The point of this project was to capture a brass ring in the center of a concrete candle holder. The form is pretty easy to construct, you need two funnels, a base board, a knockout for the tea light or candle, and a few pieces of wood to support the form during casting.
First the tea light knockout is attached to the base board with silicone. This knockout is made of polyurethane rubber and can be reused. The knockout creates the void in the concrete where the tea light will sit. The knockout can easily be made from styrofoam, or by wrapping the foil tea light holder with soft packing foam and sticking it to the base board.
The two funnels are cut down so they’re the same diameter where they need to join together. Drop the ring down onto the first funnel and then run a bead of hot glue around the outside of the joint.
The bottom funnel is stuck to the base board with a bead of silicone around the perimeter. This may be enough to hold it, but a few supports will ensure the form stays together when casting. The supports are glued down to the base board with hot glue.
Two upper supports were added just in case.
The form is filled with concrete and vibrated so the majority of the air bubbles will rise to the surface. The piece can be demolded after it has cured, in 3-5 days.
The plastic funnels need to be slit with a razor blade, being careful not to cut into the concrete. When the piece is still fresh, it will be very weak at the joint, so be careful when removing the plastic.
After both halves of the plastic form are removed, the rubber knockout can be pulled out with an awl. The top and bottom of this piece was polished flat, and diamond hand pads are used to round-over the sharp edges.
The ring is now captive in the piece, only able to be removed by breaking it in half.
