Zinc: Zinc compounds can be found in some specialized photographic processes. For example, zinc chloride may be used in certain toning processes to achieve specific colour effects or enhance image stability.
Calcium: Calcium sulfite (CaSO3) can be used in certain toning processes, such as sepia toning, to create warm brown or sepia tones in black and white prints. Calcium carbonate (CaCO3) can also be used as a buffering agent to adjust the pH level in developer solutions.
Iron: Iron-based developers, such as Ferrous Oxalate (FeC2O4), have been historically used in certain black and white developing formulas.
Iron-based toners, such as sepia toners, utilize iron compounds to create warm brown or sepia tones in black and white prints. If the developer or other chemicals come into contact with iron contaminants, it can cause staining or fogging of the film or prints.
