Easy DIY Slab Mold

A beautiful day can be a kind of a burden, as I almost feel guilty for spending it inside. On the other hand, if I go outside and just noodle around in the garden, I feel guilty for not getting my studio work done! Can't hardly win. It's too early to move into the summer studio, but I did find an outdoor clay project for today.

I've been wanting to make some slump molds for slab building out of some leftover insulating foam I have kicking around the house. It's a good material to use, as it is cheap, waterproof, and can be cut in any shape; although the cutting was the hold up. The molds I had seen had very rough edges, requiring a lot of finishing work at the leatherhard stage. A student of mine (God Bless my students -- I learn so very much from them!) who usd to be a set designer suggested cutting it with a hot wire, to get a nice clean line. The wasn't feasible (for me -- but I'd love if someone tried it, and let me know the results!) but a hot fettling knife, now, that I can do. This is strictly an outdoor project, as the fumes generated by melting the insulating board are nasty.  

Draw the shape you want beforehand. Conveniently, I am firing a bisque today, so I just stuck the blade of the knife right into the flame to heat it up. It doesn't take long - 10-15 seconds -- and if the metal is glowing, the blade is too hot. The foam will melt before the knife even touches it.

The knife when it first comes out of the flame, will cut more quickly than you expect: even faster than a hot knife through butter! But it cools quickly, and there is a relatively short period between "too hot to control" and "too cool to cut cleanly."

Here is my first effort. It looks a little sloppy but made a good mold. I think with a little practive I can get a cleaner cut. Notice the beveled edges. These are important, because the slab of clay is going to stretch at the edges of the mold. The bevel keeps the stretch from being too extreme, and tearing or (later) cracking the slab.


After draping the slab over the mold, pick up the two together and drop onto the (porous!) table top. This will cause the slab the slump down into the mold.

Voila!