In Roy Dunlaps book "Gunsmithing" which as I recall was originally written in the 1950s he gave a formula for mixing a hot salt bluing solution using Ammonia Nitrate fertilizer & Lye. I tried a batch of it once & it did give an exceptionally good blue. It had a lot of sludge in the tank & was not very efficient to use due to that. It was Economical but apparently the inert portion of the fertilizer I used was different from what he had used as he did not report any problems in that regard.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra