Alkalies are soluble salts that are effective in removing dirt without excessive rubbing. They are good grease removers, because the alkalies form an emulsion, a mixture where oily or solid particles are held in suspension. The particles do not separate from the rest of the liquid; therefore, they are not re-deposited on the surface being cleaned. Household soda and washing ammonia are both types of alkalies. Acids are beneficial in removing hard-water deposits, discoloration from aluminum, brass, bronze, and copper and iron rust stains. Strong acids also eat away clothing, leather, and some metals. They can also irritate and injure the skin and eyes. Chlorine generally is the most common bleach used in household cleaning products. Most polishes and waxes for wood furniture and floors contain a spirit solvent. These solvents are similar to fluids used in dry-cleaning processes, because they remove oily dirt.