Saltpetre
Saltpetre is a compound used in the creation of several Engineering related items. It is needed in the creation of Blackpowder, which is learned though engineering. Though it can be bought in several commodity shops, it also occurs naturally and can be picked up in swamp and jungle areas as a product from naturally decaying waste.
To create it in a more controlled manner, one can CREATE COMPOST HEAP. This will require at least 10 items of food, plants or dead animal matter in the room to create. As time passes saltpetre will slowly form. You can easily add to a compost heap by dropping more items in the room with the heap, which will slowly absorb them. The larger a heap, the faster it will produce. You can LIGHT HEAP to burn away the compost heap. This will result in the heap being destroyed when it can no longer produce anything.
Saltpetre can be found in the following areas naturally: