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