Photographer's Formulary Silver-Intensifier For Negatives is similar in composition to Kodak IN-5. The intensifier acts by depositing additional silver on an underdeveloped negative.
The results are permanent, and the new grain is surprisingly fine. Although expensive, this intensifier is probably the best that has been formulated.
The chemicals in the kit are used to make 4 stock solutions, which are mixed just prior to using. The negative is intensified in room light to the degree desired and then washed.
The stock solutions are stable for months, but the working solution is usable for only 1/2 hour after mixing.