A change in the surface characteristics of a gelatin emulsion caused by swelling the gelatin and pressing against a surface.
When a swollen gelatin emulsion is pressed against a surface the surface of the gelatin will take on the characteristics of the surface it is pressed against.
This used to be deliberately done to still photographic prints to give a highly glossy finish to the print. With motion picture film ferrotyping is the result of a tight wind and storage under humid conditions for extended periods. The resultant change in the emulsion surface has a deleterious effect on the projected image.
Anitferrotyping is the name given to the lifting of the emulsion which has started to adhere to adjacent layers. The surface characteristics of the emulsion becomes rough which again has a deleterious effect on the projected image.