The Vikings were badasses, and pretty resourceful besides. They would gather a special type of fungus that grows on trees, slice it into chunks, then boil the chunks in a pot full of urine for several days. Urine contains sodium nitrate, which permeated the fungus and reacted with it. The urine-treated fungus, when dried, could be lit and left to smolder without burning properly for a long time. This made it easy to transport coals for a fire.