Cysts of Pneumocystis jiroveci (formerly carinii) in bronchoalveolar lavage, Grocott’s methenamine silver stain.
In this induced argentaffin reaction, chromic acid oxidation forms aldehydes from fungal cell wall polysaccharide components. The reaction is similar to the Periodic acid Schiff reaction, but since the chromic acid is a much stronger oxidizer, background staining by collagen and basement membrane is suppressed (they are oxidized past the aldehyde stage). The aldehyde groups then react with the silver nitrate, reducing it to a metallic silver, thus blackening the site.
It can be a bit tricky to time since different organisms take up stain at different rates. Pneumocystis can be missed if you understain, but other organisms will come out a black blob (thus making it impossible to see the internal structure) if you overstain.