What are nitre-beds?

Answer 1

Nitre beds have been used for years to make potassium nitrate, they consist of manure, ash and urine. They stacked the "ingredients" on a bed of clay then left it till it formed a white crust, this was then removed and washed with water, this got rid of the other soluble products. They used then potassium nitrate to make gunpowder.

Answer 2

Nitre-beds are shallow pits or beds containing a mixture of animal waste, straw, and wood ash that were historically used to produce potassium nitrate, also known as saltpeter, for gunpowder production. The decomposition process in the nitre-beds allowed the potassium nitrate to be extracted and used as a key ingredient in gunpowder.