Dental plaster

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DonMcClennen
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Dental plaster

Post by DonMcClennen »

When mixing for mold making for glass casting can the "plaster" content be substituted for Dental plaster or Hydracal or Ultracal with the same strength mold ?
Don
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Bert Weiss
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Re: Dental plaster

Post by Bert Weiss »

DonMcClennen wrote:When mixing for mold making for glass casting can the "plaster" content be substituted for Dental plaster or Hydracal or Ultracal with the same strength mold ?
Don
You are messing around with a tough set of semantics.

Ransom and Randolph is a company that makes dental and glass casting plasters. These have been formulated such that you can use them straight out of the bag. The dental casters are working in a similar temperature range to glass casters.

Hydrocal and Ultracal are brands of plaster that must be mixed with other refractories to be usable for glass casting. Straight out of the bag, they will crack and fail, in the kiln.
Bert

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DonMcClennen
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Re: Dental plaster

Post by DonMcClennen »

I'm looking at Angela Thwaites suggested basic recipe eg:( 50% plaster, 50% quartz or flint)...or.....( 40% quartz, 40% plaster, 20% grog).. I have Ultracal and Hydracal on hand so I wondering if either could be substituted for the plaster component in those two recipes?
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Morganica
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Re: Dental plaster

Post by Morganica »

Yes they can be used. Used the same way you'd use #1 pottery plaster, the mold will probably get harder, denser (less permeable) and a lot stronger. I'd say go for it.

Ultracal and hydrocal have lower expansion rates than #1 pottery plaster, are harder and denser. In a 50:50 mix with silica flour, they'll probably give you harder molds that take detail well and are less likely to hairline-crack, but are also less permeable and harder to demold.

Dental plaster--dentists use lots of different plasters and investments, so "dental plaster" can mean a lot of things. The ones that my dentist considers true "dental plasters" in our sense are actually called "dental stone cements" or "dental stones" or "die stones," and they tend to have a lot more compressive strength and a faster set time than pottery plaster, ultracal or hydrocal. Common ones are Vel-Mix, Suprastone, Densite, Denscal, Ortho-stone. (it is a blessing to have a dentist who not only does a good job with teeth but also adores teaching me about this stuff)

In general, when you switch to any of these from #1 pottery plaster:
--You may want to experiment with changing binder:filler ratios
--The extra strength lets you make thinner molds, which you need anyway for increased permeability
--You may need to change firing schedules slightly, especially if you tend to make thick or box molds
--You will almost certainly change water:powder ratios and your set/cure times may change
--You may need to consider adding vents in areas most likely to trap bubbles, where before the air would dissipate through the investment
--You may want to use a different mix--or weaken the existing mix--in delicate areas, or areas where the glass is liable to trap the cooling mold material. Otherwise, the shrinking investment may crack the glass.
--Experiment with removing spent investment--it probably won't be as fragile and easily removed

BTW, Angela Thwaites has an excellent book out on casting; you'll find it at Amazon. Nice stuff in there.
Cynthia Morgan
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DonMcClennen
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Re: Dental plaster

Post by DonMcClennen »

Thanks Cynthia..for the advice and suggestions. I do have Angela's book. I will build in vents.
Don
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Peter Angel
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Re: Dental plaster

Post by Peter Angel »

Ultracal and Hydrocal are dental plaster with a small amount of white portland cement added for high strength and density and also a retarder to stop it setting up really quickly.

I have taken dental plaster and add a bit of white portland cement and made my own Hydrocal. The only problem is that it setup while I was still mixing it!

I haven't been able to find what retarder they use in Ultracal/Hydrocal.

I would suggest you use dental plaster rather than Hydrocal/Ultracal. The added cement might make it too dense to allow steam and organics to escape.
Peter Angel
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Morganica
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Re: Dental plaster

Post by Morganica »

Peter Angel wrote:Ultracal and Hydrocal are dental plaster with a small amount of white portland cement added for high strength and density and also a retarder to stop it setting up really quickly.

I have taken dental plaster and add a bit of white portland cement and made my own Hydrocal. The only problem is that it setup while I was still mixing it!

I haven't been able to find what retarder they use in Ultracal/Hydrocal.

I would suggest you use dental plaster rather than Hydrocal/Ultracal. The added cement might make it too dense to allow steam and organics to escape.
Well, there are different kinds of dental plaster, and some of them are pretty dense, too... What kind were you using?
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Bert Weiss
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Re: Dental plaster

Post by Bert Weiss »

An investment that can handle the heat must have ingredients beyond plaster. Dental investments are designed to be kiln fired with cast metal. They have other ingredients added. Hydrocal and Ultracal are plaster, with hardeners added, designed to make durable plaster stuff. You must add silica or a variety of other ingredients to make a kiln casting investment. There is a technique adding soap to hydrocal and whipping it up to create an investment with bubbles throughout the mix. You can buy this mix called hydroperm.
Bert

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Morganica
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Re: Dental plaster

Post by Morganica »

Bert Weiss wrote:An investment that can handle the heat must have ingredients beyond plaster. Dental investments are designed to be kiln fired with cast metal. They have other ingredients added. Hydrocal and Ultracal are plaster, with hardeners added, designed to make durable plaster stuff. You must add silica or a variety of other ingredients to make a kiln casting investment. There is a technique adding soap to hydrocal and whipping it up to create an investment with bubbles throughout the mix. You can buy this mix called hydroperm.
Sometimes--I have friends who use hydrocal+organic matter for smaller, very detailed molds and it works fine. It depends on what you're casting and the techniques you use. In general, refractory needs a binder plus a structural filler and...whatever additives. As Peter mentioned, the "cals" are actually plaster plus cement, not just pure gypsum plaster.

Hydroperm is an interesting beast. The stuff I tried sets up almost instantly so you have to work fast, even with fresh--I used icewater and a little vinegar or lemon juice and bought another 7 or 8 minutes of set time or I would never have gotten the detail I needed. The permeability was great; I was making 2-3 inch figures with outstretched arms (image below, they were inclusions for a larger pate de verre piece) and the dental stone+alumina mixtures I tried first were so dense that I needed to vent at the hands and head to actually get the glass into the mold. Hydroperm solved the problem. Curiously enough about half the time my frit castings came out looking like swiss cheese. Not entirely sure why. The problem seemed to go away with a slower set time, and I never had trouble casting cullet or billet, just the frit.
hostafigures.jpg
A few months back someone told me it had been discontinued and I haven't really looked for it since. I played around with adding a few squirts of dish soap to hydrocal+alumina and got something similar but not quite as permeable, so there's something else going on.
Cynthia Morgan
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Kevin Midgley
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Re: Dental plaster

Post by Kevin Midgley »

There is also the variable of what you use as your mixer. I think if my memory is good, that Boyce instructed the use of an ice cream bucket lid on a rod in a drill. It would flap and really aerate the mix..... but it could be 'messy' :lol: :lol: :lol: There may be better mixers available now.
The slightest trace of set mix from a previous batch will speed the setting time.
Peter Angel
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Re: Dental plaster

Post by Peter Angel »

Bert Weiss wrote:There is a technique adding soap to hydrocal and whipping it up to create an investment with bubbles throughout the mix.
Bert, I would like to try and make home made hydroperm as it is almost impossible to buy in Australia.

Can you estimate how much soap to add to the hydrocal/silica mix?

Peter
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Brock
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Re: Dental plaster

Post by Brock »

I don't think there is soap in Hydroperm. What produces the bubbles is the size and speed of the impeller. The instructions are on the bag. I have used it. Very good for casting a mold and filling and firing immediately. Entrained air and steam are vented THROUGH the mold via the network of Don Ho bubbles.
Kevin Midgley
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Re: Dental plaster

Post by Kevin Midgley »

jet dry, the north american brand of dishwasher anti-spotting liquid could possibly work wonders in a plaster mix instead of soap.
SilicaSynergy
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Re: Dental plaster

Post by SilicaSynergy »

Glass Casting and Mold Making: Glass Fusing Book Three, by Boyce Lundstrom has a number of mold mixed that you can make for not too much.
The key one is the 50/50 mix by weight of Pottery Plaster and Silica 200. This makes a mold that usually only lasts for one firing. Adding things like Hydroperm, or Hydrocal, would be an attempt to replace the plaster with something that is more likely to withstanding the firing, thus extend the life of your mold. Other things that can be added to extend the life of the mold are clay in various forms, as well as different types of fillers like grog, and sand. There are also other forms of refractory that can be added to help with the heat durability and not sticking to glass, such as whiting, and alumina. It all gets very complicated so let me make this easy by adding just one more formula to the book.

For anybody doing large batches of molds that they want to be able to possibly reuse several times:

Order from Laguna Clay or one of their distributors:
1 Bag (50 lbs) WC-895D Slip Cast Clay
1 Bag (50 lbs) Hydrocal 105
Add about 5% of total weight (5 lbs) of chopped up fiber (old used fiber if you have it) to the dry mix then blend (a cement mixer with a cap can be useful for this)

Place in air tight containers for later use. I like to use 10 gallon Rubbermade stack tubs, because you can still manage to move them individually, and they can take the weight when stacked.
I have also used roll around dollies with trash cans that have a good seal on them. Once again, Rubbermade makes a system that works well but is a bit expensive.
I just use dollie-carts from Harbor Freight, with a board placed across it, and a good quality trash can with a thick contractor trash bag inside. You have to make sure that moisture does not get into your mix.

For mold making:
In a 5 gallon bucket:
Add 1 part water by volume.
Add 2 parts dry mix by volume.
Mix for 2 to 3 min with a jiffy mixer, paint mixer, or glaze mixer.
Then pour into your prepared mold. Should set up in about 30 to 60 minutes depending on how fresh the mix is, how much you mixed it with the jiffy mixer, and how hot the water was that you use, as well as a few other variables.

__________________________________________
Patrick Lundstrom
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"It's All Silica To Me!"
__________________________________________
Patrick Lundstrom
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"It's All Silica To Me!"
kittyboy
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Re: Dental plaster

Post by kittyboy »

Edit Mode.
Last edited by kittyboy on Sat Jun 14, 2014 4:51 am, edited 6 times in total.
glasstech
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Re: Dental plaster

Post by glasstech »

Thanks Kittyboy, this is interesting. I have seen suggestions to replace silica flour with alumina hydrate but this is the first time I have seen alumina oxide suggested. Do you have any information comparing the two alumina compounds in terms of their application for glass casting molds - ie. refractory qualities, release from the glass, mold cracking, etc?
kittyboy
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Re: Dental plaster

Post by kittyboy »

glasstech wrote:Thanks Kittyboy, this is interesting. I have seen suggestions to replace silica flour with alumina hydrate but this is the first time I have seen alumina oxide suggested. Do you have any information comparing the two alumina compounds in terms of their application for glass casting molds - ie. refractory qualities, release from the glass, mold cracking, etc?
Aluminium oxide exists in many forms, α, χ, η, δ, κ, θ, γ, ρ; these arise during the heat treatment of aluminium hydroxide or aluminium oxy hydroxide. The most thermodynamically stable form is α-aluminium oxide.
Nikki ONeill
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Re: Dental plaster

Post by Nikki ONeill »

Great!! info. But please be clear: quatz vs quarts; alumina vs aluminum. Lots of confusion could arise.

Cheers
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