I wanted to share with you the heavy-duty plaster topped table I designed and constructed for wedging clay.
This is the finished product in my backyard.
It has a couple of benefits, the first is that you don't have to worry about putting reinforcement inside of the plaster since it is sitting on top of 3/4" ply reinforced with 2x2s. The second is that the tops are easy to swap out if you ever need to.
Step 1: Buy Materials
This is the shopping list and cost-estimate I used to purchase the materials. I saved a quite a bit of money by using scrap materials I had around. I specify PT (pressure treated) wood because I intend to leave this table outside. It would probably last just fine with non-PT wood inside. I also had a box of longer screws that I used to toe-nail the 2x4s to the 4x4 posts. The poplar was the least expensive hardwood that the store had.
Step 2: SketchUp Model
There are a few critical calculations that I've done here for you, so I don't recommend tweaking the dimensions.
Also, cut the poplar trim pieces after the lumber frame is constructed based off the real-world dimensions that you end up with.
The 20"x20"x3.5" deep plaster blocks are the size produced from one 50# bag of pottery plaster and 4.17 gallons of filtered water split equally and mixed in two 5 gallon buckets.
I used the 4x poplar to make the mold for the plaster pour and used the same mold twice. These pieces were then incorporated into the finished table, so wood waste is minimized.
For the mold: I screwed together the poplar square-and-flush, then placed it on a smooth, sturdy, LEVEL surface covered with a thick plastic sheet. I then took clay and filled the seams between the wood and the plastic to make sure that the plaster didn't leak out and to secure the mold in place.
I followed the instructions found here for mixing the plaster:
I did not worry about using alcohol to get rid of the bubbles or shaking the plaster once it was poured into the mold since I used the smooth bottom side as the finished top for the table.
Let it cure 2-3 weeks before use.