Metal Finishes - Steel

[BJREBUCK]

After reading the posts by Barry and Christopher I thought perhaps I can offer some basic information
on the subject of finishing of gun metal. The most commonly encountered finish is hot caustic bluing. This
is produced by emersion in a hot bath of caustic soda and water. The bath is heated to 290 - 310 deg F
and the parts are cooked from 15 - 60 min. This produces a black color on most alloys. It is always black.
The degree of polish determines what one percieves the color to be i.e. a course sand blasted finish will
appear as a shade of gray, a mirror polish will look like black glass. It does not fill engraving but will tend to hide fine shading. So fine bulino work is usually grayed back to make it stand out.
CAUTION: Caustic soda will readily dissolve copper, lead and aluminum.
Slow rust bluing is produced by coating the steel with a mild acid solution. It is then allowed to rust in the
air or in a humidity cabinet. When a layer of red rust has formed the parts are immersed in boiling water for
5 - 10 min this turns the red rust a dark blue black color. The parts are dried and carded with wire wheels or steel wool to remove the loose oxide and the process repeated until the desired finish is achieved, usually 5
to 10 cycles. The color will vary from a dark redish plum to blue black to jet black depending on the acid
solution used and the steel alloy. Since this process etches the steel surface it is not usually used over
fine shading or bulino type engraving.
CAUTION. The acid solutions are generally hydrochloric, nitric or sulfuric acid based. Some will attack gold
and some silver, be careful.
Charcoal blue also called machine blue: The parts are placed in a crucible and packed with a slurry of charcoal and oil. Then placed in a furnace and heated to 600-800 deg F for 6 - 12 hours. Then allowed to cool slowly back to room temp. Produces a beautiful blue black color.
Niter bluing is produced by imersion in molten potassium nitrate bath 400- 800 deg F. Colors run from light
straw at 400 deg to blue at 600 and black at 800. Beautiful colors but the least durable. Usually only used for small parts, pins and screws although i have seen it used instead of charcoal bluing for large parts but
it is not nearly as durable.
Browning is produced the same way as slow rust blue only the boiling step is omitted.
Color case hardening is done like charcoal bluing except the parts are packed in a mixture of bone and leather
char. The parts are heated to 1300 - 1500 Deg F and held for up to 12 hours at temp and then quenched
in an aerated cold water bath. Produces beautiful irridescent colors across the spectrum. Generally the
brighter the polish the better the effect with 400 grit being the minimum.
CAUTION. Because of the high thermal shock involved warping and cracking can and does happen. Seek a
highly skilled and/or well known and recommended craftsman to perform this task.
This is very basic and some detail has been left out but it should take some of the mystry out of it. Hope some find it informative and others will add to or correct any errors I may have made.

 

[Andrew Biggs]

Hi BJREBUCK

That's some really usefull information..........thanks for that, really appreciated.

Cheers
Andrew

 

[Powderhorn]

BJREBUCK, Anderw;
Some really good info, that sounds as though if should be moved to the tips section.

 

[Christopher Malouf]

I agree, this should go in the tips section but we should get some more details and as many finishes as possible in this. In addition, some basic steps to achieve the best finish out of whatever process is used.

I'd like to add the following suggestions which I know even less about ...

Nitre blue

French gray

There's also some confusing terminology like "charcoal gray" which I think is really supposed to be "charcoal blue" (don't recall where I heard this or read it but).

Regards and thanx,

Chris

 

[KCSteve]

The tips section has some of this.

Here's what I found in a quick search:

French Grey by John B

Weldon's post of tips on firearm finishes is here

Ah! This one's not in the Tips section (but probably should be)
The Finish thread

 

[BJREBUCK]

All righty then...Nitre bluing:
Nitre bluing is a heat coloring process. The colors achieved are a result of the temperature to which the steel is subjected. This is sometimes refered to as flame or fire bluing as it can be done simply by heating
the parts with a torch and observing the color change as the parts get hotter. The main problem here is
that temperature control is difficult and steel begins to oxidize at around 400 deg F. By submurging the parts
in potassium nitrate KNO3 the parts are not exposed to the air and the oxidizing doesn't occur. Also controlling the temperature of the bath is much easier, the larger mass of material helps here. The parts
should be polished very bright for best results. I usually polish to 600 grit and then buff with chrome rouge
to a mirror finish. Hold the parts with something that won't transfer heat away from the part or to your fingers. I use small ga piano wire that I bend up into clips (like a one sided paper clip) with a long handle about 8" works well. The parts should be as clean as possible. I wash with acetone and then rub the polished surfaces with paper towels. Do this just before the parts go into the pot. Don't leave the clean parts exposed to the air any longer than necessary. Bring the niter pot up to the desired temp and immerse the pieces in the bath. Watch for the desired color (a bright light over the pot helps here) when the color is right, remove and immediatly quench the part in room temp distilled water, this will set the color. Then oil the part(s) and your done. Usually takes only a few minutes to do. Tip: when the part is quenched it will turn a slightly darker shade. Sometimes the steel will gas out, small bubbles will appear on the surface these should be wiped off with a Q-tip like wiper made by wrapping some fine, de_oiled steel wool around some wire. Leave this in the pot while while you're workingso it will be hot if needed. You can lift the parts out, skim off the bubbles and return to the pot. KNO3 is hygroscopic, it will draw water out of the air when cool, this will
cause it to micro boil when first heated causing a very fine snow to fall around the pot, not dangerous just messy, so keep the pot covered when not in use.
Remember the minimum heat required to begin the coloring is around 400 deg F this is also the the range where paper, wood, etc will ignite. Anything that will burn will do so if it contacts the hot niter. Be especially
careful the the parts are clean and dry when placed in the pot as an eruption of the pot will be very bad news.
What you will need: A small container of about 1 qt size any material will work sst is probably best, cast iron or aluminum will work also. A bright light over the pot. A heat source (single propane burner will do). Heat treating thermometer 1000 deg F range. Misc holding devices. Potassium nitrate, distilled water and water
displacing oil. Except for the container and the wire clips all this stuff is available from Brownells.
Now go to it, be careful and have fun.

 

[FL-Flinter]

BJREBUCK,

No offense intended, I actually got a chuckle over the "simplified" method of rust bluing. That is one process that sounds simple but there's a lot more to it than rust and boiling water. I first read-up on rust bluing many years ago, a somewhat simplified description as yours and it sounded easy enough....

After my first attempt ended in less than appealing results, I read some more, then more and even more yet. Add in years of trial and error and it's still not "as easy as it sounds". While the simplified desciption is very much correct, there are a lot of particulars that affect the color, depth and durability of the treatment.

Depending on what acid(s) and other chemical(s) you use, when you use them and how long you leave them on will all have an affect on the end results. Another thing is surface conditioning prior to application of the acid, the finer the item is finished, the harder it is to get a deep even finish but if done correctly, it looks the best.

I've done a lot of rust bluing and I'm still learning new things. The number of variables available to produce differences in the finish are quite extensive, it's something you can constantly experiment with to produce about any color from light gray to pure black that you desire - you just have to experiment enough with it.

One word of caution to anyone attempting this process - if you use acid(s) and don't completely kill them before taking them to the boil bath, even if the part is bone-dry, the acid will re-constitute in the boil bath and come off with the steam - it's real easy to acid-burn your eyes, nose, throat, skin, ect. and in most cases you can be injured and not know it until it's too late - due caution is advised. Oh, same goes for caustic bluing as well, it's easy to get hurt if you don't pay attention to what you're doing.

 

[BJREBUCK]

FL-Flinter,
No offense taken. I did my first rust blue job around 1958 in the garage with a tank on an old camp stove.
Went well. Beginners luck I suppose. I've since discovered some of the many pitfalls along the way. My post
wasn't intended as an exact How-to, just a general description of what/how the process is applied. I"ve
been into gun work a long time and just now trying engraving. The Cafe is a wonderful resource. I learned a
lot here....still can't figure out to prepare a .005" wide channel for gold wire in-lay....sheesh.
A lot of the engravers especially the newbies I've noticed don't seem to be concerned with the proper
preparation of the metal or the final finishing. I seen way too much rather well done engraving applied thru
rust pits, dents, scratches, even old finishes...
Since the question of finishing came up I thought perhaps I could add some small info as a pay back for all
the great explanation and teaching the members share on this forum.
Brian

 

[FL-Flinter]

Brian,

I understand, and you made it clear, that your initial post was just a "generalisation" it was just the way it came across that tickled my funny bone - thinking back to when I tried rust bluing the first time and having to laugh at the disasterous result - "rust blue camo" would be a good desciption. LOL

I haven't been doing rust bluing anywhere near as long as you have but I was deliberate in my methods in trying to obtain the desired results. So despite the set-backs at first, and those that come with every new trial, it's very pleasing when it all comes together....even if it's by chance rather than design.

I'm glad you brough up surface preparation because that's one of the biggest problem areas in most any task be it with wood, plastic or metal. No matter what you're doing, the results will always revolve around the effort put into proper preparation. There is no ends to the means of putting hard labor and skill into something but yet ignoring the preparation and processing efforts required to complete the project. I too have seen very high quality wood carving and engraving applied to surface that lacked the proper preparation and it greatly detracts from the overall project.

I recently put a rant on a gun forum about this very thing. Decent quality standard production guns sporting retail price tags of $750+ put on display to prospective buyers with deep circular scratches left in the wood from an orbital power sander that now stand-out under the polyurethane finish like thousands of little neon signs that scream "no craftsmanship". While many see this as minor issue, anyone who appreciates quality craftsmanship sees it as an indication that the rest of the gun may have the same lack of craftsmanship and/or serious quality issues. Overall quality and craftsmanship is even more important when it comes to putting out custom/semi-custom work.

As it applies to rust bluing, surface preparation is only half the battle as process procedure is what will make the difference in being victorious in the battle or ending up a POW. Speaking from experience, you can put much effort into preparing the surface to perfection then not use the proper finish development procedure only to end up with a complete mess. As you know, it's a balancing act that isn't easily described, just the same as over or under carding a brown finish will directly affect the end results. Knowing how the particular alloy reacts is also a must, 4140 steel alloy will not respond the same as A-36 standard mild steel and while it's close, the procedure must be adjusted slightly to obtain the desired color while a completely different procedure must be followed for A-541 alloy.

I agree whole heartedly that this is an extremely informative forum with so many masters who are not only willing to share their hard-earned knowledge but also willing to take the time to offer it at will. I'm no engraver which is why I read far more than I write.

Mark

 

[dlilazteca]

The tips section has some of this.

Here's what I found in a quick search:

French Grey by John B

Weldon's post of tips on firearm finishes is here

Ah! This one's not in the Tips section (but probably should be)
The Finish thread

Links are not working

Carlos De La O III