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Newsgroups: rec.crafts.metalworking
Subject: Re: Blackening steel?
From: John De Armond
Date: Mon, 17 Jan 94 21:52:04 GMT writes:

>In article <>, 
> (Bruce Lucas) writes:

>> "The Home Machinist's Handbook" recommends coating the part with motor
>> oil and then baking in the oven at about 350 degrees for 10 minutes to
>> obtain a "hard, durable black finish".

>I tried it.   It didn't work.   I also tried higher temperatures
>and longer times.   All I got was an annoyed wife.

Motor oil absolutely will not work.  It contains additives specifically
to PREVENT it from oxidizing in that temperature range.

The most durable coatings are either black oxide, blueing or Parkerizing.
One of the easier ways of applying black oxide or blueing (thinner oxide)
is the so-called "nitrate blueing" process.  

This involves simply immersing the chemically cleaned steel object in 
molten potassium nitrate (saltpeter).  As the temperature is brought up,
the surface turns a straw color which proceeds to blue and then black.
If you stop at the blue stage, the color will be a gorgeous deep royal
blue not achievable any other way.  Very expensive custom guns are often
blued this way.  

Brownell's (a wholesale supplier of gunsmithing tools and supplies)
sells kits for hot blueing, nitrate blueing and Parkerizing.  They
are reasonably priced and work well.  No, I don't have their number
handy. They advertise in most gun magazines.

To do nitrate blueing, you'll need a sturdy steel pan or tank and a 
Burner capable of heating the mass to the required 600-800 deg F.
Brownell's sells a wide variety of sturdy welded steel trays and
tanks.  I welded short legs to mine and applied a layer of castable
refractory to the sides in order to minimize heat loss and speed
the initial melt.  I use an old burner from a gas furance.  A digital
pyrometer (Omega Engineering has 'em for < $100) measures the 

Chemically clean steel is a must.  I first dip a part in hot concentrated
sodium hydroxide (lye) to saponify and remove any oil residue.  After
a water wash, a short dip in hot (not boiling) hydrochloric acid
removes any inorganic surface contamination and rust.  A second rinse
with hot distilled water followed by drying with a hot air blower 
finishes the prep.  This must be done fairly rapidly to prevent 
re-oxidization.  If you can't IMMEDIATELY wash, dry and blue a part,
store the cleaned part in a zip-lock bag purged with argon or nitrogen.

After blueing, let the excess saltpeter drip off, allow the part to
cool and then wash with warm water to remove the residue.  Immediately
apply a fine oil film.

One small caution:  Hot saltpeter will vigorously react with organic
materials and will start fires.  After all, it is an oxidizer.  Molten
saltpeter dripped on grass or paper will ignite it and the resulting
fire burns vigorously.


Newsgroups: rec.crafts.metalworking
Subject: Re: Blackening steel?
From: John De Armond
Date: Sun, 23 Jan 94 05:33:52 GMT writes:

>Since motor oil is not suitable, what kind would work best?

I don't really know much about oil blackening.  Assuming you want 
a carbon/varnish film (only think I can imagine at the temperatures
involved) maybe a cooking oil would work?  At least it turns skillets
black :-)

>1) The darkest, blackest color.
>2) The most durable coating.

Well there's little doubt that Parkerizing would be the most durable.
Neither blueing nor black oxide are very thick, after all.

>I just got the Brownell's catalog and see that they have
>a variety of blueing kits.   I don't want a blue color, 
>so I am a bit confused by references to "blueing".   Could
>you comment on this a bit?   I guess that gun fans all
>know this sort of thing, but I don't.

The blue is an oxide coating just like when steel turns blue as it is
heated.  The difference is the chemical process controls the oxide
consistency much better than mere heating.  And just as heated steel
will go through blue to black, so will hot and nitrate blueing.  Hot
blueing involves immersing the piece in a hot water solution of chemicals.
Brownell sells an excellent chemical kit.  

FWIW, I had a fire a few years ago and many of my tools were severely
corroded by the smoke residue.  I bead blasted those which could be
functionally salvaged and then nitro blued them to an almost black
color.  This has been VERY durable even on such frequent use tools
as pliars.

>In your opinion, what is the best book on the subject?

I don't know of a best book because I've never had one.  I do know
books on gun finishing are available from advertisers in gun magazines.
I learned by moonlighting in a friend's gunsmith shop and by reading
the Brownell instructions.

73 John

Newsgroups: rec.crafts.metalworking
Subject: Re: Blackening steel?
From: John De Armond
Date: Wed, 26 Jan 94 20:00:06 GMT

Missout Gilles <> writes:

>In article <> John De Armond, writes:
>>This involves simply immersing the chemically cleaned steel object in 
>>molten potassium nitrate (saltpeter).  As the temperature is brought up,
>>the surface turns a straw color which proceeds to blue and then black.
>>If you stop at the blue stage, the color will be a gorgeous deep royal
>>blue not achievable any other way.  Very expensive custom guns are often
>>blued this way.  

>do I have to prevent the solution to come inside the barrel or there is
>no problem?

No.  The oxide coating is so thin as to have no effect.  You DO have to
protect the barrel if you are parkerizing.  We use a piece of all-thread
and a couple of rubber-lined steel washers.


Newsgroups: rec.crafts.metalworking,sci.materials
From: (Russ Kepler)
Subject: Re: Blueing steel
Date: Sat, 21 Sep 1996 19:46:44 GMT

In article <>,
Jeffrey Prothero  <> wrote:
>Does anyone know what blueing salts are?  And does anyone know which
>fertilizer can be used for blueing  ( possibly the same question )?

The most easily obtained chemicals are sodium hydroxide, sodium
nitrate and sodium nitrite.  Industrial grade chemicals are fine,
even prilled sodium nitrate fertilizer will work.  The percentages
are 50%, 25% and 25%, adding to water until the boiling point hits
(as another poster said) something like 295 degF.

Bluing can be a hit and miss proposition, if you don't have good
temperature control, a good prewash for degreasing and some
experience.  Most simply steels (10xx, 11xx and 12xx) will blue
nicely, even 12L14 comes out nice.  Chrome-Moly steels blue well as
well (naturally, with all those 4130 barrels out there but steels
with much nickle, or too much chromium will either not blue or blue
with some funny colors.  If you run into one of these (a post-64
model 94, as an example) you need to play with the bath time and
temperature to get a good blue.  Most time about 20 minutes at
295 degF will give a good blue.

Don't blue something with want to have low friction - iron oxide is
not known for low friction.  Aluminum will last a couple of seconds
in a bath, and copper will "kill" the salts, making for an expensive
replacement.  A wrapping of electrical tape will protect a surface
from bluing, as well as some paints.  Keep a bottle of white vinegar
around to neutralize the NaOH.  NaOH at 300 degF on exposed skin
really, really hurts.  If the "blue" is purple toss it back in and 
raise the temperature 5 degF.  Cast iron will blue eventually - 
leave it in the bath for a few cycles.  Boil what you pull from the
bath in water to remove the salts at least 10 minutes if there are
threads connecting anything.  Use a polarized oil after removing the 
clean blue metal, hang up hot to dry remaining water.

Don't get me started on phosphating.

Russ Kepler

                    Please Don't Feed the Engineers

From: (Chris Kantarjiev)
Newsgroups: rec.crafts.metalworking
Subject: Re: bluing - how and what is it?
Date: 3 Mar 1998 18:48:03 GMT

I recently made some T nuts and "blued" them with sodium thiosulfate, as
per discussions here a year or so ago.

Sodium thiosulfate is photographic fixer; you can get it at your local
darkroom supply store ready to mix with water, in quart and gallon
pouches for less than $5.

I degreased the parts in a boiling tri-sodium phosphate solution for 15
minutes, rinsed with hot water, then simmered in fixer for 60-90
minutes. I think I started out with the temp too low - by the end of
the time I had the solution at something between simmer and roiling
boil. You'll want to cover the pot - I used a galvanized painting pot
that is now dedicated to this purpose. Rinse with hot water and coat
with boiled linseed oil.

I got a variable finish - some areas were very black, and some looked
more like color case hardening. I think that was a surface prep issue -
I probably didn't get the TSP off completely.

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