Arcturus was an alchemist that lived in England in the 800's. He made several notable advances in his field, but none that surpassed those that were later made by his pupil, Merlin.
Like most alchemists, Arcturus' ultimate goal was to transmute base substances into gold. Arcturus never managed to achieve this, but he did discover methods of purifying various metals, to form stronger, more durable weapons, tools, and armor. He also discovered a method of turning gold into lead, but deemed this discovery a step in the completely wrong direction, and started over in his work. Merlin, however, would later take this method and reverse engineer it, to discover a cost-effective way of producing gold.