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terrim8

Mold Making - Hydrocal vs Pottery Plaster No.1

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I am making a large mold using one of those large plastic buckets that you get from the hardware store. The model fits well except it only has about an inch around the sides for one small portion. I have heard that hydrocal (plaster w a bit of portland cement) is stronger than pottery plaster No.1 and I'm wondering if I should use this because of the thin area. Anyone know? Does it adsorb the casting slip as well as pottery plaster No.1? (The store that sells it isn't open 'till Jan8 so I can mull it over till then.)

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From my understanding of making casting moulds, the thickness is to do with even casting as much as strength.  A thin wall will get saturated with water from the slip more quickly than other thicker walls in your mould.

This might or might not be a problem, depending on the overall size of the mould.

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Sounds like I should try a slightly wider mold - have to go check out the hardware store for flashing or a crazy carpet. I'll keep the buckets for slip. 

One more question : will paper-plaster mix leave holes where the paper slurry bits are? I'm also trying to reduce the weight of this thing. But I have to keep the mold surface smooth, so that is another decision to make before the store opens next Tuesday - Calgary is such a one horse town - no instant gratification here!

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27 minutes ago, terrim8 said:

Sounds like I should try a slightly wider mold - have to go check out the hardware store for flashing or a crazy carpet. I'll keep the buckets for slip. 

One more question : will paper-plaster mix leave holes where the paper slurry bits are? I'm also trying to reduce the weight of this thing. But I have to keep the mold surface smooth, so that is another decision to make before the store opens next Tuesday - Calgary is such a one horse town - no instant gratification here!

When you get this sorted out, I would love a photo.  I am having trouble visualizing.  And I made a bunch of plaster molds this summer.  In the heat,wind,sun.  I am impressed that you are doing this in the winter!!

 

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31 minutes ago, Roberta12 said:

When you get this sorted out, I would love a photo.  I am having trouble visualizing.  And I made a bunch of plaster molds this summer.  In the heat,wind,sun.  I am impressed that you are doing this in the winter!!

 

Will do. New Year- New Project :D I even considered using an ice form as the model but first things first- get the basic process done.

Edited by terrim8

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1 hour ago, terrim8 said:

Will do. New Year- New Project :D I even considered using an ice form as the model but first things first- get the basic process done.

That is an interesting idea!  I wonder if the plaster, when it is heating up, would melt it's mold???  Hey, let me check airfare to Calgary....I gotta see this!

r

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I don't know about airfare but our dollar is in the tank right now - floating upside down.  So things would be cheap for you on this side of the border.

Have to sort out the process though. The ice would melt & maybe with some crazy shapes! Maybe a process where I can get the hardening rate of the plaster to be a tad faster than the melt.  More hair brained ideas to keep me experimenting :rolleyes:

 

Edited by terrim8

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2 hours ago, Stephen said:

I think I might be confused and of course both ceramical and cottle boards would not be of any value for a slip casting mold.

Cottle boards are used and I have some, but this item is bigger than my boards , so I've had to find something  else. Just picked up some crazy carpets at the hardware store - (the kid's tobogganing plastic sheets). They are stiff enough to form a wall around my model.

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When I saw Trudy Golley give a paper plaster demo a long freaking time ago, she did a skim coat or a milk coat of regular plaster over the master before adding a subsequent layer of paper plaster. That way you don't get the texture on the working surface if you don't want it. 

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2 hours ago, Roberta12 said:

That is an interesting idea!  I wonder if the plaster, when it is heating up, would melt it's mold???  Hey, let me check airfare to Calgary....I gotta see this!

r

You can stay at my house and we'll both go help her. We're having ham and pretzel buns for New Years dinner. 

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When she did it, she flicked wet plaster with a gloved hand onto the master she was using, creating just a fine enough layer that the inner surface of the mould was going to be smooth. As soon as she finished the milk coat, she started mixing the paper plaster to apply over top of that.

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Because of the ease of creating ice models from various types of molds in our climate, many people experiment with this. An extreme end member of this activity is ice carving competitions and ice "hotels". The less experienced artists use ice molds - homemade or purchased silicone molds. You've probably seen the floating ice centre pieces used in punch bowls or the ice lanterns along walkways into homes. 

Plaster mold making is a new-ish thing for me and I like to see what is possible while learning how to use this material. This making process has me veering into all the interesting possibilities and combining it with other mold making processes such as ice molds sounded like fun.  This involves experimenting with types of plaster to use - hydrocal or pottery plaster No.1 or paper plaster. Then seeing if you can use ice as a model and testing to see if the plaster then can dry well for slip casting.  My first experiment last night proved a simple ice form can work. Too many bubbles in the plaster so that needs work -  but its a start. 

I'm still making the large model and mold, using a crazy carpet that's wider than the plastic bucket & I'll use the paper plaster to make it lighter - but still haven't decided on pottery plaster no.1 or hydrocal. As usual, I'll probably choose the less expensive option first. I'll run the ice -plaster experiments on a very small scale (paper cups) at the same time until I get it working. The crowd sourced input helps - like being in a room with everyone's experience!

 

 

Edited by terrim8

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