I find that the black oxide forms very happily at anything over 92 degrees.
Toby, that is very good info, I had no idea the black oxide would form at that low a temp. Common practise seems to have always been for boiling or steaming. Perhaps the main reason was for the parts to dry quickly.
I have not done any rust bluing for some time, but when I did, I am in the country & have spring water. I could not get a decent job using that water so just bought the bottles of distilled water like is used for steam irons etc. Current price on this is less than $1.00 a gal at my local Wal-Mart. My wife still gets it for her high priced steam iron.
