lot more exact when we are mixing it. The investment is then allowed to set for an hour and a half. After this, we number the tops of each can and then rip off the rubber bases (not the other way around …. Trust me, you don’t want to take off the bases before numbering… that is not a fun game)
The cans then get placed in our ovens overnight and go through what we call a burnout cycle. During this cycle, all the wax melts out of the cans, leaving us with spaces inside each can where the wax once was – this is why the process is called lost wax casting. The burnout cycle is an overnight process which steadily increases the oven’s temperature in different ramps mixed with hold temperatures – it’s quite a technical process, but it essentially ensures that the plaster mould does not deteriorate from the wax burning out too quickly or from the water in the investment evaporating too quickly. By gradually increasing the oven’s temperature, we are able to get lovely smooth castings.
The following morning our cans are all ready to cast with our metal. Yes, believe it or not, we are finally at the casting stage!
The first step in the morning is to weigh up our metal using a combination of clean scrap and fresh grain - we cast in sterling silver, any colour and carat of gold and platinum. We then turn on our casting machine and set our casting temperature. We use a small vacuum casting machine; however, in the past, we used to use a centrifugal machine (that was before my time here, though).
We place our metal in the machine’s crucible and slowly start to heat the metal. Depending on which metal we are casting with, our metal casting temperature can range anywhere from 990°C to 1800°C. Once the metal is just about molten, we take our cans, which are held at a temperature of 620°C and place them in the casting chamber. We then vacuum the machine to remove any air inside the mould, and then the machine pours the molten metal into the can. We let each can cool for around 10 minutes after casting before plunging it into a bath of cold water, which breaks all the plaster away from the metal castings.
The castings then get cleaned in our wet-blast machine, which is like a teeny powerful pressure washer. We then cut each casting of the metal base and take them up to the main workshop, where they have a quick bath in our pickle to clean up before being allocated to orders and then dished out to all of us jewellers.
Casting is a very methodical process that does take quite a bit of time, but it is so important for us here at the workshop, and we couldn’t do what we do without it! I think I could go on for hours about casting, all the different steps as well as technical bumf, but I believe I have made my blog long enough for now. I will, however, leave you with the note that although I have been casting for 3 years, the process still surprises me and I am still very much left scratching my head sometimes, but I think that’s what I love most about it, as I’m always able to learn more about the process.
