Cleaning methods generally are divided into three groups: water; chemical; and abrasive or mechanical. The selection of a cleaning technique is best done by testing several methods. The tests should begin with water washing and move up to harsher methods only if the simpler ones do not produce a satisfactory result. WATER CLEANING Cleaning masonry with water is generally the simplest operation, the safest for the building and the environment, and the least expensive. The most commonly used water cleaning methods, from the simplest to the most complicated, are handscrubbing, spraying, pressure washing, and steaming. They are particularly effective on calcerous (calcium-based) materials such as limestone and marble and on some lime-based bricks. Water cleaning can cause some problems. Water can seep through faulty seals around wall openings. Within the wall, excess water can cause corrosion of metal elements. CHEMICAL CLEANING The main types of chemicals used for cleaning masonry are acids, alkalies, and organic compounds. Acidic cleaners are used for cleaning most granites, sandstones, and all noncalcerous stones and unglazed bricks. The acid loosens the dirt, which then can be rinsed off easily. Alkalies (caustics) can be used on acid-sensitive masonry materials such as limestone or marble, glazed brick, and glazed terra-cotta. Poultices can remove oil, tar, plant materials (lichens and algae), graffiti, metallic stains, and some types of efflorescence. The paste is made up of an inert absorbent material that has been saturated with a solution to dissolve the specific type of stain. ABRASIVE CLEANING Abrasive or mechanical cleaning techniques, such as sandblasting, are unacceptable cleaning methods for old and historic masonry.