Quick Answer
Rocks may not be heating due to insufficient exposure to fire, inadequate insulation, or poor rock selection.
Choosing the Right Rock
Select rocks with high thermal mass, such as basalt or obsidian, which can retain heat for longer periods. Avoid rocks with high water content or porosity, as they will absorb water and reduce their heating efficiency. A good rock should be dense, dark in color, and relatively flat.
Insulating the Rock
Wrap the rock in insulating materials like dry leaves, grass, or moss to retain heat and protect the rock from cold surfaces. Create a reflector using a piece of shiny metal or a smooth rock to focus heat onto the rock. This will increase the rock’s temperature and reduce heat loss.
Firing the Rock
Build a fire large enough to heat the rock to a temperature of at least 500°F (260°C). Monitor the rock’s temperature and adjust the fire size as needed to achieve the desired heat. Avoid over-firing the rock, as this can cause it to crack or shatter.
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