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Old 10-03-2017, 05:29 PM
Chris Lowry Chris Lowry is online now
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Plaster silica

So anyone out there do plaster silica kiln casting? I'm trying to order some supplies but not sure what to order.

I know dental plaster is good. Is that the same as hydrostone plaster? I read that somewhere but not sure if it's true.

I know casting plaster is good as well but again is that the same as 20 minute casting plaster?

Everyone calls things by different names so it's always hard to figure out what to use.

Thanks for the help
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Old 10-03-2017, 07:45 PM
Rick Kellner Rick Kellner is offline
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Although I haven't tried all of them in terms of kilncasting, I suspect that they will all work to one degree or another.

From my experience in the dental field though, I will say that the stuff you want is known as "dental stone." The dental plasters are still pretty soft (for articulating models), whereas the stone is some seriously strong stuff. You will usually find it marketed in various pastel shades (i.e. yellow, blue, green) compared to the white plaster.

There are many formulations, some of which apparently have resinous binders to enhance the formulation.

Throw some sand in with dental stone to give it refractory properties, and I believe you will have a combination that would be hard to beat for casting with glass.
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Old 10-03-2017, 09:42 PM
Rosanna Gusler Rosanna Gusler is offline
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I find that using alumina hydrate instead of silica makes for better castings because of less flashing from quartz inversion issues.
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Old 10-04-2017, 08:35 AM
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I keep getting bags of calcium aluminate cement from Hi Temp in Fenton. It's the stuff that makes castables set up. I'd add some to the plaster.
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Old 10-04-2017, 09:40 AM
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Are you just trying out kiln casting? Or are you specifically trying to use dental plaster? If not and if you start with small molds, I would just use a basic 1:1:1 ratio.

1 cup of Pottery Plaster or Plaster of Paris
1 cup of Silica Flour - otherwise known as Flint (300 mesh)
1 cup of water.

Get the plaster and silica from your local pottery supply store
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Old 10-04-2017, 11:03 AM
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Franklin Sankar Franklin Sankar is offline
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I asked if you can add expired refractory cement to the silica plaster mix and the answer was no.
Not sure why.
Franklin
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Old 10-04-2017, 01:01 PM
Chris Lowry Chris Lowry is online now
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Yea just trying it out for the first time. I have to order wax as I live on an island and it's easier if I can get everything at the same place. I was going to use the 1-1-1 ratio.

The store sells many kinds of plaster and just trying to decide on which one.

Does anyone know about hydrostone specifically?

Pete, how much... instead of silica?
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Old 10-04-2017, 01:09 PM
Marc Leva Marc Leva is offline
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Quote:
Originally Posted by Jacqueline Knight View Post
Are you just trying out kiln casting? Or are you specifically trying to use dental plaster? If not and if you start with small molds, I would just use a basic 1:1:1 ratio.

1 cup of Pottery Plaster or Plaster of Paris
1 cup of Silica Flour - otherwise known as Flint (300 mesh)
1 cup of water.

Get the plaster and silica from your local pottery supply store
I think that is a good place to start. I've used about everything out there and finally settled on whats readily available. I use hydrocal white and silica.
One part hydocal
two parts silica
two parts dry mix to one part water

I found that technique is more important than what materials you use.
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Old 10-04-2017, 02:08 PM
Chris Lowry Chris Lowry is online now
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Marc just to clarify are you saying...

Mix one part plaster to one part silica together well. Then when adding to water it's one part water to two parts dry material?
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Old 10-04-2017, 03:02 PM
Marc Leva Marc Leva is offline
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Quote:
Originally Posted by Chris Lowry View Post
Marc just to clarify are you saying...

Mix one part plaster to one part silica together well. Then when adding to water it's one part water to two parts dry material?
I use one part hydrocal white and two parts silica. One part water to two parts dry mix.
We use this for pate de verre because it holds up well wet. We introduce a lot of water into our molds before firing and we generally have fragile undercuts. After firing it becomes very friable.
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Old 10-04-2017, 05:16 PM
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I can only say that the calcium aluminate makes old castables way 'mo bettah.
Let me look into the chemistry when applied to molds.
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Old 10-04-2017, 05:17 PM
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When you're talking proportions it's important to designate by volume or weight. The silica to plaster ratio is 1:1 by volume. By weight it's 60% by 40%. I can never keep the weights straight, so I always just do it by volume. Yes, the ratio of investment to water is 2:1. Jacqueline's formula is accurate.

Use No.1 pottery plaster. Avoid plaster of paris from Home Depot. Don't bother with hydrocal/hydrostone. More $ with intangible benefit IMO.

I use chopped fiberglass for reinforcement.

There are many formulas. Right now I'm working with one that also involves EPK kaolin and pearlite. I'm having good luck with it so far. Got the formula from someone else, so I don't know if they'd be okay with me sharing it here. In the end I've found a lot of it is just voodoo. Experiment and find what works best. I agree with Marc that technique is more important. Mixing, pouring, and setting are important. Mold cure is critical. Most cracking and mold failures occur because molds are not properly dried, and are then brought up too fast. Some pros that I've talked to said they bring their molds up to temp over days as opposed to hours. Longer than the anneal cycle in some cases.
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Old 10-04-2017, 06:22 PM
Marc Leva Marc Leva is offline
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I always measure by weight. One to one and I never cure.
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Old 10-14-2017, 07:50 PM
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Franklin Sankar Franklin Sankar is offline
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Is there an assumption?
I am trying to find out how to determine What proportions would you use to make 1cup of liquid plaster. if I use volume ....1 volume of plaster and 1 volume of silica is 2 volumes. But 1 volume of the plaster mix and 1 volume of water is??? 1 volume liquid?.
1 cup is 32 ozs
From GN pg 63...castables can be used as a back up. .quote..they are trowelled onto the mold face as a backup material about an inch or two thickness.
Which surface is the face.? Just to make sure.

Franklin
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Old 10-14-2017, 08:15 PM
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Is dental plaster the same as plaster of Paris?
Franklin
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Old 10-14-2017, 10:07 PM
Steven O'Day Steven O'Day is online now
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No, plaster of paris is a generic term for gypsum plaster. Dental plaster is an ambiguous term for plaster that is used in dental mold making. Some of them are heat resistant, some set up quickly. It is difficult to determine what someone is referring to when they specify dental plaster unless they specify a manufacturer.
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Old 10-14-2017, 10:20 PM
Steven O'Day Steven O'Day is online now
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Quote:
Originally Posted by Franklin Sankar View Post
...proportions would you use to make 1cup of liquid plaster.

Franklin
Here is a plaster calculator for excel. It automatically downloads. It will calculate water and plaster amounts for a given volume. Starts out in cubic inches and ends in grams, so American.

https://alfredgrindingroom.squarespa...Calculator.xls
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Old 10-14-2017, 10:30 PM
Steven O'Day Steven O'Day is online now
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If you want to get repeatable results you should weigh both the plater and water rather than using volume.
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Old 10-15-2017, 05:17 AM
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Franklin Sankar Franklin Sankar is offline
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Thanks Steven that is wonderful. Answers many questions.
No pottery plaster here but many Dentist.
Franklin
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