Stone heat holders are heated soapstones to keep you comfy and warm. Soapstone can become very hot and retain this heat for a longer period of time.
The stones are heated true a hanging system on a wood stove. When the stones are heated, you put the stone in a felt cover using gloves.
The felt cover protects the user against burning and contributes to retaining the heat of the stone.
This drawing comes from a 3D scan of one of the soapstones which I performed to investigate how you could manufacture the stones, lids and hanging systems industrially what turned out to be possible.