素材
Refractory Plaster
Japanese: 耐火石膏
Refractory gypsum materials used in casting and mold making, such as castable, lost-wax, and Pâte de Verr. Their heat resistance, shrinkage rate, and reactivity with lead-based glass vary depending on their composition.
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Refractory plaster (investment) is a fire-resistant gypsum material used in mold-making processes such as glass casting, lost-wax casting, and Pâte de Verr. By blending silica, alumina, kaolin, zircon, and other materials with its main component, gypsum (CaSO₄·1/2H₂O), it is given the heat resistance necessary to withstand the maximum temperatures of glass (800–900°C). Commercially available products include Bullseye 50/50 (50% plaster + 50% silica fume), R&R 910/965 (North American standard), C-2G (Yoshino Gypsum), and G-2 (Noritake).
The most critical factor in selecting fire-resistant gypsum is its sulfate content; products with high sulfate content react with lead-based glass, causing sulfidation and blackening (see §5.1). Rei Chikaoka (2012) published a DIY formula of 1:1 (by weight) burnt gypsum to silica sand, which continues to be used as a practical, inexpensive, and readily available alternative to commercially available products. Since crystalline silica is classified as IARC Group 1 (carcinogenic to humans), be sure to wear a dust mask rated N95 or higher when handling the powder.
Related Concepts
Lost-wax casting, fiber-mold casting, sulfide blackening, talc, and ceramic fiber, and understand the guidelines for selecting mold materials.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.