Why was salt peter used in prison?

Saltpeter, or potassium nitrate, was historically used in prisons primarily as a preservative for food and as an ingredient in gunpowder. In some cases, it was also believed to have medicinal properties, helping to prevent scurvy and other ailments among inmates. Additionally, its use in the production of explosives made it relevant in the context of prison security and control. However, its application varied widely and was not universally adopted across all prison systems.