Saltpetre - food-grade Potassium Nitrate

Saltpetre (salt peter) cures meats, like ham, bacon, etc. Key ingredient in Morton's cures, above, but without the sugar and other ingredients.

Nitrite and/or nitrates are used in curing meat to counteract the undesirable effects of salt upon color. Not only is the color of fresh meat protected from degradation, but the pigments react with nitric oxide to produce the stable pigments characteristic of cured meat. These pink pigments are important to the acceptability of most processed meat products.
Both nitrite and/or nitrate are used in meat curing for color stabilization.

The end result is the same in either case, although the pathway for stabilization of color by nitrite is more direct. Since nitrite reacts quicker and less is required for color stabilization, it is being widely used in place of nitrate.

Click for more information on potassium nitrate.