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Hi I am hoping that this is the right place for this question. Is there someone out there that is a wizz making moulds. I am having problems making moulds off a plaster master,it is a large mould and I am wasteing a lot of time and materials not having success. most of the mould is ok but I am having problems with small craters in some parts of the mould. I think the problem is air coming up through the master and not allowing a nice smooth surface.

I first soak the master , then soap it up 3 times then carefully tip the plaster mix into the prepared  frame. 

I wonder could I spray the master with some sort of "varnish" or something that would seal the plaster master, I would like some advice as I do not want to spoil the master as it would be very difficult to reproduce.

Thanks to anyone who comes up with advice on this 

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How do you pour your plaster? If you pour it directly on top of the master it will definitely leave air pockets. Most start pouring the plaster in the corner of the mold slowly and let it roll over the master, this usually helps. It also doesn't hurt in my experience to take a small soft brush and gently tap the actual model after pouring a thin layer of plaster. That way you can break  any bubbles for a smooth mold surface without damaging the master.

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Thanks, the mould is round and takes almost 2 gallons of plaster to fill, I did pour at the side so it came up and over the master and I did brush it. Do you know of anything I can seal the master with before I try again. 

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I have been learning from my 16 yr old granddaughter the modern way to make molds. With a silicone mold kit for under $50 at Amazon you can brush a 1/4-1/2" thick coat of silicone. When it's cured, but before you remove the mold you make a two-piece plaster coat about 1" thick to keep the silicone rigid when you pour into it and for storage. The silicone can be peeled from your master without damaging it.  Saves a LOT of plaster weight. The products we've been using are by SmoothOn. They have many useful videos on their website. Granddaughter sculpted a wolf mask for Halloween and made the silicone/plaster mold herself following the videos. I cast it in resin for her for the mask. 

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On 20/01/2018 at 8:11 PM, Min said:

I would dry the master out, seal it with a good quality shellac, then use the mold soap/lube.

Thanks min I think I will put it in the dryer and give this a go ,do you know if I will need more than one coat of shellac

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1 hour ago, Rae Reich said:

I have been learning from my 16 yr old granddaughter the modern way to make molds. With a silicone mold kit for under $50 at Amazon you can brush a 1/4-1/2" thick coat of silicone. When it's cured, but before you remove the mold you make a two-piece plaster coat about 1" thick to keep the silicone rigid when you pour into it and for storage. The silicone can be peeled from your master without damaging it.  Saves a LOT of plaster weight. The products we've been using are by SmoothOn. They have many useful videos on their website. Granddaughter sculpted a wolf mask for Halloween and made the silicone/plaster mold herself following the videos. I cast it in resin for her for the mask. 

Thanks Rae can't quite get my head around this method to produce a mould for slip casting from what I have but off to look at the smooth on website.

 

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26 minutes ago, juliad76 said:

Thanks Rae can't quite get my head around this method to produce a mould for slip casting from what I have but off to look at the smooth on website.

 

Would depend on complexity and thickness of casting for a peel-off one-piece. They can also be two-or-multi-part molds. 

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5 minutes ago, Rae Reich said:

Would depend on complexity and thickness of casting for a peel-off one-piece. They can also be two-or-multi-part molds. 

Oh, duh! What was I thinking? Of course it wouldn't absorb water. But you could duplicate your master in plaster, just-in-case. Sorry for the brain-blank.

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You never said what your master is made of-most masters are made from rubber-I feel yours may be plaster???

I use as well as  pros Polyurethane parting compound-this may be hard for you to come by-a gallon lasts about a lifetime . Its made to seal plaster and rubber masters.

I would do as Min suggests above dry and shellac.

Then what has not been suggested is just after pouring your plaster from Corner of mold vibrate the whole table to let the air bubbles rise -a small hand sander works well for this or you can tap or beat on table. Make sure your mold can take this vibration. This is a tip from a pro I learned long ago.

I had a mold business-getting for a decade and we did this every year with all working molds-they where all 3 piece.We cast aroma therapy lamps for Frontier Herbs (they are in most Health and organic markets with herbs today)for just over a decade-cone 10 wares in porcelain-made way over 10'000. of them.A small aroma therapy company near me contracted us to supply them with lamps and they got bought up by Frontier after a few years for 5 million$. I have 4 left on a shelf to remind me never to get into slip cast business again.

Edited by Mark C.
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1 hour ago, juliad76 said:

do you know if I will need more than one coat of shellac

Probably. Spray shellac isn't going to go on as heavily as brushed on so what you use will make a difference. I'ld put on a coat, let it dry then dribble a little water on it. If the water beads up and runs off then you're good to go, if not then put another coat on. 

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