Plaster/Quartz Mould

Well I’ve finished my latest mould I don’t have too many pictures of this process as I was elbow deep in plaster most of the time, but I’ll describe it here as best I can. I started out with the wax, with the negative stalactite shapes in it, now, as discussed in my previous post, the plaster that fills these shapes needs to be slightly softer than the rest of mould, so that when the the piece has been cast as the glass cools down and contracts, the plaster needs to be soft enough to flex and give way to the glass, otherwise the piece will crack. But of course this softer plaster needs to be strong enough to hold up to all the processes that happen to it before it gets fired, so in order to reinforce it I have taken the advise of far more experienced casters Heike Brachelow and Katya Filmus and used cuts of copper wire embedded into the mould as it was set.

I hand built the mould  in one part, so the first step was to prepare the copper wires to fit into the stalactite shapes, I’ve never used this method before myself, I’ve only heard it described to me, and I was a bit concerned about the wire being smooth and having no tooth, so I used one piece of slightly thicker wire about 1mm and wrapped a fine copper wire around it, about .02mm thick to give it a sort of thread. I cut them down to a length that meant they stuck out of the stalactite a bit and would be sticking into the main part of the mould. I prepared a small mixture of the weaker plaster and quartz (normally I use a 50/50 ratio of plaster to quartz, just taking alternating handfuls of each, for the softer mixture I just added an extra little bit of quartz) I poured (well dribbled really it was such a small quantity) that into the stalactite shapes and as the plaster set I wanted to ensure that the wires were in their correct positioning. I didn’t want the wires to be all the way at the tip of the stalactite because when the glass is cast, if any of the copper touches it, the copper oxide can cause the glass to turn blue, so I had to keep adjusting the wires to the correct and centered position as the plaster was setting.

Then I proceeded with the usual 50/50 mixture hand building the mould, I did a couple layers and then put a chicken wire cage around it and put on another couple layers. Once it had set, it was time to steam out the mould. I’ve spoken to a few people about how long one should wait for a plaster/quartz mould to set before they steam out the wax, I was initially taught that it takes 24 hours for the chemical reaction to be 100% finished and if you steam out before then you could lose surface detail. But in my own experience I’ve never had much problem steaming out once the mould has cooled off. For this particular mould I decided to wait a week, when I have attempted this shape in the past, what I found was that in the steaming process the stalactites in the mould fell off, because they got so soft from all the moisture. So this time I wanted the mould to be well set and also to dry out a little. While it was steaming out I checked and rotated it often to ensure that one area wasn’t getting over saturated. And Voila! So far so good, the mould has come out just fine, and its now drying out, getting ready to go in the kiln. All I’ve got left to do is order the glass!


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