The ways in which minerals and mica detectors respond to chemical etching are described. While tracks with angles of inclination less than a critical value will be lost from minerals, a steady state is reached. With continued etching all tracks in an external detector can be destroyed. Methods are proposed and numerical expressions presented by which the number of tracks lost during etching external mica detectors can be determined. An example of the process applied to automatically counted tracks in an external mica detector is given.