Casting Concrete in a Plaster Mold

Here is a Question I Was Asked by a Reader. 

I hope everyone will find it useful.   

Hi Del,  I have a plaster mold and wanted to know if it is possible to cast concrete in that mold and how to do it.  Thanks,  Gabriella.   

Hello Gabriella:  I would like to thank you for visiting my mold site.

Now for your question about casting concrete in a plaster mold.  I presume you mean that you have a mold that was made to cast plaster in, not a mold that's made out of plaster?If the first assumption is true (mold for casting plaster in) then I don't see any reason you couldn't cast concrete in it.

Is it a plastic mold?  I think that if it was made strong enough for plaster you should not have a problem doing concrete.To be safe you could set up a "bed" for your mold in a box with wet sand.  Wet the sand until it will hold it's shape when you squeeze a handful.  Use this sand to pack around your mold to help support it.

You will need to use a release agent in the mold - I use 30 weight engine oil and recommend it to everyone.  If you go to Wal-Mart and get the cheapest stuff you can find. A quart will last forever.  Maybe not that long if you do a lot of casting, but you get the idea.  Paint it onto your mold and let it sit about 5 minutes.  Then wipe off any puddles.  Puddles will leave marks on your casting.  Don't wipe off all the oil - just mop up the excess.

Now you can mix up your concrete according to the instructions on the sack, maybe use a little more water than it says.  You want a mix that is about as thick as pancake batter.  Pour this into your mold starting in the center and letting the concrete find its' own way to the outsides of the mold.  If you have to you can move some by hand with a big spoon or something.  Try do it evenly so you will not bring any air into the mix.  Now cover the mold with a piece of plastic so it will cure slowly.  I say at least 2 days, keeping it damp all the time, 3 or more if the temperature is below 70 F.  If you are not sure it is cured wait a little longer. 

There is nothing worse than breaking your casting because you where eager to see how it turned out.  Believe me - I have broken more than I like to admit. 

Good luck to you and best wishes.  Del