How can I make plster of paris harder? I was thinking if I put some morter mix in with the plaster it would streagthen it. Does anyone know if that would work?

Use patching stuff.

What kind of paching stuff? Do you mean the stuff like fix-all?

This may be a little off-the-wall, but can you "bake" it?

I dont think you can bake this kind of plaster.

Hey, I am all about experimentation!

I believe Keith uses something called dental plaster. I think its a type of plaster used by dentists. Its supposed to be harder.

As far as regular plaster of paris that you already have - try rebar. Cut lengths of wire to stiffen your stucture.
If you want simply harder plaster to help with etching detail - hmmm. Try less water in the mix.
Household spackle is what I think Howitzer is talking about. Might be another thing to try.

Ya fix-all. It as hard as hell.

Thanks for the help guys! I think I will get some fix-all and try that.

Hey Slodder, I was going to suggest that as a joke. It really works?

I've done a lot of research on this... a key finding is that PSI rating not only indicates the amount of strength of the final pour, but also the level of detail... much like resin, which has a PSI in the 22,000 neighborhood, the more dense the material, the far more dense and detailed is your final cast object. Let me share my findings:

Plaster of paris("PoP") has a breaking point rated at 5,000 PSI... not bad, but baking only adds about 10% to that - so you now have a harder, but more brittle shape. Good news is, it''s quite cheap... about $5.00 for a half gallon jug.

HydroCal - a substance popular with the Model Train crowd, has a slighlty higher rating of 8,000 PSI. But it doesn't bake well.... it BURNS if it gets too hot. And it costs about 14.00 per quart jug.

Dental Stone There are several companies that make Dental Stone... the one I use costs about $1.40 per pound if you buy in bulk, but it has a rating of 18,000 PSI... the stuff is amazing! Even without bakling, you have something that has superlative detail, and just darn HARD to break.... It will also set under water, which helps if you are using a hydro-based mold material like Cast-aPlast... normal PoP won't ever harden in a wet mold... it get's solid, but never firms up well enough to pull out without breaking. I use Plast-acast to make my "first run" mold... to make sure that everything conforms well and pulls cleanly before I waste a hugh chunk of RTV Silicon Rubber ($$$).

There are several companies out there that sell dental stone, but the brand I finally settled on after experimenting with several is one I've used for quite some time now. It pours almost "black" but by the time it fully cures (10 minutes working time, 4 hours pull time, and 24 hours absolute cure), it is a light grey in color, and paints very well.

If anyone is interested in buying dental stone, I'd be glad to share... we can buy in larger bulk and get a better price... just drop me an email. Of course, the stuff is heavy so shipping will be a bit more, but I think I could probably ship a 10LB bag of the stuff almost anywhere in the continetal US for around $22. Great stuff, but not cheap!