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Plaster mold making problems

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Hi Folks,

Pretty new to making plaster molds for slip casting. I have had some success but one recurring problem... sometimes when I take the item I am casting out of the mold, the plaster seems to tear, leaving an uneven surface. Many little bits of plaster are stuck to the original item, leaving lots of little divets in the mold. What is going on here? Am I demolding too quickly? Not mixing the plaster thoroughly enough? Mold release not working?

I demold when the mold is warm to to the touch, about 40 min after pouring. 

I mix the plaster with a paint mixer on a drill, 2 min for 0-20lb, 4 min for more. I do not have a good system for sifting the plaster into the water, however.

For mold release I brush on several coats of 50/50 Murphy's soap and let it dry. This has worked well for me, after many early failures with other methods. What I found in those failures though was very hard to remove the item from the mold, not that the plaster would fail in this way.

Any insight appreciated!

 

 

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Several ideas

1st and most important is your #1 pottery plaster new or old???It is #1 pottery plaster??

2nd no need to seive it-which leads me to belive its lumpy and old .

You should add it slowly to the water while mixing.-Two personsreally help here-shake it into the spining water in a larger bucket

I use a jiffy mixer in a cordless drill.

If its lumpy toss it out and get NEW plaster for molds-old plaster will be trouble alway the way along-Are your molds made from old plaster???

I'm not a fan of murpys oil soap-use lube  made for mold relase which also will not clog the plaster pores

this is made for that job

https://www.axner.com/pure-lube.aspx

I was in the slip cast business once and making quality molds that work is the whole deal. The right ingredients make the right molds

3rd Neil posted this in another thread

(With #1 Pottery Plaster, you'll need 16.316 grams of plaster per cubic inch. Plaster to water ratio is 10:7 by weight.)

this is the fomula you need to use always-weight the ingredients

4th I assume the plaster coming loose is in the working molds ???not the master?

Master are made from rubber and will not shed plaster

if thats true  that the working molds are shedding  sounds like molds where made with old plaster or are super wet when poured?

to dry your molds blow air over them with a box fan

any of this help?? let us know

Edited by Mark C.

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I’m not sure of the terminology but my post is about making the mold itself, not pouring slip into the mold. Most recently I was making a mold from a large turtle shell. I am using #1 pottery plaster and it is not old— the place I buy from seems to run out frequently so I assume it is fresh. I mix by weight, 7 parts water to 10 parts plaster. I can try adding more slowly... instructions I found are tight on time though. It does not seem lumpy when I pour it.

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Ok 

Does the mold have some small undercuts which are catching the plaster?

Turtle shells have under cuts-so are you just casting the top of shell with no undercuts.

I would let the mold cool off before pulling it as its expanded when warm and will be harder to remove.Also the plaster needs time to get hard and its soft when warm still.

I would still add the paster fast as its a time thing and you need to be fast.

I drop the bucket on a firm surface after power mixing to raise any air bubbles to the top before pouring

as to pouring start filling slowly so air does not get trapped in mold. Once covered our faster.

 

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+1 to everything @Mark C. has said. 

Another thing you could try, apply the first coat of plaster with a large soft brush, to make sure it is well covered, and no air bubbles.  When pouring the plaster, make sure it does not hit and bounce off your model, but hits the baseboard or the side walls/cottles.

I don't unmould until the plaster is cold.  I do occasionally get a mould with tiny pinholes, but they don't affect the finished pot.

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I think you need to take the first impression not with plaster.  I was buying silicon products from Smooth On but found ammonia latex works better.  You just build up 4-5 layers and then it peels right off.  Then you get the plaster off the latex mold.  Of course, now you're working with a negative, so you might have to go twice.  I've been using this approach to take textures off some difficult rock specimens, like crystals and it really works.

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Thanks all for the adivice! I think I may be demolding too early, it seems like the problem is the plaster not setting well or not fully set. Somewhere I read that I should demold when the mold is warm, but perhaps that’s wrong, has anyone else done that? Could also be too much soap, thats a good thought. I have been avoiding oil-based mold release agents on the advice that the oils can clog the pores of the plaster making the mold less effective, but maybe it’s time to give those a try.

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It probably works for me because I seldom (never) do slip cast objects and do mostly press molding with my plaster casts, I’m only interested in the quality of the mold surface and if the object releases from the mold, dryings not an issue and so far I’ve never had issues attaching parts or with the firings because of the oil. I guess I’ve used it for years because it good on the skin and was handy one time when I needed something not knowing any better, I’ll have to try the soap.

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