Formation of iron-sulfur compounds produces an amber colour. Reducing Purple Manganese (such as pyrolusite) Around 3%[1] Oxidising[1] Blue and green Copper 2%–13%[1] The natural 'aqua' shade can be intensified with the addition of copper. During the Roman period this was derived from the recovery of oxide scale from scrap copper when heated, to avoid the contaminants present in copper minerals.[1] Copper produced a translucent blue moving towards a darker and denser green.