Synthetic pigments were introduced as early as the second millennium BCE. White lead (basic lead carbonate, (PbCO3)2Pb(OH)2) is one of example. and blue frit (Egyptian Blue). Another early synthetic pigment is blue frit, calcium copper silicate, made by heating glass with a copper source, such as malachite. Later premodern synthetic pigments include vermilion, verdigris, and lead-tin-yellow. Vermilion, a mercury compound, was favored by old masters such as Titian. Indian yellow was once produced by collecting the urine of cattle that had been fed only mango leaves. Dutch and Flemish painters of the 17th and 18th centuries favored it for its luminescent qualities, and often used it to represent sunlight. Since mango leaves are nutritionally inadequate for cattle, the practice of harvesting Indian yellow was eventually declared to be inhumane. Modern hues of Indian yellow are made from synthetic pigments. Vermillion has been partially replaced in by cadmium reds.