Edward used a staining film and paper developer to process his images. He used Pyro to develop his film and Amidol to develop his paper. Amidol was the chosen developer for Edward, Brett, Cole (when he was printing his father’s work) and grandson Kim Weston (for a period in his career). They are highly staining agents and it is why there are pictures of Edward, Brett, and Kim with black fingernails.
Pyrogallol was one of the first developing agents, and was dominant until metol/hydroquinone were introduced. In a low sulfite environment, pyrogallol produces a stain in proportion to the silver image density, and tans/hardens the emulsion proportionally. Staining and tanning are useful characteristics and can be exploited to enhance image definition, and improve the printing quality of a negative.
Amidol is a colorless crystalline compound with the molecular structure. It is a dihydrogen chloride salt and is used as a photographic developer. It was introduced as a developing agent for photographic papers in 1892. It is unusual amongst developing agents as it works most effectively in slightly acid conditions rather than the strongly alkaline conditions required for most other developers.
One night during a dinner party, Brett and Ansel Adams were having a friendly discussion over photographic chemicals. Ansel used Dektol and told Brett,
Wikipedia. Amidol. https://en.wikipedia.org/wiki/Amidol
Digital Truth Photo. A New Age of Staining Developers. https://www.digitaltruth.com/articles/stainingdevs.php