[0cf8612b2e1e]

Granted, I have only ever had access to modern materials, but that feels like a basic a QC check during manufacturing. Pour boiling water straight into the cup. Designs that fail need to be reworked to be thinner/thicker/baked differently.

[analog31]

Essentially, the materials hadn't been invented, or were not widely known. The people who were manufacturing hard china kept their recipes a secret. Adding bone dust to the ceramic made it tough enough to withstand the temperature swing.

Also, mass manufacturing was in its infancy.