I have heard that vintage crystal microphones and phono cartridges that contain Rochelle Salt Crystals are subject to damage from high heat. Is this true?
Yes! Here is a warning provided with a Shure crystal phono cartridge made in 1940:
DO NOT EXPOSE TO HIGH TEMPERATURES
Rochelle Salt Crystal Devices should not be exposed to the rays of the sun in hot weather for any considerable length of time - or left in closed automobiles parked in the sun during hot weather, as the temperature inside the automobile may easily build up to over 125 degrees F and permanently damage the crystal. When the crystal device is placed in a cabinet or enclosure, be sure that the temperature will not build up to more than 125 degrees F.
DO NOT APPLY HIGH VOLTAGE
This Rochelle Salt Crystal device may be seriously damaged if accidentally connected to a loudspeaker or power supply outlets carrying high voltage. Check connections carefully. Read data sheet carefully before installation.