How are glass diodes produced?
The silicon wafer or bead can withstand 900’C but the critical process is the low temp interlayer powdered glass that is now Pb-free using Bismuth and other metals mixed with Silicon Dioxide. This passivation layer reduces the high temp leakage and limits most diodes to 200’C or less. The outer layer of glass is then molded with powdered SiO2 to form the shape that appears.  The trade secrets are in the passivation blends of powder , cleaning processes and  standard wire bonding to the melted gold bond to the crystal.