Hi, Picking up in the middle of this thread. There are four types: 1 Chemical reaction types (like the light stick) where all the reactants are sealed in separate packages inside a larger one. The inner package(s) are broken and the contents mixed to make a reaction which provides heat. 2 Chemical reaction types which use atmospheric moisture to work. Similar process to above, but involves removing from airtight sealing somehow. Last ones of these I saw had sawdust as a carrier to prevent a runaway reaction. 3 Fuel types which actually burn to provide heat. 4 Types which use the Latent heat of crystallisation. These have an esoteric substance sealed in a malleable plastic container. Some sort of activiation system causes the liquid to crystallise, releasing heat. These alone can be regenerated easily, buy reheating the (now solid - and cooled) substance in boiling water until it "melts" again. All of the above have their dis- and advantages. My preference would be for the last. mike p.s. After being away for three weeks, I note that the Archive is up to speed. Big tick and a star for the Mail Archive personnel. - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
