Efflorescence appears in various forms on masonry surfaces. It sometimes is crystalline or crusty; at other times it is powdery or fluffy. In its worst forms, it appears as a scum or as unattractive green or brown stains. Efflorescence results when moisture comes in contact with soluble salts within masonry units or mortar. The moisture dissolves the salts and gravity, hydrostatic pressure, evaporation, or a similar mechanism transports the salt solution to the surface, where it appears as efflorescence. HOW TO PROCEED Determining the type of salt causing the efflorescence can help you select a cleaning agent that effectively dissolves the efflorescence, but has minimal impact on the masonry itself. Many proprietary cleaners have been developed to remove specific types of efflorescence. If the wrong cleaning solution is used, it can cause soluble salts to become insoluble, creating an even greater problem. Improper cleaning of masonry surfaces also can significantly change a wall's color, eat away cementitious material, leave the aggregate exposed, and cause additional efflorescence to occur. To minimize the effects of cleaning on a wall, always start with the gentlest method possible and progress toward harsher measures as needed. The gentlest method usually is dry brushing. If that's not effective, try using a brush dipped in a bucket of water, or use a gentle water spray. Proprietary masonry cleaners, acids, and other chemicals should be used only when gentler techniques prove unsuccessful.