In my last post, I mentioned using cardboard tubes as curved molds for slabs of clay. Here’s an example of a vase I made using this technique. I rolled out slabs of clay, then laid them into the half-tubes. Once the clay was stiff enough, I took it out of the tubes to work with it. I found the cardboard tubes to be problematic, though, because the moisture from the clay caused them to lose some curve. So today I decided to make some plaster hump molds. Hump molds are designed to lay slabs of clay over, as opposed to slump molds which allow you to slump a slab of clay inside.

First, I laid slabs of clay into the tubes. Then I put more slabs at the ends and sealed up the cracks with coils of clay, then smoothed it all out to make a nice surface for the plaster. I let those sit until the clay was a little past leather hard, not dry yet, but not quite pliable.

Then I assembled all of the materials I needed. Everything must be within reach when working with plaster. It starts to set quickly so I didn’t want to mix it until I was ready. I measured the water and weighed the plaster - two gallons of water, 22 pounds, 13 ounces of plaster. Then I mixed the plaster and poured it into the molds. Here is a picture of the large mold, then the measured water and plaster, and the molds once the plaster was poured:

Here they are after setting up and being removed from the molds. At this point they are still pretty wet, and have some clay residue left on them. I’ll sponge them off and allow them to dry for a couple of days before I use them.
The finished products were one large and two small half-round molds and a tile mold. One great advantage of using a plaster hump mold is that a slab of clay will dry much faster than another material. The plaster actually pulls water out of the clay. These molds will cut the waiting time down when making handbuilt items.