[/quote] "Jerry"
According to Thwaites the ludo may cut down on cracking because a) the ludo has various sized particles and helps with packing and b) chunks of ludo act as aggregate to stop cracks running through the mold. Also ludo is classified as a refractory or a modifier rather than a plaster (or binder) because the gypsum has already "gone off" and has no binder abilities (though it does speed up set time.)
Who is Thwaites?Thanks
Katia[/quote]
"Delores"
Angela Thwaites wrote mixing with the best based on her research with Keith Seybert.
Jerry, Once the plaster/silica is fired above 1500 doesn't it become cristobilite? I think they (Thwaites/Seybert) considered the fiberglass and vermiculate (perlite as a modifer which adds in the elimination of physically and chemically bound water to escape out of the mold while adding strengthing to the mold. But Bert is right when you get into the higher weight pieces and longer holds his perlite (vermiculite is virtually the same principle) is definately a stronger mix then with just fiberglass. I felt the perlite and vermiculite is so easy to get it wasn't worth putting my health at risk with the ludo but I think it was in the early 90's that Lucartha Kohler was using ludo and obviously she gets stunning results along with her zircar molds so it obviously works well.
Kathia....Have fun experimenting there lots of cool ways to get to the end of the road and in the end it's the type of work that you're doing that will help you be guided by the best way for you. Fortunately there is many possabilities to try until you get what works best for the art you're trying to create. If your casting at high temperature for extended holds a pate de verre mix probably isn't your best bet, I try a stronger mix. Good luck and I hope your project turns out great.
Sorry, volume weight problem again...
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Delores Taylor
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Hi Katia
My method of mixing plaster is to mix by volume 50/50 plaster and silica together dry, then lightly sprinkle the dry mix over the entire surface of the water (no specific ratio of water to plaster/silica) until small peaks of plaster/silica sit on the surface. Tap the sides or bottom of the container to break up any lumps of dry mix, without disturbing the water too much. Do not stir the mixture yet! Pour off any excess water that has settled on the top then you may stir the mixture gently. The amount of water that you start with will depend on the size of your mould. The strength of the mould is determined by how close the plaster particles set together so if the mixture has too much water the particles will set further apart making a weaker mould.
My method of mixing plaster is to mix by volume 50/50 plaster and silica together dry, then lightly sprinkle the dry mix over the entire surface of the water (no specific ratio of water to plaster/silica) until small peaks of plaster/silica sit on the surface. Tap the sides or bottom of the container to break up any lumps of dry mix, without disturbing the water too much. Do not stir the mixture yet! Pour off any excess water that has settled on the top then you may stir the mixture gently. The amount of water that you start with will depend on the size of your mould. The strength of the mould is determined by how close the plaster particles set together so if the mixture has too much water the particles will set further apart making a weaker mould.