International Academy of Cosmetic Dermatology
A crust can be caused by skin injury. It represents a mixture of proteins and other tissue components that work like a tap covering a wound. Crusts are also formed when certain bacteria, such as Streptococci, attack skin. In such cases, the crust has a yellowish colour that is caused by the mixture of cell debris, bacteria and, white blood cells. The yellowish crusts may be contagious. Such crusts need to be removed to avoid the spread of superficial bacterial infections.
Black or brown crusts are usually formed after superficial abrasions of skin. They may also occur following medical procedures such as surgery, curettage, or laser surgery. The black or brown crusts eventually disappear after the skin defect is closed during wound healing. Often, it is unnecessary to remove them actively; indeed this may cause pitting scars or infection if done not properly.
Uwe Wollina, MD