After constructing the wooden forms, we use a concrete mix consisting of Portland Cement (made partly from ancient sea shell deposits), sand (for strength), and rock salt! When the structure is placed in the river, the rock salt on the surfaces melts and leaves "nooks and crannies" ideal for vulnerable oyster embryos to hide from crab predators until they can form a shell.
See the following pics to see the process.
Vegetable shortening is used to grease the wood surfaces to help the concrete slide out when ready
Rock salt (intended for home water purification filters) is added to the concrete mix prior to adding water
Form for 4 oyster blocks filled with concrete mix. Wooden blocks are used to support the form when it is inverted.
Once the concrete has hardened adequately, the form is inverted and the plywood and outer boards are removed by unscrewing the deck screws.
The blocks are set out to harden further before being removed from the internal portion of the forms. Removing the blocks can be problematic. If the internal forms are not square and have an outward flare of any of the four walls, the blocks will crack as they slide off the form. It is important to construct the internal form nearly perfectly square or at least tapering to a smaller diameter at the bottom end of the block so the top of the block will slide off the end.
The addition of rock salt changes the chemistry of the concrete mix somewhat and makes the concrete dry out/harden more slowly. Once the blocks are removed, they are exposed to rain/water sprinkler daily to wash off the high pH dust that forms on the surfaces. This process is called "curing" the concrete. Cured concrete makes the block immediately habitable to oyster embryos before they are placed on the river bottom.