Certain developing agents tan the gelatin in proportion to the image density. Hydroquinone, catechol and pyrogallol are the ones we commonly use. I use a process of bleaching and redeveloping in a staining-tanning developer for negative intensification. I sometimes repeat the process. Once I did it one too many times and tha emulsion cracked in certain places as a result. It surely had a strong stain image, though.