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Thread: Hot Bluing

  1. #1
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    Default Hot Bluing

    A friend of mine has a number of old firearms and asked if I would have a go at the hot bluing process for him, so yesterday I mixed up a small scale amount of salts from the simplest formula I found online (5lb sodium hydroxide, 2.5lb potassium nitrate, 1gal water, I actually used potassium hydroxide because I have it on hand) and ran a couple of test pieces.

    The first was a newer looking P&N HHS drill bit that took the blue quiet well considering the only prep work I did was wipe it down with some metho.
    DSC_3307.jpg

    The second test was a piece of hot rolled mild steel I turned and polished up, it came out a little bit blotchy at one end, I suspect the cause was while I was bluing some water evaporated from the salt mix exposing areas of the part to air, I also noticed the galvanised tie wire I used to hang the part left a mark so I'll have to use something else from now on.

    The area where the blue took is perfect, I couldn't ask for better, I just need to get it even over the whole part.
    DSC_3315.jpgDSC_3314.jpg

  2. #2
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    I've always wanted to try hot bluing, but can't get hold of potassium nitrate ... where did you get it?
    Cheers.

    Vernon.
    __________________________________________________
    Bite off more than you can chew and then chew like crazy.

  3. #3
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    Hi Vernon, I got a 25kg bag of potassium nitrate fertilizer a few years ago from the nearest rural supplies store, cost me about $40

  4. #4
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    Its very difficult to get potassium nitrate now - if at all. It used to be a common fertiliser. Farmers also used it mixed with diesel to surgically remove tree stumps from the ground. It cn also be used as a rocket propellant and for fireworks..... So now you need a license to buy it and farmers went on to alternative fertilisers instead.....
    Cheers, Joe
    again completely retired - more time to contemplate projects and spend more shed time....

  5. #5
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    Hence why I was asking ... I thought there may be an alternate supply small quantities.
    I have been using parkerizing instead.
    Cheers.

    Vernon.
    __________________________________________________
    Bite off more than you can chew and then chew like crazy.

  6. #6
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    To be honest I was very surprised they sold it to me, I think the only reason I was able to purchase it at the time was because I chanced upon a staff member who didn't know any better

  7. #7
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    Umm, I think you blokes might be thinking of ammonium nitrate, potassium nitrate is pretty harmless on the scale of things and as far as I'm aware freely available.
    Quick google found this:

    Potassium Nitrates

    I would have thought the green place would have it in smaller quantities, try looking for saltpetre or perhaps potash, I can't remember but potash might be a different potassium salt.
    Cheers,
    Greg.

  8. #8
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    Yeah, you'll never get your hands on ammonium nitrate and I'm sure it won't be long before potassium nitrate suffers the same fate, better go buy another 25kg bag

    Potash is potassium sulphate

  9. #9
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    With a little time and patience it can be extracted from urine.

  10. #10
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    Stop taking the Bob.
    Cheers.

    Vernon.
    __________________________________________________
    Bite off more than you can chew and then chew like crazy.

  11. #11
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    Had another go at bluing today, this time I degreased with acetone before submerging in the bluing salts, I think the results speak for themselves
    DSC_3330.jpgDSC_3337.jpg

  12. #12
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    (5lb sodium hydroxide, 2.5lb potassium nitrate, 1gal water, I actually used potassium hydroxide because I have it on hand)
    I got a 25kg bag of potassium nitrate fertilizer a few years ago
    What did you actually end up using?
    What temperature and what length of time?
    I'd like to have a go at it. Looks really nice.
    Cheers, Joe
    again completely retired - more time to contemplate projects and spend more shed time....

  13. #13
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    Hi Joe

    I used 5lb potassium hydroxide, 2.5lb potassium nitrate, 1gal distiller water (although some water did evaporate while bluing), the time was approximately 25 minutes and the temp hovered around 140 C, it climbed a little as the water evaporated.

    I switched out the sodium hydroxide in the formula for potassium hydroxide because I have some on hand and I doubt it makes any difference, if I were to scale up the process I would use sodium hydroxide because it's considerably cheaper than potassium hydroxide

    Cheers

  14. #14
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    Results look fantastic! What are you actually heating the solution in?

  15. #15
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    it should be done in a black steel/iron vessel and alloys should be avoided due to the risk of leaching that could contaminate the salts, but for testing I've just been using a stainless steel pot on the stove

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