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  • Black Oxide finish

    Some of the members here wanted a quick run down of how to do a Hot Black Oxide finish so here it is. It's very easy to do you just have to be careful with the salts, they are very corrosive. You'll want to wear some protective gear, I wear a face shield, gloves and respirator when I mix the salts.

    What your going to need.

    The salts are mixed from a combination of Lye, Ammonium nitrate and water. The original formula called for 5 pounds of Lye to 2.5 pounds of Ammonium Nitrate, to 1 gallon of water. I have been successful in scaling that mix to a lower quantity, just keep the ratio.
    I could not find Ammonium Nitrate in Canada so I used Potassium Nitrate, you can also use Sodium Nitrate. They are all supposed to work equally well.

    You need a pot big enough to fit the parts you plan to finish with enough room for a vigorous boil. I use a 2 gal enamel coated pot, it can't be galvanized or aluminium the salts will eat it and render your salts useless at the same time. I don't know if stainless will work I didn't want to waist the $$ to find out.

    A burner to boil the salts, I use a propane fired Coleman camp stove, it's cheap and easy to store. Whatever you use make sure it will get hot eough, you will need a vigorous boil to get the deep black look. The first time I tried this I had just a "slow" boil and the parts came out brown, so it needs to be hot and move'in.

    A few bucks of clean hot water to rinse your parts with after the boil, and to clean your pot, stove and anything else that comes into contact with the spray or boilover.

    The hardest part and most important is the finish and cleaning of your parts. Black Oxide finish will magnify all the imperfections in the work piece. I found finishing with 100-120 grit on a belt sander to be about as course as you'll want unless you want a machined look. You must also clean the parts thoroughly, any oil will leave spots that will not take the oxide finish. Wear rubber gloves when you handle your parts as even the oil from your hands will be enough to wreck the finish. I start by giving it a quick wash with lacquer thinner and then a thorough cleaning with wax and grease remover. And finish off with a dip in hot water and wipe dry. From what I've read anything that removes/clean grease will work (fantastik, simple green etc..) just make sure there is no oil/grease.
    Cleaned parts with the hanggers ready for the boil.




    Now you can mix the salts. You'll want to do this outside where your going to be boiling, don't do this inside as the steam is corrosive. Start with cool water and gradually mix the Lye in until it is fully dissolved. Now mix in your Nitrate, it will gas off as you mix it in (wear a respirator) Mix it slowly as this will heat up as you mix. Now you ready to bring it to a boil.

    Here's a picture of the setup I use. I save the haggers for the next job, makes everything go faster and they are clean from the last time.



    This is the Lye, Potassium Nitrate and a scale to get the ratios correct. Be careful handling the Lye it is very corrosive, wear safety gear



    Clinton
    Last edited by ClintonH; 03-16-2009, 01:59 PM.

  • #2
    This is the setup outside and a shot of the salts boiling.






    The length of time you leave the parts in the bath will determine how dark your parts will turn out. I tried to take a picture of the various times and depth of colour but it's hard to see the difference in the picture. 5-10 minutes will give you a light brown finish . 15-20 minutes will give you a deep brown finish. 25-30 will give you the black finish 30 being the darkest. I did not see any better or darker finish after 30 minutes.
    When I remove my parts from the bath I rinse them in hot water, you should do this quickly as the salts will ant to dry on. Don't worry if your parts have a red coating, this will wipe off with a rag or a toothbrush is what I use. I usually leave the parts in the hot water until I take the coating off.
    This is a picture trying to show the various depth of colour.




    Clinton

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    • #3
      The red coating you get if you parts touch the sides or bottom of the pot.



      Once the patrs are cleaned and dried they should be oiled. I use WD40 or ATF and soak them overnight.
      Cleaned and dry.



      Oiled and wiped down.




      After you have removed your parts form the bath you must add water to replace the boil off. Take the salts off the burner and add the water slowly as it will boil as you add even off the burner. It takes awhile to cool down, don't let it solidify/crystalize. You cannot remelt and the mix will be shot. Store is a plastic or glass container (not metal), they should be good for another 10-15 uses.


      Clinton

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      • #4
        Nice work Clinton. That would have to be a summer time job for me. I do not like to work in the snow. JIm

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        • #5
          Originally posted by JBL37
          Nice work Clinton. That would have to be a summer time job for me. I do not like to work in the snow. JIm
          Nether do I but It seems like it's never going to stop here.

          Clinton

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          • #6
            Will it work on Aluminum?

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            • #7
              Thanks!!

              I was going to use cold blue, but this looks to be the way to go.
              Thanks for your effort in posting the process, now back to lurking.
              Freedom is not free!!
              Bill in SE Idaho
              With enough time & motivation anything can be fixed

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              • #8
                Nice job. And thanks for the info, I want to try that.

                Do you mean ammonium nitrate is not even available for agricultural/gardening purposes in Canada? Good grief!

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                • #9
                  Your welcome guys. No you can't get Ammonium nitrate in Canada, I had a heck of a time finding Potassium Nitrate.

                  I meant to include this info as well.

                  I have tested what will and will not work in the salts.

                  Aluminium - Not to finish just to see how it reacts. It does not take the finish incase anyone wants to know and it will dissolve in the salts.

                  Brass - Doesn't seem to be effected but it was a quick test so don't hold me to that. It may erode as it boils.

                  Galvanized - The galvanizing is dissolved instantly and the salts are shot, with my test anyway.

                  Stainless steel - Didn't really take any colour, maybe a ting of brown. Doesn't seem to effect the salts. I don't know what a large part or using a pot made of stainless would do, I tried a few washers. For someone that could get the mix cheap it might be something to try, this stuff is not cheap for me to mix.

                  Clinton
                  Last edited by ClintonH; 03-16-2009, 03:31 PM.

                  Comment


                  • #10
                    Originally posted by ClintonH
                    I had a heck of a time finding Potassium Nitrate.
                    Stump remover, found in garden stores is reasonably pure potassium nitrate. I'm not sure that it's pure enough for this use, but may be worth a try. My understanding is that it IS available in Canada. Amazingly enough, given my unorthodox use of other household chemicals, I do actually use it on stumps.

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                    • #11
                      Originally posted by Just Bob Again
                      Stump remover, found in garden stores is reasonably pure potassium nitrate. I'm not sure that it's pure enough for this use, but may be worth a try. My understanding is that it IS available in Canada. Amazingly enough, given my unorthodox use of other household chemicals, I do actually use it on stumps.
                      No it's been pulled off the shelf, at least here in Alberta.

                      Clinton

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                      • #12
                        Originally posted by ligito
                        Will it work on Aluminum?
                        It will EAT the aluminum and "kill" the salts.
                        Ignorance is curable through education.

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                        • #13
                          Nice post. How about disposal?
                          Jon Bohlander
                          My PM Blog

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                          • #14
                            I'm collecting the parts to do just this... A propane deep fried Turkey cooker looks like the best choice, and out in the garden.

                            The salt mixture boils at around 285F (IIRC), so it's really easy to get a "steam explosion" if you add a wet part or particularly one that has a screw hole filled with water. Lye will take your eyes out... boiling or not.


                            So.. to be safe(r), I'm going to lower my stuff in from a few feet away - line and pulley, wear safety glasses and have a garden hose running to douse me if stuff happens.

                            Comment


                            • #15
                              For disposal you can dilute and pour on the ground or down the sewer. None of the ingredients are environmentally hazardous. Lye, sodium nitrate and potassium nitrate are all available in food grade and ammonium nitrate is fertilizer.
                              Free software for calculating bolt circles and similar: Click Here

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