Note 1 — II THE BEGINNINGS OF STAINED GLASS
Reading edition
Pliny's word "nitrum" does not mean what we call nitre, which is potassium nitrate, but natron, or natural carbonate of soda, of which deposits are found in the Nile Delta. It is this that is meant in the passage in Jeremiah: "Though thou wash thee with nitre, and take thee much sope...."