The brown spots which frequently appear on old paper, commonly referred to as foxing, can be very disfiguring to an image. There are various causes including mould and tiny bits of metal in the paper, particularly iron which has oxidised and turned to rust. Any specks of metal need to be removed or the stain can spread. A mild bleach is used to reduce the brown spots. If they are very dark it is sometimes better to leave them paler and less obvious than to try to remove them altogether. Continued bleaching weakens the paper structure and could lead to creating small holes in the paper where the spots were.