doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com French gray finish...

Do these acids increase the C content of the steel? I doubt that they do, but I could be wrong. I think that they just etch on the surface not necessarily increasing the strenght.
I also thought that the French Grey finish is obtained by removing the colours of the case hardening process.

That's the way I've always used French Gray, after case hardening.
This custom Garbi 20 ga. was pack hardened by Doug Turnbull then the colors removed with a soft mechanical abrasion technique leaving the glass hard skin. Just coloring the metal (or removing the color) without the hardness would be suspect (to me) as far as corrosion resistance(??)
Years ago I "grayed" one non-hardened action because the engraver insisted, but that action did not need hardness, and I laquered over the "gray" to prevent rust. It has held up over the years but I wouldn't do it that way again and I've encouraged the client to revisit the metal finish.

French Grey or just grey finish?
As I remember Mr.Gaddy told French Grey was allmost the same case coloring process, but no shields over metal parts and quench liquid was Calcium Chloride(??)water solution.

Acid does not add any carbon to the steel. It only imparts an etched surface. Highly polished is to shiny for most shooters' taste. A mechanical or acid etch of very fine "grain" produces a more grey matt appearance. Case hardening does not automatically produce colors. However, as said above, removal of the color layer will produce a grey base metal.

Case hardening in itself does not impart colors. It is the impurities imparted by "the pack" which results in coloring the metal. Case hardening is done every day to all sorts of tools and parts. To simply case hardened metal, there are various compounds available. None of them imparts a color. Including Kasenit and Cherry Red:

http://www.midwayusa.com/viewProduct/?productNumber=119479
http://www.youtube.com/watch?v=9tlsq2ESQz0

Early gun makers would heat a part and then quench in linseed oil, which imparted colors, see "The Gunsmith of Williamsburg" with Wallace Gusler. http://www.armoryhill.com/review_gusler.html

Here you can see receivers being packed in charcoal at the Marlin factory prior to heating.


The charcoal imparts carbon, ie, acts as the carburizing agent. In adding carbon to iron, we produce steel. This is from Oscar's articles in DGJ showing a scanning electron microscope image of the surface after case hardening with charcoal.
http://www.doublegunshop.com/doublegunjournal_v7i4_9.htm


This surface color is only a few microns deep. It can easily be removed. It happens to most guns over the years just by contact with the hands. Ken Hurst simply uses some toilet bowl cleaner and a scotch-brite pad to achieve a french grey after the part has been case color hardened.

Pete

Pete, it's not colors only, it's protection from corrosion. If you delete case colors, the surface of steel will be open for corrosion. That's why surface has to be sprayed with trasparent laquer. The idea of French grey - surface is protected from corrosion just after quenching in water solution.
I heard gunsmiths in Ferlach use argentation to imitate and to "restore" French grey finish with yellowy patina.

From J.V. Howe (of Griffin & Howe), Formula No. 27, a GRAY color on steel is achieved by immersing the work in a heated solution of:
10 gr. (avoir.) Antimony Chlorid
10 gr. gallic acid
400 gr. ferric chlorid
150 cc. distilled water
The first color to appear is pale blue, then darker blues to purple, and finally to gray. If immersed long enough the metal will assume the gray color.The intermediate colors are better produced when the solution is cold. The above is considered one of the "bronzing solutions."

Geno, per a discussion with Dr. Gaddy, the case color film is very thin and loosely bound. It provides very little corrosion protection; the bulk of corrosion protection comes from the high carbon content of the case "skin." The use of a clear coat (wax/drying oil/lacquor/poly/etc.)is for abrasion protection for the color layer. As above, there are many ways to make a grey finish on steel.