I wanted to share what I thought was a rather clever trick involving Legos and moldmaking. But as I was taking pictures of the process, I realized that it might not make sense unless someone already knew how plaster medallion molds were made. Well, that and I wasn’t completely sure that my “discovery” was anything new to other moldmakers, because when I figured it out it was rather obvious. Maybe I am just slow!
So I decided to post a general how-to on ceramic medallion molds, using Legos instead of traditional mold boards. This is going to be rather long, so I’ll split into two posts. In this first one I’ll talk about slipcasting molds in general, and in the second I’ll start posting the step-by-step pictures.
The first rule of mold-making is that something has to give. Our plaster molds work because the leatherhard clay inside will give. If it didn’t, the casting would remain inside the mold unless it (or the mold) was broken. Likewise, when we make the initial plaster mold the original must give. It is possible to make a plaster mold from a plastecine clay original, but not from a hard resin copy. The resin cannot give, so removing the original will damage the plaster mold. This is our first constraint – one half of the equation (the mold or the casting/original) must be made from a flexible material.
Our second constraint is the fact that slipcasting wears plaster down rather quickly. The plaster wicks the water from the slip, which erodes the surface of the plaster. Over time this will eliminate detail. The difference between a new mold and one with the detail eroded is what makes some Hagen-Renakers “crisp” while others are softer. With artist-produced ceramics, we stop using the molds when they start to lose detail – when they no longer produce “crisp” copies. That means our molds are good for around 20 copies. This dynamic is why some ceramic horses have been released in extremely small editions. In those cases, a plaster mold was made from a clay original. Since the original is usually destroyed in the process, there was only the one mold with a very limited number of castings.
If we want to make more than twenty horses, we have to make a production mold. In short, we need a mold to make more molds. That is where rubber molds come in. Rubber gives, so we can pour plaster on it. And unlike the clay original, it survives the process so we can use it again and again. All we have to do is replace the artwork with a rubber copy, and stabilize it so that it gives just enough to make the mold, but not so much that it distorts.
That’s what these are – production, or master, molds. They are rubber molds with a plaster back to keep them stable. In the next post, I’ll start the step-by-step of how one of these is made, only we’ll replace both the mold boards and the stabilizing plaster with Legos.