From Rust to Salt Blue...

[Hugh]

Heinz & Viggo,

<img border="0" alt="[typing]" title="" src="graemlins/yltype.gif" /> I don't know why I have a perverse desire to get into the middle of this, but this information may be of help to those interested.

Quoted from INTRODUCTION TO MODERN GUNSMITHING by Harold E. Macfarland, pages 235 & 236, discussing rust bluing:

"The object of any bluing process is to oxidize the surface of the steel with a minimum of the common red or brown oxide known as ferric oxide, or rust, and with a preponderance of the desired black ferrosic oxide. This is a black magnetic sesquioxide. It occurs in nature as "black sand" and is commonly found in connection with placer gold. Ferrosic oxide is impervious to rust. Surfaces so blued will subsequently rust only because the film is non-continuous, "porous". Hence a high degree of polishing on the steel surface is indicated, since it helps prevent rust by making the film continuous.

Essentially, the desired ferrosic oxide is formed by nitrogen compounds. Sodium nitrate is commonly used. Potassium nitrate is more desirable for several reasons, but more expensive. Ammonium nitrate is sometimes recommended but should be avoided, since it is highly volatile; loss from the tank is great, in the form of ammonia fumes. Sodium or potassium nitrate will promote the oxidation when used within a critical heat range. Sodium hydroxide (lye) is added, partly to raise the boiling point of the solution to that critical heat range and partly to promote the chemical cleanliness on parts, permitting uniform oxidation. Sodium nitrite is the first product of the decomposition of sodium nitrate. Oxidation is slow until some nitrite has been formed. Hence we age the bath and promote oxidation by the addition of a small amount of sodium nitrite.

For the benefit of experimenters, other oxidizing chemicals are potassium dichromate and potassium permanganate. Manganese dioxide or manganese acetate will act as a catalyst, to speed the reaction, while also serving to hold the unwanted red ferric oxide in solution. I find that the addition of a small, non-critical amount of tri-sodium phosphate will hasten the bluing, while giving a deeper color."

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