I have been reading this forum for some time – trying to learn enough so that I can create my own projects from concrete. I have read several places about the question of “can you use ceramic molds with concrete?” I have about 1500 ceramic molds – I do pottery and ceramics with slip. I did have about 2400 mold but have been selling them for 1.00 each so that I can have some room in my shed and I can’t lift heavy molds any more.
I decided to do an experiment with some flower pot ceramic molds that I am not that attached to. I mixed up some Portland cement and sand (didn’t take the time to follow the directions on the proper mixture for “hypertufa” at that time because I was in a hurry – it was getting dark). With one mold (the first photo) I put a 13 oz trash bag into the mold and pulled it down around the outside of the mold to protect the mold – there was so much of the trash bag left over that it worked out well that way. I then put a bowling ball into a Wal-Mart bag and sat it on a small medicine bottle that I had placed in the bottom middle of the mold. The medicine bottle kept the bowling ball from settling in the bottom of the mold making the concrete too thin in that location. I then poured some of the concrete mixture down into the bottom of the flower pot and around the bowling ball. I let the concrete dry over night.
The next day I opened the mold and it came out very easy BUT the concrete had picked up the print of the wrinkled trash bag instead of the mold. Also, I could not get the bowling ball out even by pulling on the Wal-Mart bag it was wrapped in BECAUSE the top of the flower pot MOLD curved in slightly. So I tried chipping around the top of the bowling ball but this eventually broke off edges of the concrete. Once I finally got the ball out I chipped off the top part of the concrete and just made a half ball – but it will still hold a plant. I can fix the broken top piece (see photo) with mortar because I plan on covering it with mosaic.
The mold was not damaged because the bag protected it. Next time – instead of using a bowling ball, I would use a ball that I could let the air out of OR I would use a Wal-Mart bag and fill it with sand so that I could pour the sand out once the concrete dried enough. I would also be very careful to follow the directions for mixing hypertufa.
The second photo shows the second smaller flower pot that I used vaseline on instead of the trash bag...