Re: Hydroperm Plaster


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Posted by joe wizard on December 29, 1999 at 17:17:36:

In Reply to: Re: Hydroperm Plaster posted by Jesse Brennan on December 29, 1999 at 10:14:07:

Jesse, the one to one ratio may not be the best. However there is no sign of left over water, like one would see with Plaster of Paris.

My guess is that variable mixing time/violence is the way one controls the amount of permiability
One necessity seems to be to continue mixing untill the batch turns into a mildly stiff cream, able to support a little 'peak'. This prevents the heavier stuff from migrating to the bottom of the mold. That takes about 10 minutes of mixing with a one to one ratio.. too long a time, if you ask me. My first under-mixed batches seperated.. a layer of heavies at the bottom, a layer of water, and above that, the mass of expanded plaster foam.

There is so much air in it that, the hardened mold is soft to the point one can easily push a pencil into it. I think that any cores have to be well suported by metal pins or the like.

First burn out was a flop so i didnt pour (100% plaster, no grog/sand .. the model was a hollow ball.. the core, supported by a 1 inch hole through the wax ball, seperated, probably cracking off during the burnout).

There was no exterior mold cracking. I broke open the mold, and examined the interior surfaces. Some surfaces were nice and smooth, but some were all bubbles. By the way, this stuff absorbes 95% of the wax, so dont wait around for it to drip into a pan. I just set fire to the smoke and let it all candle away in the oven.

I'm looking forward to your further posts. In the meantime, I will make some spare time, I'll do some further experimenting with water ratios/mixing times, etc..

While you are in SF, visit Douglas Sturgess Co, 730 Bryant Street ( . Lots of molding supplies and some interesting hard to find things.

See ya.

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