Lead occurs as galena (PbS), cerisite (PbCO3), and anglesite (PbSO4). Galena is the most common. Lead ores are usually roasted and then reduced in a blast furnace with coke. Purification is carried out by adding zinc, which removes silver (Parkes process), or by electrolysis in a solution of lead hexafluorosilicate and hexafluorosilicic acid (Betts process). One of the most useful properties of lead is its ability to absorb nuclear and X-ray radiation. Lead has good chemical resistance but is attacked by nitric and organic acids. Lead is extremely resistant to atmospheric corrosion as it rapidly becomes covered with an impervious film of oxides, carbonates, and sulfates.