Characters with this skill have a reasonable understanding of the principles of chemistry, although their knowledge will often be in a mystical or magical form.
As chemists, these characters have the ability to recognise various types of mineral ores, naturally occurring chemical compounds, and prepared chemicals. They also know where to look for these items (e.g. that sulphur can be obtained from volcanic pits, etc.). Chemists also understand how to extract and refine chemicals from mineral ores or compounds. They can isolate an element from its compound to produce iron, copper, silver, etc.
It is left up to the gamesmaster to guide players running characters with this skill. They should be permitted to manufacture certain compounds, but must be restrained from trying anything too complex or unlikely. TNT is definitely out! Gunpowder and various flammable mixtures should be permitted. Gunpowder comprises a mixture of sulphur, saltpeter, and carbon, whilst most of the 'Greek Fire' mixtures of ancient times contained naphtha and usually sulphur, as well as various bitumen substances and oil. A test against Int should be used, with modifiers to reflect the complexity of the task, as the gamesmaster sees fit. Serious failures can be expensive, messy, and painful.