These concrete jars are simple and cheap to cast, using plastic tupperware forms from the dollar store and a pigmented concrete mix. The wood lids here were cut on a CNC machine, but they're just as easy to turn on the wood lathe, and really - round shapes are better suited for the lathe, it eliminates the tedious hand-sanding needed to clean up the tool marks from the CNC router.

The trick is to find two shapes that fit inside each other without making the walls too thick (1/2") or too thin (1/8"). These were the best I could find, but the piece is still about 3/4" - 1" thick at the base, which makes it a bit heavy for a kitchen jar. The wall thickness is about 3/8", which is plenty strong for a regular concrete mix.

To cast the piece, the form is filled with concrete, about halfway. Then the inner form is pressed down into the mix, and then weighted, making sure it stays centered before it's left to cure.

The inner form is removed after the piece has cured. This is usually pretty easy to do, just pry on the plastic around the sides and pull it straight out. Then the top of the piece is polished flat, using the edge of the outer form as a guide. There's not much draft on the outer form, so it may need to be broken in order to get the piece out.

With the rough polishing done, the top of the piece is taken up to a high polish with a variable speed polisher and a series of polishing pads. The edges are rounded over with diamond hand pads so the concrete doesn't chip and spawl in the future.

Based on the inner and exterior diameters of the piece, a CAD file was made in the shape of the lid with Inventor. This is a simple revolve, and is easy to do in any modeling program. The CAM file is prepared, determining the speed and path that the router bit will take when cutting the wood. The wood is about 3/4" thick and screwed to the router platform at the corners. A roughing pass and a finishing pass are made by the router before cutting is finished. They are cut out with the band saw and the edges are finished on the belt sander. There's a lot of hand sanding to get the lids very smooth.

The lids have a dish in the bottom, and are tapered on the sides to match the profile of the concrete jar.

These jars don't have an air tight seal, and they aren't as light as ceramics, but they were fun to make, and after using them for a year, they work just fine for me. I really really really really like the color of Peruvian Alder, which is shown here in the first and last pictures.