Signal speculum

Signal specula were magically heated plates.

Description
A signal speculum was a metal plate that was on one side insulated and bare on the other. The plate was heated with magic heating the bare side.

Powers
The metal plate was heated, in other words, it looked like bright light to creatures who had infravision. Using this property, signal specula were used as signaling devices by creatures with such vision.

They were created by wizards and clerics but could be used by anybody.

History
Signal specula were magic items associated with the drow. They used it as signaling devices but also to treat chilled, wounded, or otherwise immobilized people with warmth, to warm drinks or limbs, or warm torn or cramped muscles.