The Lye used as a degreaser or neutralizer is very weak as compared to that in a hot blue solution. One formula I have seen for mixing your own hot blue calls for 5 lbs of lye per gallon of water, Far more than would be used for degreasing or neutralizing.
As I understand it the lye is an economical method of raising the boiling temperature of the mixture, the nitrates do the oxidizing. The mixtures generally operate around 285° F which allows the nitrates to provide rapid oxidizing but is still below the melting point of solder.
In industry this process is generally known as Black Oxiding & has many uses & is the process used on most modern firearms which have a "Blue" finish.
In the machine shop I worked in on several occasions we made Hollow parts by machining a piece of aluminum to the inner dimensions, having this piece nickel plated thick enough to give the outside dimensions. We would then finish machine the outside, place the works in a strong lye solution & heat it. In a few hours the aluminum was completely disintegrated & we had the nickel shell left.
I am not absolutely certain if the lye works on the tin or the lead in solder, but believe it to be the lead.
Thanks.
