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I want to create a necklace made from concrete, but use a brass crimp tube for the chain hole. Is there anything chemically wrong with doing that? Will either the concrete or the brass corrode over time by mixing them?

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    I doubt concrete will be as much a corrosion problem as the salt and moisture on a persons skin which will cause the brass to develop a green patina. I guess you could get silver tube . Dec 11 '20 at 1:09
  • Untreated concrete is porous, and air is an essential oxidation agent. So I think there is some risk of corrosion over time, but you may be able to use that to your advantage in your design. Dec 11 '20 at 8:47
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Different metals have different issues. For example, aluminum is highly reactive with the alkaline concrete (the reaction is used to create foamed concrete). Iron rusts and decomposes all the way through. Besides losing its integrity, the rust can crack the concrete.

Brass is much less affected by the concrete. The concrete can accelerate development of the patina and pitting on the surfaces in contact, which would not be visible. This won't affect the integrity of either the brass or concrete. And concrete covering one portion of the brass won't affect the exposed, visible brass.

Perhaps a more important issue is just that brass tarnishes, even when it's well protected by a finish. If the finish is at all permeable to air, the brass will tarnish underneath and the finish just makes it harder to polish the brass again. If you want the patina as part of the intended design, great. But if you want the brass to look polished, the concrete will make life difficult. Concrete is porous, and cleaning the brass later will stain the concrete. You could try to seal the concrete to facilitate cleaning the brass later.

If you go this route, don't embed the brass in the concrete when you mold the concrete; that would make it almost impossible to seal the concrete where they meet (and they expand and contract at different rates). Instead, mold or drill a hole for the brass. Seal the concrete, and you may want to seal the brass. Then put the brass in the concrete. I wouldn't try to crimp the brass after it's in the concrete, as it will create stress that could break the concrete (it has very little tensile strength). I would glue the brass in place if it is necessary to hold it there.

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  • Could one use painters tape to separately seal the concrete and brass instead of adding the brass later? Or are you saying the issue is sealing the non-visible area where the brass and concrete are touching?
    – rfii
    Dec 27 '20 at 2:01
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    @rfii, I wouldn't trust masking the surface with something like painters tape. And yes, the issue is sealing the non-visible area. A microscopic gap can develop over time between the surfaces via expansion and contraction. Then you get capillary action drawing in liquid or stains around the brass and creating a dark ring.
    – fixer1234
    Dec 27 '20 at 2:56
  • Thanks really helpful elaboration! When you say mold the brass, do you mean simply putting some release agent on it and pulling the brass off once the concrete sets then putting it back after sealing?
    – rfii
    Dec 27 '20 at 4:35
  • @rfii, if I was doing it, I would try to avoid initially embedding the brass. It could get buggered up taking it out, and the hole should be slightly oversized. Also, the brass would then need to be cleaned before it could be sealed. If it needs to be glued back in, any residual release agent in the hole would interfere. Instead, I would use a temporary placeholder in the concrete mold that concrete doesn't stick to. The author here was talking about a brass tube, so I might use something like a plastic or silicone rod, maybe incorporate it into the mold, itself.
    – fixer1234
    Dec 27 '20 at 5:38

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