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Hello everyone!

I've been looking over some old notes I have on in pate de verre and I came upon something that was written in the late 90's about Christina Kirk's methods for pate de verre. It refers to packing glass over a mold (e.g like a hump mold) which is something I've been trying lately with varying degrees of success. Kirk's recipe for investment is part silica, plaster and talc. The addition of talc making the mold a bit softer with the idea of having less potential for glass cracking as it contracts. Anyone have any experience with this?

Thanks!

Not sure it would have that much effect unless you used a lot of talc and very little plaster/silica. When I need to weaken a refractory mix, I use organic material as an additive. If you've access to an office paper shredder, that works well, but you can use grass clippings, kitchen scraps, or similar. I'll first lay down a thin face-coat (3-4mm max), and then layer on a mixture of plaster/silica and my organics. I'll have chopped them into rice-size pieces, mixed them into a looser-than-normal plaster-silica so that it resembles lumpy oatmeal, then applied.

I extend the ramp up time a bit to allow for extra burnout (and it will burn out--it will smell like autumn leaves and create some smoke). The organics will vaporize, leaving a mold full of holes, like swiss cheese. It'll be strong enough to process the glass at temperature, but the holes will allow the refractory to collapse in on itself as the glass contracts around it.

Thanks!

Oh, one other thing: Your organic material should NOT include anything that expands when it encounters water or steam, such as rice, spaghetti, etc. Discovered that the hard way, when I was steaming the wax out of a mold containing some rancid old brown rice. Rather spectacular watching it implode, but since it was a one-off sculpture, very disappointing.

Thought I'd report back on my experience with this.

When mixing up the dry ingredients with water, the mix started to thicken very quickly so I felt like I had little working time. But the mold was quite smooth and it held up very well. However, it stuck to the glass like crazy after firing. I slowly broke/dug the investment away. My piece was exceptionally thin and I ended up breaking a bit of the edge when removing the investment. When there was just about 3/8" left I submersed it in water and interestingly a few seconds later the remaining investment separated and slipped away from the glass all in one piece.

Geo, I know of glass slumpers who use talc, working at 1140ºF. I have always been advised that it is not good for hotter work. Calcium carbonate (whiting) is used at 1250ºF. It too is not good at higher temps. Alumina hydrate is the refractory material that holds up best at higher temperatures, but it is costlier.

Thanks, Bert. I took this to 1375 and it held up well. Not sure that it's the right answer for my original problem though. I think I'm just lucky that the glass didn't crack. I also did a comparison test with a similar piece packed over a mold made of just plaster and silica. Those pieces turned out fine as well. Who knows?

I use only 50/50 #1 pottery plaster and 200 or 300 mesh silica (depends whIch plaster your local supplier carries) Seattle Pottery has 200 mesh silco-sil. I use talc as an inner core and find the above recipe with fiberglass shorts easy to get of off a thin walled vessel. The heavy high temperature firings I do I use a different mix. A third aggregate makes for a beefier mold.