…’Cause it really is not the actual casting bit.
I’m finding the actual casting of these current tokens very frustrating. I would really like to know from other casters out there what the failure rate on your casts are, cause at the moment I’m getting about 2/3rds failures. I’m also not exceptionally happy with the quality of the other 1/3rd that I have been keeping and often just give up on a particular pour all together.
Is the mould meant to get so hot you can’t actually hold it with welders gloves? Does preheating the mould actually make a real difference? If so… what Temp do you preheat to? I have found that I tend to get the best results with a cold mould that has a big enough opening to allow a full pour without allowing pockets to form, though I have also found that it is very hard to get that to work when you are casting thin tokens as the pewter freezes before it can get into all the nooks and crannies.
Advice anyone?
Hmmm… interesting to hear your results. I’ve got a series of similar tests going on for replacement mould materials. Using plaster was going to be next!
You CAN have a mould too hot. If it’s hot enough to prevent the pewter cooling down fast enough then there can be results. As I just found for myself whilst testing another medium for similar capabilities. It wasn’t so much the medium but the effect it had on the metal’s cooling abilities. It needs to be hot enough to encourage flow but not so hot that it prevents reasonably quick cooling to retain image detail.
My experiments thus far -
So I poured pewter into a silicon rubber mould made of Sikaflex, the plumbing gunk. The moulds were a few years old but I can’t imagine these moulds deteriorating much at all and they looked good to start with. I’ve poured nice wax models from these in the past so I know the mould produces good results with the right filler.
Found that the silicon slowed down the rate of cooling to a destructive point. The surface of the metal had bumps all over it much like the moon’s face. The button was liquid for aaaages. I tried pouring at *just* melted temp and the bumps smoothed but were still present. The inside of the mould became sticky and I believe was starting to ‘meld’ with the pewter after only 1 pour!
I have a laser engraved disk of nylon I had made elsewhere. It has fine lettering and detail on it. This is my test piece as there’s a mistake on it. So I’m subjecting it to a series of tests. First I pressed it into plasticine to see what it would look like in reverse as this commission is for a seal.
Next I pressed it into Das Terracotta, an air drying clay. It didn’t give a good image in the clay as it seems to be made from a fine papier mache mixed with something resinous. The paper fibres were large enough to destroy the fine impression of the image. It’s dry now and I’m still going to pour some pewter in it, to see what happens. It may not be good for fine detail but it could be good for larger detail or simpler shapes like drinking cups perhaps?
After cutting the nylon model in half (to keep one half in it’s original state), I’ve pressed it into Fimo polymer clay, and baked it. Left it in the oven overnight so need to look at it and see if the fine details have sagged. Well the image has shrunk a little. 1.5mm over an original 40mm diameter. The wording is now too fine. But I’ll pour pewter into both the Das and Fimo moulds tomorrow night to see how they hold up.
The plaster might work for me on another project to make 10 prize tokens. Will it hold up to that quantity do you think? I was going to carve them out of soapstone but, as soapstone is harder to get in a fine grade, I’m looking for alternatives that aren’t as expensive as a vulcanising rubber mould set up. If i use a medium temp plaster mould, with a large sprue opening, I could be prepared for the non-events that can just melted later on anyway.
Hey Tig,
I believe the pockmarks you are seeing are the result of a slight out-gassing of the mould. This occurs in two situations, either too much moisture in the mould, or the mould material itself not being able to withstand the heat of the pewter. I have also seen this result, and I have also seen it in soapstone on humid days, when the soapstone just seems to instantly soak up moisture in the air. I suspect that with your Sikaflex, there is an element of the mould that has either not cured completely, or just cannot take the temp and is melting or vaporising or even just the heat is causing a chemical reaction that is causing it to create a small amount of gas, this then forms the flat bubbled look.
I suspect that the best material for doing a press mould would be plain old potters clay and have been meaning to get some for ages. Clay used to be used not only for push moulds, but also for lost wax. The clay was pressed into the details building up a very rough mould around a wax object, then that was left to dry. Once dry it was placed in the fire to burn out the wax and after that was used to cast from. I believe there is a small amount of shrinkage with clay… something like 5%? But I can’t really remember back to my art days at school.
As for your other experiments I look forward to hearing your results, though I do have some suspicions of how some of them will turn out. I believe the Das one will work, but will start cracking after 5-10 castings, it will also start burning, which will make the inside of the mould loose detail as it becomes charcoal. I believe the Fimo mould won’t take the heat at all, and will start melting from the inside out, probably with a lot of bubbling burning plastic. I know… I sound negative, but I’m more than happy to be proven wrong and stress that those opinions are only my own opinions and they have no basis in experimentation.
The plaster will depend on how many tokens you are casting, and the mass of the pewter going into the mould. If you are pouring small tokens with a small button, then I suspect you will get more tokens, though Plaster is hard to get to pour correctly. It seems to resist the molten pewter not allowing details unless the cast is pressurised. In fact pressurisation or increasing gravity in a centrifuge seems to be the answer to alot of casting frustration, and is something I have turned my mind to constructing. However I diverge… Plaster seems to not be able to take the stresses of the heating and cooling cycle of pouring pewter. I have yet to work out whether this is simply because plaster seems to be a good insulator and therefore expands more where the heat is than a small distance away where the heat has not reached. This in turn causes uneven stresses in the mould itself and over time causes the mould to crumble. I suspect that the best way to use plaster as a mould is to use it in parts of the mould that do not really matter that much, such as creating a two part pouring button for a one sided soapstone mould, where it doesn’t matter that the plaster crumbles after 10-20 pours, because it’s easy enough to make a second pouring button, and the face of the token is in soapstone and won’t change.
I am so tempted to construct a centrifuge/spin casting machine so that I don’t have to ever do gravity casting ever again. That way I could also cast Festival type numbers without too much problem.