These are the two concrete dishes that I've made so far. I used an expensive concrete mix designed for repairing cracks in the driveway and such. I suspect that any cheap sack would work fine also, but I wasn't sure before I tried making these. I would recommend though, that you choose a finer grit over a courser one. This is a new experiment for me and I'm learning as I go. It seems to be working pretty well.

For a mold I used a plastic wash tub similar to the green one in the picture that I'm using for a steam chamber, only it's larger and made of a thicker material. As you may see, the dishes are very slightly concaved. After steaming the plywood blanks, I place them between the two dishes and let them set for about 15 minutes.

The dish on the left, I made first. I added some bits of styrofoam to the concrete mixture to make the dish more massive, yet lighter for easier handling. I found out that this was somewhat impractical, having ended up with all the little craters on the surface. Fortunately, The weight of the concrete in the plastic tub yielded a nice subtle convex shape on the lower surface, which came out plenty smooth. For the second dish (on the right), I used straight ready-mix and placed the first dish on top of it to mold a better and more shallow concave on the top of it. I still would have liked it to be smoother, but this one has turned out to be sufficient for its purpose. After pouring the second dish, I placed a thin sheet of plastic between the two, to keep them from sticking together. John V. This is the previous method I used for tuning steamed boomerang blanks. It works too, but timing is more critical.

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