Back in his day, a few scientists had been tinkering with staining tissues, especially with a compound called carmine — a red dye derived from the scales of a crushed-up insects.
Gerlach and others had some luck using carmine to highlight different kinds of cell structures, but where Gerlach got stuck was in exploring the tissues of the brain.
So one day, he tried making a diluted version of the stain — thinning out the carmine with ammonia and gelatin — and wetted a sample of brain tissue with it.