US equivalent for Keramicast

[Nikki ONeill]

angela Thwaites recently published a delightful small book "Mould Making for Glass," a follow up from her thesis work in GB, detailed on her CD "Mixing with the Best." She describes Keramacast as a good, high-density alpha-type plaster for mold making. Does anyone know of a US equivalent?. I need a particularly strong mold that has a relatively slow setting time, for handbuilding (she notes that Keramicast sets in about 30 min). Maybe USG's hydrocal would work similarly. Has anyone had experience with this material?
Cheers,

[Morganica]

Hey, Nikki;

Depends on what you're doing:
--How slow a set time are we talking about, and do you mean you need slow-setting so you can continue to work the refractory (i.e., potlife), or slow-setting because your design is liable to produce more cracks with a fast set/cure?
--How much strength (compressive strength) do you need?
--How much glass is going into the mold, what's the size of the mold and how long will it be in the kiln?
--Is expansion/shrinkage a factor?
--Does your design have any delicate outcroppings (I'm thinking of your habatats) that might be an issue if the plaster is still strong/hard when you demold?
--Can you get me a copy of that book? (just kidding)

I don't know of anything called Keramacast, but Ceramacast is a line of aluminum oxide refractory systems that has very high compressive strength (usually) with reasonable potlife and moderate shrinkage. It's an interesting line because almost it relies on aluminum oxide or zirconium oxide instead of silicas and therefore should be less likely to stick or scum up the mold. It's also pretty expensive. Now that I'm getting back into pate de verre casting I'm looking at it for thicker facecoats instead of the custom mix I usually use.

One thing--if you're hand-building the mold anyway, you get a lot more control over mold performance (especially strength) with a layered build and reinforcement in the outer layers... Might save some money that way.

[Brad Walker]

Keramicast is very similar to USG's Pottery Plaster #1, found at most ceramic supply places. Both are gypsum, CaSO4·_H2O.

The Angela Thwaites book is easy to find (Amazon has it) and worth getting. It's basically a book form of the "Mixing with the Best" CD, which came out about 10 years ago and is now unavailable.

[Morganica]

Keramicast is very similar to USG's Pottery Plaster #1, found at most ceramic supply places. Both are gypsum, CaSO4·_H2O.

The Angela Thwaites book is easy to find (Amazon has it) and worth getting. It's basically a book form of the "Mixing with the Best" CD, which came out about 10 years ago and is now unavailable.

I was kidding about the book. I've got the CD and it's got some good stuff on it.

[Brad Walker]

I was kidding about the book. I've got the CD and it's got some good stuff on it.

I thought you might be. The CD is better than the book, but I'm a compulsive book buyer so had to have both. As you know, there aren't many books on casting.

[Jeff Wright]

A quick Google search reveals that "Keramicast" is manufactured by Saint-Gobain. Here a link to the physical properties of Kermicast. The site says "Keramicast is a formulated plaster used in the ceramics industry to produce working moulds for the slip casting of tableware. It is also used in the general casting of decorative plaster elements."

My thought is "No. 1 Pottery Plaster" is very close.

Of course, the Thwaite's book is available on Amazon. I just spent two weeks with Richard Whiteley and Heike Koehler at Corning. Heike's work and process pictures are all throughout the book. I didn't realize it when I'd purchased the book. I had Heike autograph it for me. That was pretty cool.

[Nikki ONeill]

The CD us a tome! I tried printing it out and stopped after 60 pages. there are a lot of gems to digest in there.
The reason I'm asking for anyone's experience is that Angela suggests using a "hard" plaster for casting float glass. I want to cast fairly large pieces,20-30 pounds, and need a strong mold to withstand the higher temperature needed to cast this glass. She lists pottery plaster, casting plaster, and fine casting plaster as beta type plasters. She lists dental plaster and high-density (Keramicast) as hard plasters that are the alpha type (prepared from gypsum at higher temperatures). So, if there is a difference from pottery plaster, I'd like to use it. She says the working time is 30 m inutes..nice for hand-building. Maybe a little more research is in order.

[Nikki ONeill]

Found an answer to the question of finding a strong refractory plaster, similar to Keramicast, for casting float glass. Found this among ages-old old notes from Warmglass:

"FYI: Dental Stone (and all variant names of such, e.g. FGR-95) is also known as Hydrocal. This is the alpha hemihydrate form of plaster, made by calcining gypsum in an autoclave. It is stronger and denser than the beta form (AKA Plaster, Pottery Plaster, etc.)" John Kurman

So good old hydrocal should work just fine to make a strong mold, along with a softer face coat. R&R 910 and 965 would work too, probably, but are more expensive.

Cheers,

[Morganica]

Hydrocal is only a USG brand name--there are stronger dental plasters. When I cast in the 30-50 pound range I don't rely on the plaster for strength--I have a big puddle of sandy crystal in a bin because I figured stronger plaster would keep the mold intact. Thank heavens I'd sanded the kiln floor before casting, but it still cost me a few hundred bucks in wasted glass and mold materials.

Hardness and strength aren't necessarily the same thing--a material can be very hard but brittle, not strong at all. I think when Ms. Thwaite refers to "hard" plaster she's talking about an alpha plaster. What you're really looking for is high compressive strength.

Anyway, when you're making bigger molds, it's better to build them in layers than to rely on a single pour--reinforce the layers as you go and you can pretty much dial in mold strength selectively. Pour on your choice of facecoat, then build up a second coat with a chop or ludo additive. Check your mold for corners/angles or protrusions (wherever the glass is likely to break through), then soak fiberglass strips in plaster and smooth them over those areas. After that, cover the whole thing with a sheet of fiberglass mat soaked in more plaster. Add whatever final coats you need to finish off the mold.

You'll get extreme detail at the glass surface and an extremely strong mold that can hold up to a lot of moving glass. Plus it'll be easier to get the mold off the glass than it would be with a single, very hard plaster mold.

[Nikki ONeill]

Cynthia, thanks for the tips and reminders! We met at BCon at the pate de vere and casting conference and talked a bit about mold making. Everything you said resonates. It's a bit of work (and fun) to instill or re-instill casting and mold making details back into the brain, after not having done any serious casting for a few years. It's interesting to read Angela's interviews with prominent mold makers and glass casters around 2002, when she did those interviews.
Onward..
Nikki