What causes bluing to turn " Plum "??

[wlw-19958]

Hi There,

The older gunsmithing manuals (prior to 1950) I have read blame
the purplish tone on some gun parts one re-blues comes from either
bluing salts becoming too hot (270°F is the approximate bluing temp
but as the solution boils, the water is driven off and the resulting
solution becomes more concentrated and hence, the boiling point
goes up. The purple starts in the 305°F temp and above).

The other cause is very hard high carbon/chromium steels that are
highly polished. They don't like to take a blue and sometimes a higher
temperature is the only thing that will work. At least, these were the
believed causes for the difficulty in taking a blue and thus a plum
color results. At least so the older books say. One technique to improve
Hard, Highly polished parts in taking a blue is to etch the surface in
a weak Nitric Acid solution (10%) for a few minutes or just long enough
to knock some of the 'shine' off.

I have an old Lyman copy (Uberti?) of the 1858 New Model Remington
and it is Purple.

Cheers!
Webb