Subject: Re: Blackening steel?

From: [email-masked] (Bruce Lucas)
Date: Sat, 15 Jan 1994 16:53:06 GMT

"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".

Bruce Lucas

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From: [email-masked]
Date: 16 Jan 94 08:02:40 PST

> 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.

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From: [email-masked] (Jher)
Date: 16 Jan 1994 20:14:04 -0600

I had some old chainmail that had rusted and once I fired it in my
oven at about 600 deg it turned black.  Not all of it mind you, but most
of it turned black.  I didn't oil it or anything.

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From: [email-masked] (John De Armond)
Date: Mon, 17 Jan 94 21:52:04 GMT

>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 
temperature.

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.

John

-- 
John De Armond, WD4OQC, Marietta, GA    [email-masked] 
"Would you fire on US Citizens while in the process of confiscating their guns?"
    A question on a survey given to members of the U.S. counterterrorist 
    unit Navy SEAL Team Six by their commanders last September.

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From: [email-masked]
Date: 21 Jan 94 12:03:39 PST

In article <[email-masked]>, 
[email-masked] (John De Armond) writes:

<good replies about blackening>

Thank you for your answers about blackening techniques.
I still have a few remaining questions, however.

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

Of the other techniques, I am still a bit unsure which
one gives:

1) The darkest, blackest color.

2) The most durable coating.

I have some lathe toolholders made by KDK that are an
intense, deep black color that appears to be quite durable.
This is the sort of thing I would like to duplicate.

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.

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

Thank you for your assistance.

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From: [email-masked] (John De Armond)
Date: Sun, 23 Jan 94 05:33:52 GMT

>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 :-)

>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.

>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

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From: [email-masked] (Michael J. Edelman)
Date: 24 Jan 1994 12:49:52 GMT

In article [email-masked],  [email-masked] writes:
 
>you comment on this a bit?  

I suggest the Brownell's "Oxpho Blue", as it delivers a nice and extremely
durable blue-black. Multiple applications intensify the color. And no 
special processes are required- you just wipe it on. I've used it on a
number of jobs.

The cheaper "quick blue" formulas use a system of precipitating copper on
the surface. They result in a much lighter blue which rubs off easily. 

 --mike

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From: [email-masked] (John Kopf, X6305)
Date: 24 Jan 1994 20:21:54 GMT

There are two common forms of iron oxide -- ferric and ferrous (I just can't
remember which is which...one is black and hard, the other is red and softer).
In thin layers, the black form looks blue (in even thinner layers, you get the
"colors" used in tempering).

I believe that "oil blackening" coats the iron with oil and then heats it in
an enclosed container...I think that this makes a "reducing" environment,
and the black form is produced.  The oil also seeps into the oxide coating 
and eventually "cracks", forming a "tar" which helps seal the oxide and also
makes it darker (like a wet highway).  THis finish is commonly used on clock
springs, etc.

The "bluing" *compounds* available produce the same color, without the hassle
of trying to get the heat evenly distributed.

Any chemists out there who could verify this?

JK

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From: [email-masked] (James Foster x2912)
Date: Mon, 24 Jan 1994 21:36:56 GMT

Okay, so what exactly is the process in Parkerizing and
how would one go about it in a home shop environment?

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From: [email-masked] (David Hoopes)
Date: 25 Jan 94 15:56:19 GMT

>Bruce Lucas

I have a knife that I made from a railroad spike.  The handle was left
looking like the head of a spike.  This was my first seccessfull attempt
at making a knife.  When I was grinding and polishing it, I managed to
slip up and remove the black on several large areas of the handle.  When I
hardened the knife I used motor oil as the quench.  After I wiped the oil
off with a rag, the polished areas on the handle where back to black
(along with everything else). 

That was more then three years ago.  The black has stayed on the handle
and has been pretty durable.  I have been carried it on all of my camping
trips for at least 3 years (and I tend to be pretty hard on a knife). 
When I looked at it this weekend, there where only 1 or 2 minor scratches
in the black. 

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