The reason that plaster moulds work for ceramics is because the clay gets pressed in while soft. The plaster then sucks moisture out of the clay, the clay shrinks slightly, and lifts out of the mould easily. So the mould can be used over and over again.

If you cast concrete into a plaster mould it doesn't act in the same way. Even if you use a release agent (like vaseline, or clay slip) it will be difficult to release the concrete cast without smashing the mould.

A way round it might be to cast a concrete tile from her mould, smash off the plaster, clean up the concrete tile well. Then roll out a thick slab of clay and use the concrete tile to make impressions all over the clay (use the concrete tile as a stamp). Then cast concrete into those impressions. Cover with plastic. When the concrete has cast, strip off the clay, clean the clay off and knead and roll it out into a slab to re-use it.

PS, if you are going to use her plaster mould for concrete, make sure that the plaster can't suck the moisture out of the concrete you pour into the mould. Soak the plaster well beforehand, and coat the inside of the mould with vaseline.

Laura,
I have been successful using a plaster mold such that you might use in making a clay tile. The plaster however does need to be treated with polyurethane or a spray urethane/shellac first, then with a oil of somesort or release agent. The other things to consider are that the mold must not have any undercuts and for the most part needs to be able to be turned over and have the clay fall out when it has been used for clay. Clay does shrink away from the plaster mold.
If your friend was throwing the mold out, first make your self a clay positive, then try the plaster mold after treating it. If it has to be broken you have a positive to make your own latex or silicone mold from then put a plaster back onto it to create a 'mothermold' to hold the latex or silicone mold.
Dena

Thanks for the information. Maybe I will get better at this soon!
Laura

Ive used pop molds and it worked pretty good. Lots of coats of shellac is important. I also have never gotten more than 3 uses out of the pop molds i used. They start to break apart on me. Good luck and let us know how it goes.

carey

I've done several molds for concrete pavers I wanted to look like carved stone. The finished product has been in use for almost 5 years now and still looks lovely even though the hard edges have been worn down somewhat. The design was cut from sheet craft foam and glued to a piece of parchment paper of the type used for cooking. Thin coats of liquid latex were applied until the resulting sheet back was a solid flat piece. The foam design pieces were peeled out and discarded. The rubber sheet is laid on the bottom of the paver mold design side up and concrete is poured into it. Once the concrete is removed fom the form the rubber sheet can be peeled away leaving a sharp relief. Sandy

I'm trying to make a series of 6 "tiles" (6"x6") that are lightweight and can be painted (with a textured spraypaint) and then glued to my kitchen cupboard doors. They need to be fairly thin but also initially soft enough so that I can imprint them with herbs, flowers, leaves (e.g., lavender spires, thyme, oregano, mint, a pine spray, etc.).

Would pop work for something like this? In my day, we didn't make handprints in pop, so I've had no experience with it. Pop handprints I've seen however look thick and rough, which makes me wonder if pop can take the more sensitive imprints of plants.

I tried googling for a tile-making kit and found zillions of references but they all involved embedding a photo in some medium, not making a brief imprint with leaves and stems.

I'd be grateful for any help.

Thanks,

Birch Wind