Differential staining uses multiple stains per slide. Previously, it was viewed using simple staining methods such as H&E stains. Pigments in paints and stains protect against UV damage. The ETFE material is resistant to most stains, which simply wash off in the rain. The stains turned out to be paint stains. They have mild abrasives which aid in the removal of stains on enamel. Oxalic acid is particularly effective at removing iron stains from wood. Eosin also stains red blood cells intensely red. This stains clothes permanently and irritates eyes. Another witness testified to spending much of the day cleaning blood stains off the wall. This allows for removal of superficial stains in the enamel. It also stains mast cell granules brown. It is harder and more resistant to stains than granite. The sap sometimes stains yellow on fingers and clothes. Some bacteria have cell walls which are particularly adept at retaining stains.