plaster

[Laurie Spray]

What is the best release for a plaster mold taken off of a plaster master? I have lost one and do not want to lose another!!!! Thanks!

[Lauri Levanto]

I have used several alternatives..

Liquid soap is a good one.
If the piece is dry enough, any oil works well.
On a wet surface oil may separate and leave gaps.
Even a slurry of clay may work, if you manage to open when it is still wet.
It is the most difficult, as it deters air flowing into the seam. It also tends to mask details.

[Jim Murphy]

What is the best release for a plaster mold taken off of a plaster master? I have lost one and do not want to lose another!!!! Thanks!

Hi Laurie, typically a plaster master is first sealed with a suitable liquid soap [e.g. green tincture soap, or Murphy's Oil Soap], then
a release is applied such as vaseline or commercially available silicone emulsion.

Plaster-on-plaster though may be impossible if there are undercuts. Enter rubber moldmaking to the rescue!

Good luck!

Jim

[Kevin Midgley]

spray paint the plaster and then use the above suggestions.

[Laurie Spray]

Thanks.....no undercuts.....all good ideas.
Never thought of paint first. So that sort of seals the plaster?

[Peter Angel]

Thin clay slip (very watery clay) is the best. Nothing beats it.

Paint the thin clay slip on, then immediately apply the plaster. Wait until the plaster hardens then dunk the whole lot in water. Wait a little while and it will separate out easily.

[Lauri Levanto]

Thanks Peter,
as a self tought, or really by reading a lot.
I have had trouble with the clay slip, because i had not learned
the "dunk in water phase.

-lauri

[rosanna gusler]

spray paint/laquer and silicone spray over that. rosanna

[Laurie Spray]

Hmmmmmm.....dunk in water.......so do you "dunk" the mold then paint on the slip? Let it dry then spray? Interesting. I never hear about the dunk!

[Bert Weiss]

One trick I have seen used, is compressed air. You blow strong air at the seam and it gets in there and pushes the pieces apart. Of course, this assumes you used an effective separator system. I was impressed by this, as the 2 parts were not going to just pull apart by hand. I saw this done at Pilchuck.

I am a big fan of rubber molds. They are relatively easy to work with, flexible, and can be forgiving. I have worked with rubber molds, with undercuts, that did break the plaster. If you need a forgiving mold, thin rubber backed up with plaster, for stability can work.

[Morganica]

Compressed air works, slip works better (especially if you first apply butchers wax or liquid soap and THEN apply the slip).

Personally, I'm not a huge fan of plaster to plaster mold making. Plaster masters have very little tolerance for undercuts or even 0-drafts so I typically wind up re-engineering the model for plaster (and given the number of undercut details I typically use, that's a major headache). Plaster molds can be heavy and awkward to work with once you're working with larger pieces, they're fragile to store and I always seem to knock them at exactly the right angle to pop off a detail, so they don't seem to last.

I much prefer using flexible RTV silicone/urethane for a master mold. If I'm trying to make a positive mold (i.e., a mold of a mold), I'll make the negative with elastack. It's a remeltable flexible mold material (rather like Gelflex from the UK, or composimold). Then I pour the silicone into the elastack mold, pull it out, and melt the elastack back down.

(On the plus side, plaster molds are cheap and they make great wax molds--wax poured into a wet plaster mold has a beautiful surface and if you dunk the whole plaster in water the wax will float right out)

[Laurie Spray]

Thanks! I do know the air compressor trick. My problem project was pottery plaster to pottery plaster with mold soap in between....... Obviously i will do more next time! Mold soap alone did not work.

[Morganica]

Thanks! I do know the air compressor trick. My problem project was pottery plaster to pottery plaster with mold soap in between....... Obviously i will do more next time! Mold soap alone did not work.

Well, the big deal is the what you're making the mold of...if neither side is flexible, really watch your shapes.

[Laurie Spray]

It is shallow with no undercuts...just some subtle texture...should be so easy. I will be doing it again today with suggestions from here!