I am attaching two pdfs for you. The Bray book addresses exactly what you're asking for and gives recipes. I'm also including a chapter on crucible casting from "Glass casting and moldmaking" written by Boyce Lundstrom, but I'm not sure it will be that helpful.
These two articles are from the bibliography I sent you. If you'd like to see them, you can request them at your local library and ask the librarian to get them through Interlibrary Loan.
Rehren, Thilo. "Cutting Edge Technology -- The Ferghana Process of Medieval Crucible Steel Smelting." Metalla (Bochum), v. 7, no. 2, 2000, pp. 55-69, ill. 0017-1050. Includes illustration of "a glassy slag cake," p. 64. German summary, pp. 67-68.
Roddis, Jim. "Hints for Setting Crucibles." British Artists in Glass Newsletter, no. 9, July 1979, pp. 19-20.
Online various crucibles are sold that are made of fused silica. If these suggestions aren't quite technical enough, I'd recommend that you contact The Scholes Library at Alfred University, New York State College of Ceramics. They have a substantial collection of technical information about glass. Their reference desk number is: (607) 871-2951. Alfred’s library catalog is available for searching on the web: https://scholes.alfred.edu.
or
The Glass Division of the American Ceramic Society (Columbus, Ohio) Website: https://www.acers.org/ The American Ceramic Society, P.O. Box 6136, Westerville, Ohio 43086-6136 ; Phone: (614) 890-4700 ; FAX: 614/899-6109 ; email: info@acers.org
As our Library focuses on the art and history of glass and glassmaking, it is beyond our scope to answer technical questions in detail. Feel free to email me at the address listed below if I can be of further assistance.
