Maleic Acid

Maleic acid is used as a pre-wash in the use of physical developer for wet or partially burnt porous material in fingerprint development.  Maleic acid stabilizes the reaction by neutralizing the alkaline components, such as chalk, calcium, calcium carbonate, and other known alkaline substances in paper. If these compounds are not neutralized, the chemical reaction will create background coloration of dark gray to black in color. This coloration hampers the contrast and partially obscures the friction ridge skin deposit.

During the 5–10 minute pre-wash, the acid lowers the pH of the paper and reacts with alkaline fillers noted above (which will often cause bubbling). Once the bubbling stops, the paper is rinsed with distilled water, preventing the paper's intrinsic fillers from interfering with the silver nitrate-based developer.