Step 7 - Partial Curing
Place the molds in a shady, cool location and cover the molds in plastic. The plastic will allow the mix to cure slowly, which will create a stronger product. Drying too quickly could result in cracks. If you are having hot weather, mist the surface of the mix every few hours with water.
Step 8 - Carving
After 3-5 hours, it is safe to carve a millstone pattern into the top of the hypertufa mix. The surface should feel slightly firm to the touch, but easy to make an indentation with your finger or a utensil. With the pointed end of a metal file, carve your desired pattern into the top of the millstone. I googled millstones, and found all sorts of pictures of antique millstones, most having different patterns. I chose to do a simple curved pattern.
If you make a mistake in your pattern, simply spray the surface of the piece with water and smooth the surface with a gloved hand until you no longer can see your carving. Let the piece sit 30 minutes and try again.
Step 8 - Unmolding
After 24 hours, the mix should feel firm to the touch and it should be safe to unmold the pieces. Turn the pieces upside down and gently work the piece out of the mold. The pieces are very fragile at this point and if they are dropped they will crack and break. They should come out fairly easily if you used enough oil on the molds in the beginning. If you wait beyond 24 hours, you may not be able to drill out the center hole as the hypertufa will be too hard.
Step 9 - Softening edges and drilling holes
Once the pieces are out of the molds it is very easy to soften the edges by running the metal file along the hypertufa mix. Be gentle, you don't want to take off too much, unless of course you are going for a more distressed look.
With a ruler, find the center of the millstone and make an indentation with the pointed end of the metal file at the center. Attach the hole saw bit to a drill and drill out the center of the millstone using a slow to medium speed. You may have to stop halfway through and remove any hypertufa mix from the hole saw bit.
You can use any size hole saw bit you like, depending on what you want the finished product to be. If you are not sure, you can start with a smaller hole. You can always drill out a bigger hole if you think it's too small.
Step 10 - Curing
Once you have the edges knocked down and the hole drilled out, spray the pieces with water and lay them flat in the shade with plastic on top of the pieces. Spray the pieces a couple times a day if temperatures are above 80 degrees. After 7-10 days, you can display the pieces in your garden, but they will not be fully cured for 30 days. If you are making these to sell, wait the full 30 days to help prevent cracking and breaking.
Step 11 - Maintenance
Once your hypertufa millstone is cured, there is basically no maintenance involved. In fact, you can even leave it outside in the winter. If properly cured, you should not have any trouble with cracking.
If displayed in the shade, any hypertufa piece may begin to grow moss if it's damp enough. Either move the piece to a dryer location or allow it to grow.