I had so much fun making the original hypertufa millstones that, using the information gleaned from that project to improve my design and methods, I made three more.
In the first trial, I used a method of placing the middle mold (a tin can or plant pot) in the center of the large round, and covering the entire thing with a large sheet of poly film.
Two things happened here; one, the wrinkles in the poly were too big, and ended up actually encased in the hypertufa mix as it dried, and two, when I tried to pull the wrinkles out, it caused large chunks of the mix to come out too.
This could be an issue if the mix isn't strong enough at this stage - you could lose the entire project to catastrophic fracture.
Here is the new method of placing the inner mold - I've wrapped it in its own piece of poly, and although it could still have smaller wrinkles, I don't believe that it will be as much of a problem when removing it. Time will tell.
Important; wiggle the inner mold around before the mix sets up. This ensures that there is enough room to remove it. Ask me how I know this.
The textures, like this traditional one from an example of a real millstone, are scraped in while the mix is still soft - this was done after a few hours, then again the next day. One more scraping will take care of the lumps.
One of my favorite designs is a fern leaf - I use this motif a lot in my crafts, so why not here too?
Organic swirling lines are another of my favorite textures.
Here are my collection so far; there are seven millstones gracing the edge of the pond. Winter will be here soon, but I'm not worried - there is nowhere for water to collect and freeze, so there won't be any cracking.
Next year they'll start to show some age - maybe I'll even plant some moss on them.