Warmer can not be used if warmer has hardened without being opened.
If the warm patch is hard before use, either the product has expired, or the product packaging has been leaking and air contact, or is damp. For safety reasons, warm stickers harden before being taken apart and cannot be used again. 
Under normal circumstances, warm paste after the use of hard lumps, because warmer is the use of iron oxidation reaction heat to heat. According to the principle of oxygen absorption corrosion of iron in humid air, while using the strong adsorption of activated carbon, there is water vapor stored in the loose structure of activated carbon, water vapor liquefied into water droplets, outflow and contact with air and iron powder, under the catalytic action of sodium chloride more rapid reaction to produce iron hydroxide, release heat. And when it's oxidized, it hardens.