When using mildly hardened papers, the gelatine can swell strongly if the development times exceed three to four minutes. Subsequent toning may then cause the gelatine to color.
The hardener concentrate can be added to the stop or fixing bath, with the disadvantage of prolonged washing times.
It makes more sense to use a hardener bath (one minute at dilution 1+5) after a pre-washing of approx. 10 minutes. After hardening, the surface is closed, protected from mechanical damage and the image whites remain clean when toned. The hardener is acidic, before toning the acid should either be neutralized by a slightly alkaline watering aid, or removed from the layer by a short watering.