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Getting rid of red flux stains on brass

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  • Getting rid of red flux stains on brass

    Whenever I silver solder brass parts, I use a paste flux recommended by the silver solder manufacturer.It is made by Harris, and is simply called "StaySilv white brazing flux". It works great but wherever it melts and runs to as a result of heat from the torch, which liquifies it, it stains the brass red. Not a vivid dark red, but still red enough to be quite visible. The attached picture shows two test parts immediately after silver soldering them together, and the reddish color around the soldered area is quite visible.
    Last edited by brian Rupnow; 12-19-2014, 12:52 PM.
    Brian Rupnow

  • #2
    From the photo, it looks like the flux (perhaps an acid) is making metallic copper leach out of the brass surface. Could you try another flux? Maybe borax - that's what jewellers have used for centuries.

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    • #3
      Yesterday, I read on the internet that making up a cup of white vinegar and a teaspoon of salt, then heating it would yield a solution that, along with "brisk scrubbing" would remove the red stain. This is the result after a half hour soak. Although the parts are undoubtedly cleaner, the red stain is not removed, in fact doesn't seem to be affected at all.--Okay, so much for internet solutions!!!
      Brian Rupnow

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      • #4
        Originally posted by Euph0ny View Post
        From the photo, it looks like the flux (perhaps an acid) is making metallic copper leach out of the brass surface. Could you try another flux? Maybe borax - that's what jewellers have used for centuries.
        It is quite the opposite of this. The flux is leaching out the zinc from the brass leaving a surface rich in copper.
        I think the only way to clean of the red stain is to abrade it away, using abrasive, to get back the parent brass alloy.
        Mike

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        • #5
          Very fine soft stoning, like finishing cavities and cores for molds.

          With skill you can make any surface beautiful!
          Has to be soft stone. Has to be feather light touch.
          And food-oil will work wonders to keep the stones clear
          and not turn your skin red like kerosene does.

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          • #6
            Hi Brian
            After the solder has set and while the piece is still hot, wipe of any excess flux with a damp rag. When it cools, hit it with some #0000 steel wool.

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            • #7
              Originally posted by RichR View Post
              Hi Brian
              After the solder has set and while the piece is still hot, wipe of any excess flux with a damp rag.
              When it cools, hit it with some #0000 steel wool.
              +1
              internal corners can be addressed with pop-cycle sticks or clothes-pins
              to work the wool into places it doesn't want to go.

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              • #8
                Surely the manufacturer is the number one resource for this question? If they don't have a satisfactory answer, then try home remedies from the forum?
                "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                • #9
                  The acidic flux is fine for construction but not so good for art. Search for a non-acidic flux such as used by jewelers and crafts people.

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                  • #10
                    It is indeed copper remaining after the zinc has been removed by the flux. The only cure is to abrade it away back down to the full alloy metal surface. Chemical treatments are unlikely to work since any acid is going to react more with the zinc than the copper which is just what the flux did.

                    RWO

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                    • #11
                      Brian- I still maintain that for most of the things I see you silver soldering, a soft tin/lead solder would be more than enough.
                      Lower temp, cleans up easier.
                      Master that skill!!

                      Sid

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                      • #12
                        There are special brass alloys available that resist dezincification. For example...


                        ...http://www.nbmmetals.com/c35200-dezi...ine-brass.html
                        "...do you not think you have enough machines?"

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                        • #13
                          My main method of post silver braze clean up on brass/bronze/copper is to dip the part in an acid bath, car battery acid diluted 7 to1 with distilled water. The weak solution will clean it right (10 to 15 min.) up but will also leave a matte finish on the part. This can be restored with a bit of polishing.

                          Caution - standard acid handling rules apply.
                          The shortest distance between two points is a circle of infinite diameter.

                          Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                          • #14
                            That why I don't like brass... If you can get bronze, and be careful here as many industrial alloys called bronze actually contain zinc, you won't have the zinc depletion problem, no zinc. Classic bronze is only copper and tin, I alloy my own for casting, no nasty zinc fumes and the metal doesn’t change every time you heat it.

                            I don't know if I would describe the fluxes used for brazing as highly acidic, they are mostly fluoride salts, and following the advise of jewellers, I would use the absolute minimum of flux need to carry out the operation. The flux will flow wherever the metal is hot enough to melt it and the solder will follow the flux if the temperate is high enough and there is enough brazing filler.

                            If the parts allow it, you can drill a small hole somewhere inside the the joint that will be hidden once assembled. Place a small piece of brazing metal in the hole and flux and assemble the pieces. Hold it together with wire or a fixture, and heat the area. The flux will melt first, and the solder will then melt and be drawn into the joint by the capillary action. If the amount of material is figured correctly it will fill the joint and that’s it. This would work with brass or bronze or steel for that matter.

                            The flux is a chemical that reacts with the oxides on the surfaces of the materials, and has a limited useful life at those temperatures, so it's beneficial to mechanically clean the surfaces, flux them sparingly, flux the solder bits and assemble them and heat the assembly without a lot of screwing around. If the surface condition are correct the joint will fill and everything will be done in no time.

                            You can estimate how little metal is actually required by calculating the volume of the joint. say .75 square inches times .002" would be 0.0015 cubic inches... Its not how much solder you put in, it's how well the preparations are done. You could also try silver foil that is used to braze old school carbide lathe bits to the holders if you have some.

                            BTW +1 what Sid said, don't overlook low temperature solders.

                            paul
                            Last edited by ironmonger; 12-20-2014, 12:27 PM.
                            paul
                            ARS W9PCS

                            Esto Vigilans

                            Remember, just because you can doesn't mean you should...
                            but you may have to

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                            • #15
                              A place I worked at once where I often silver brazed parts had a solution for cleaning after brazing. It was called a Bright Dip. Can't remember what was in it but if you Google "Bright Dip" then you may find what it is.
                              Nev.

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