Complex sentence: In conjunction with this skill, extensive woodland on the Jura mountains around the town provided the raw material for charcoal, a form of carbon free of sulphur and other impurities that make most coal unsuitable for iron working.
Split into simpler sentences: In conjunction with this skill, there was the presence of very extensive woodland on the Jura mountains around the town. This produced the vital ingredient of charcoal, a form of carbon which is free of sulphur and other impurities which make most coal unsuitable for use in iron working.