素材
Cullet
Japanese: ガラスカレット
A general term for glass scrap and fragments intended for reuse or recycling. Used as raw material for cast pâte de verre. The COE must be consistent; mixing different types can cause cracking.
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"Glass cullet" is an industry term referring to glass scraps, fragments, and waste; it is a general term for glass scraps and defective products generated during the manufacturing process, as well as glass that has been intentionally crushed or broken into pieces (treatise §10.4). In kiln work, it is used as a filling material for cast molds, as a raw material for pâte de verre frit, and as a layering material for pot melt. To make your own cullet, crush sheet glass or rod glass with a hammer, or grind it in a mortar to a frit-like consistency (Chikaoka 2021, p. 145: details on how to make frit from sheet glass using a mortar are provided).
The most important consideration when using cullet is ensuring that the COE values match. Cullet made by mixing glass with different COE values will accumulate stress due to differences in thermal expansion after cooling, which can cause the glass to break later (failure below the strain point). While commercially available cullet products clearly specify their COE—such as “COE 90 cullet” or “COE 96 cullet”—the COE tends to be unclear when using scrap glass or waste glass as raw materials. By checking for residual stress using a polariscope test, you can retrospectively evaluate the safety of mixed cullet. Additionally, safety precautions (such as wearing an N95 mask) are required when handling lead-based glass cullet, including disposal and dust management.
Related Concepts
Kiln casting, pâte de verre, pot-melt zone, COE, sub-strain fracture, polariscope test
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.