[ Ref. Bob Caletti in 150520 MMDigest ]

Oh, Bob, you came to the right place.

First, I hope you are asking about how to blue steel.  That blue
surface is an oxide coating that can be produced in a several ways.
The most common was is to use a chemical bluing liquid, but I've
never been happy with the results; it's sort of a gray-black coating
more suitable to guns.

If you want something that is really outstanding, you need to blue
the part by using heat.  Below are links to a couple pictures of the
process.  It's fairly simple to do.

First you bring the surface up to a mirror polish: file, sand and
buff it.  Then you slowly heat the part up to about 560 degrees F.
Don't bother to measure the temperature -- just watch the color.

I use a hot air gun because it's easier to control.  If you use a
torch, the color of a flame tends to obscure the color of the object.
With hot air you can watch as the color progresses up the scale.

It can take a little practice because the temperature/color tends to
drift up a little when you remove it from the heat.  In the last
picture, you can see that the last screw on the right is still slightly
purple in the center while the rest is a nice blue color.  I should
have heated the shank of the screw a bit first so the whole head
reached 560F at the same time.

After you get it to a nice blue color, remember to add a bit of oil so
that it won't start to rust on you.

Give me a call if you want to discuss the process.  These pictures are
from a demonstration I did last year for the NAWCC.

Have fun.

Craig Smith

 [ Blued washers on a music box
 [ http://www.mmdigest.com/Attachments/15/05/21/150521_195901_d.jpg
 

 [ Four stages of color
 [ http://www.mmdigest.com/Attachments/15/05/21/150521_195901_g.jpg
 

 [ Temperature vs. Color chart
 [ http://www.mmdigest.com/Attachments/15/05/21/150521_195901_n.jpg