Was that a genuine concern? Boiling water has a fixed maximum temperature. Not sure that room temp->90C is all that different from room temp to 100C for ceramic.
I visited a fascinating museum with a ceramics collection, and it included an exhibit with a history of ceramics. Earlier ceramics had this problem. Apparently, bone china -- with some bone mixed in -- created a tougher material that was less prone to cracking under the rapid temperature shock. As ceramics improved over the decades, it became a non-issue.
I suspect that letting the heat spread out over a larger area at the bottom of the cup might have alleviated cracking. That's a speculation of course.
I wonder if this is why the Russians drank tea from glasses. My mom has some lovely Russian tea glasses with silver holders.
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.
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.
When you pour hot tea into an empty cup, the innermost layer of the cup goes from room temp to ~80C within a fraction of a second.
When you pour hot tea into a cup that already contains a fair amount of milk, it goes from $milkTemp to ~60C gradually over a few seconds as the tea mixes with the milk, giving enough time for the cup to expand evenly.
With a very thick glass, you definitely can crack it with boiling water. I didn't know if earthenware is similarly susceptible, but anything that is both brittle, and has a high enough rate of thermal expansion would be.
Sure, thermal expansion can crack ceramic. More my question is, did this really happen? The operating parameters for a ceramic cup are well known. A cup that is ok for 90C tea vs 100C tea seems like terrible design which would run you out of business if your competitor could make cups that were not known for breaking if you did not let the water cool long enough.
