High Specific-heat Capacity Materials
Heating packs can also be made by filling a container with a material that has a high specific heat capacity, which then gradually releases the heat over time. A hot water bottle is the most familiar example of this type of heating pad.
A microwavable heating pad is a heating pad that is warmed by placing it in a microwave oven before use. Microwavable heating pads are typically made out of a thick insulative fabric such as flannel and filled with grains such as wheat, buckwheat or flax seed. Due to their relative simplicity to make, they are frequently sewn by hand, often with a custom shape to fit the intended area of use. In rare instances, these types of pads have been known to ignite during or after the microwave process and cause fires.
Often, aromatic compounds will also be added to the filler mixture to create a pleasant or soothing smell when heated. The source of these can vary significantly, ranging from adding essential oils to ground up spices such as cloves and nutmeg, or even dried rose petals.
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