Powdering is a term used to describe a substance that appears on vegetable tanned leathers, which usually signifies red rot.
Powdering was not an easy term to research as the term powder is used to describe a lot of different conservation issues. The term powdering seems to come up most in the discussion of vegetable tanned leather and red rot. Red rot occurs on leather objects that have been exposed to high humidity, high temperatures, and pollution (Applebaum). The disease presents it self in a red powder that is also accompanied by structural weakness and a felt like consistency.
Though powdering and red rot are not reversible, the spread of it can be stopped through the application of a consolidant that has been coated with a sealer (Mahoney and Pearlstein).
Applebaum, Barabara. Guide to Environmental Protection of Collections p. 185. Sound View Press, 1991 Mahoney, Caitlin and Pearlstein, Ellen. 2014. “Evaluation of Consolidants for the Treatement of Red Rot on Vegetable Tanned Leather”. Retrieved From: http://www.conservation-us.org/docs/default-source/annualmeeting/2014am_poster13_evaluation_of_consolidants.pdf