Safety is relative to usage and location. A surface accessible for long-term contact by children should not exceed a temperature that will scald, that might be 105F to 130F. A surface only reachable by intent can be hotter depending on location and audience...
The safety of a given temperature is also dependent on its conductivity, which is how fast heat travels through it. Metal is highly conductive and can cause a burn quickly, while cob is less conductive and will take considerably longer to cause damage on contact. Soapstone is relatively conductive and may need to have a lower surface temperature than cob to be safe.
Do the next thing next. That’s a pretty good rule. Read the tiny ad, that’s a pretty good rule, too.
Grow a Salad in Your City Apartment E-book - By Rosemary Hansen