Mordancage Images are created by chemically removing parts of the gelatin underneath the emulsion of a Black & White Gelatin Silver Print. The process was first popularized in Europe by the French Photographer Jean Pierre Sudre, then introduced into the US by Craig Stevens and Elizabeth Opalenik.
The chemicals are toxic without strong ventilation, and the use of a dual respirator, so I won't discuss the process. It is discussed in the book on Alternative Processes by Christopher James, pp 314-317.