Nothing has to be added before use.
Lower contrast can be obtained by using a soft-acting developer like Kodak Selectol Soft. We do not recommend variable-contrast filters because they greatly lengthen exposure time. Phenol, also called carbolic acid, emits toxic fumes having a distinctive sweetish odor. Other brands use liquid phenol as a preservative. In our opinion, liquid phenol has absolutely no place in darkrooms because when prints are drying, the buildup of toxic vapor can be hazardous to health.