Gabriella Emailed Back...

Nice picture - not relevant, but it's my site so...

Hi Del, Sorry not to be more descriptive. 

The mold is made out of plaster, I think to cast ceramic.  So I wanted to know if I could try concrete in it. 

The other thing is I have been casting stepping stones in plastic molds and I am using a mortar mix with no gravel should I be using gravel? 

In sealing the stones do I just seal the top side? 

Thanks,Gabriella

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Hello Again Gabriella:

Thanks for writing back with your question.  I was looking for a topic for a new article and I think I found it.  I hope you don't mind if I use your name, if you do please let me know and I will substitute another name.

First - your plaster mold. - plaster molds are used for ceramic like you said.  The plaster absorbs the water of the ceramic mix, that is how it cures.  You don't want to have the water absorbed out of your concrete mix or it will have no strength.  Water is essential for curing concrete. 

In order to prevent this you'll have to seal the plaster mold.  My suggestion is see you get a real good epoxy paint and coat the inside of the mold with it.  To get a nice smooth finish I suggest a spray on and use a couple of coats.  Once you've done that she should that will pour concrete in your plaster mold with no problems.  You'll still need to use a release as I suggested in our previous conversation.

Second - re: casting the stepping stones-you should definitely be using some gravel.  They should be cast at least 2 inches thick, three would be better.  Mortar mix by itself has very little strength.

Third - as far as sealing them goes I personally don't do any sealing.  If you're doing it to get that nice shiny appearance is no need to do both sides. Just do the top and the sides.

I hope this helps,   Let us know and maybe send a picture. 

Regards,   Del