On 1 Apr, 21:07, "Rick Hughes"
wrote:

People used to call at *houses once a week to collect it ............ I seem
to recall it was boiled down to make saltpeter


I think the process was more like composting, there's details on
wikipedia;
http://en.wikipedia.org/wiki/Potassi..._of_production

The saltpetre men could dig up any stable or outbuilding floor for
material and often dug up the earth floors in houses.