mending cracks in plaster mould

[H Palmer]

HI there... i just dried out a large mould in my oven,  I  think it was hotter than I thought, or the mould too close to bottom of oven as it was only on 50..Anyway,  end result is that there are a few cracks in the bottom of the mould... they are on the underside of the mould as well.  I have looked up how to mend plaster moulds;  nothing for mending cracks so what do you all think?

'Mending a broken mould'  advice suggests soaking the mould in a bucket of water until completely saturated, then using a plaster mix as paint to stick the broken pieces together.  i am unsure what would be the best way forward... soak the bottom of the mould a bit and add a thin layer of plaster perhaps... 'mending cracks in plaster on a wall' advice suggests opening up the cracks then adding filler to the cracks... to me that sounds like good advice, I might open up the cracks, wet the plaster and then fill the cracks with plaster mixture... 

Its worth a try before throwing it away I think... 

 

 

[Mark C.]

If say you break a mold in 1/2 you can use waterproof wood glue to glue it back together . I do that with throwing plaster bats. In terms of repairing a cracked (not broken mold) I know of no fix that will work well so try anything but expect it to fail. Molds should not be overheated but you now know that. Ceramics is hard lessons learned and you are on the right path.

[Jeff Longtin]

Heating a mold - so you know, 120 degrees is the highest temperature you should expose plaster too. (Beyond that it starts to break down.) 

The cracks could indicate a problem that simply gets worse, and a new form needs to be made, or you may get lucky, and they don't increase?

I find Elmers glue to work well to temporarily repair cracked molds. (Elmer's is a simple, white, water soluble glue, widely available in the US.) However, once the plaster becomes sufficiently wet, the glue sometimes loses its bonding ability and the pieces separate again.

[Mark C.]

The glue that is a step above Elmers is Titebond wood glue. Its like Elmers in all reguards except its waterproff when dry

[H Palmer]

Trouble with using glue on the inside of the mould is that it will make the plaster un absorbable .... I'll try a thin layer of plaster i think. .. glue on the underside is probably fine.. 

 

Edited May 19, 2023 by H Palmer

[Mark C.]

Glue is for broken molds (two pieces ) not repairing cracks. I do not think its possible to fix cracks. Spend your time make a new mold

If you fine a miracle cure let us know.

[Jeff Longtin]

Mark makes a good point. Glue is for broken molds, two pieces, not "cracked".

Cracks, on the inside of the mold, can sometimes be filled with a thin slurry of plaster, but sometimes not. It's usually best to apply the plaster in a water like consistency and build it up in layers. This repaired crack will likely be a spot of sticking, for the cast piece, but after several castings the sticking should decrease.

 

 

[RobinFlanders]

What type of mold is it? If it's a two part, and the cracks are somewhere that won't affect the mold locking together you can moisten the plaster with water, and make a small batch of plaster and using a small tool like a stick, just kinda dot the plaster in to fill in the cracks, and then swipe the surface to make it even. It's not a perfect method, but I've used it to fix little issues before.