How to darken this light-golden metal

Question

I have these rods with rounded ends, originally from a light fixture, with a full golden color of medium brightness and shine. I'd like to use those ends, and perhaps file more semispheres from the material. However, sanding one (to remove circular manufacturing traces), the cleaned material is not only quite light but hardly yellow, almost silvery with just a tinge of color.

What I want is to make it golden again, but not too dark nor dull, though high shine isn't necessary, nor matching the original finish. And while there's the option of finding another alloy, I want to learn about this case.

First I simply let the piece lie around, but this particular stuff hadn't darkened after a few days. (That was in cold, dry weather; current mild conditions actually do the trick. So I'm satisfied, but I let the question stand.) So I put it in water (really the last puddle from a bottle of ozone/oxygen disinfectant which had been standing around for months and smelled no more), thinking that would speed oxidation; yet the result of watering was still light and also actually coppery, shown in the picture.

(This photo doesn't bring out the lightness difference that well. Think of the tip as two notches lighter rather than one; and that was also so before the reddening.)

Then I went to read up on aging brass, thinking that was the material. I don't want to use strong chemicals or buy anything, so the options seemed to be salt sludge, vinegar solution (possibly salted), and coffee or coffee grounds. Yet all those articles aim at a pronounced patina, whereas I just want back a saturated golden hue like the original finish has, though the precise color doen't matter. Hence I'm wondering what to use and how to control the effect, also in relation to the times I'm at the workshop.

• How do I get my gold back? That is, which agent do you recommend,
• and how far to dilute it so that appreciable action will take either up to 3 or 4 hours, or at least 21 hours?

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People indicated in comments that the metal might be pot metal or else low-copper brass because it's so bright and doesn't catch fingerprints, neither on the show surface nor nakedly. Density is around 8.6 g/cm³. It feels slippery under saw and file but is tougher than typical brass.

Below's a bulk color comparison after two or three weeks of cold, dry air exposure vs. newly sanded. It had darkened pretty nicely. Note that the bright one isn't quite silver and the copper red shown above is unmistakable I guess.

^Sanded

^{Air exposed}

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FYI, the application is giving this friendly fellow eyes, golden orbs (with slit pupils of course) on painted wood.

Update My most elaborate, and prettiest, piece is finally done, after letting it lie a few times out of fear of botching what I had ;)

Answer 1

I'll get some ideas started. Some thoughts (updated):

• It isn't really clear what the rod composition is. The gold color on the surface appears to be different from the metal comprising the rod (not just oxidation of the rod material). It looks and behaves like brass or bronze plating, rather than something like a lacquer coating. The implication is that you couldn't restore the same gold color on the dome by letting the metal oxidize.

  If it was brass plating, I'd expect it to be covered with dark, oxidized fingerprints from handling. It could also be something like titanium nitride, which has a gold color and is often used to coat things like drill bits.

  The rod is a silvery metal that isn't soft, darkens a little with oxidation, and it looks like the oxidation takes on a bit of a coppery tint. It isn't steel (non-magnetic). It probably isn't aluminum (which can be anodized with a lot of colors, but it's soft, oxidizes quickly to a grey color, and you would have noticed the light weight). The density estimate also pretty well rules it out.

  It's even possible that the dome and the rod are different metals. The rod could be made as tubing and the dome attached to the end.

  The question has been updated and talks about the exposed metal darkening and taking on a gold color. I'm not aware of anything that tarnishes as a gold color. So that's great if it will restore itself, but hold off on the champagne until the color matches the rod surface. If it doesn't get there, I'll leave the information in this answer to provide some options.

• Estimating the density looks like the effort was too crude for that to be helpful. An estimated density of 8.6 g/cc is in the ballpark of what some of the likely metals would be. If you think it's actually very close to that number, it would rule out something mostly tin. Mostly zinc might still be within the margin or error. But it sounds like the weight estimate is not reliable, or precise enough to differentiate the composition in a way that would be useful. In case you're able to refine the estimate, here's a table of metal densities for comparison: Density of Metals.

  It could be an alloy, but for this application, it would probably be something variable and nondescript (pot metal), rather than a specific alloy with targeted properties. It might be mostly zinc or tin, although those are soft metals. As Chris H suggested in a comment, it could be low-copper brass. The presence of a little copper might explain the coppery colored oxidation.

• The type of metal makes a difference in several ways. Plating will stick to some metals better than others. You often need to plate a piece with a layer of one metal and then plate that with the finish metal. You could always just try to plate it directly with brass and see what happens. If Chris H is right about it potentially being low-copper brass, brass plating ought to stick.

• There is at least one method that involves heat, and the melting temperature comes into play. For example, tin melts at 232°C (you could melt it in a toaster oven), zinc at 420°C, and most of the other metals it could be (or that are likely to be alloyed in), melt at much higher temperatures. If you have a small butane or propane torch, you could heat one of the cut ends, or shave off some small pieces and heat them, to see if melting starts at a relatively low temperature. If you happen to have an infrared thermometer, you could measure it and get an idea of the likely predominant metal. Here's a table of melting points: Melting Point of Metals & Alloys.

• At this point, I probably wouldn't put more effort into identifying the metal. If the color doesn't restore itself through oxidation, I'd just focus on picking a solution that will give you the appearance you want and that you're in a position to try. If you don't like the result, remove it and try a different method.

So how to restore the gold color to the dome? Since we don't know what the finish on the rods is, the resulting color on the dome isn't likely to exactly match the rest of the rod. If that's important, you might want to refinish the entire rod (or both if they will be used together). If an exact match isn't important, consider the fast and easy solution of a brass or polished bronze spray lacquer. The small area might not be that noticeable and a little overspray will help it blend.

If you want the look of a bare yellow metal (no glossy lacquer), there are a few ways to do that.

• Gild it using an imitation gold leaf, which is usually a brass alloy and the color may be reasonably close. Gilding a curved surface is hard. You would need to do it as a collection of small areas, and could end up with some tiny gaps that are difficult to fill.

• Brass or bronze powder: This technique involves applying something to the metal that dries a little tacky, at least to fine powders. Then the surface gets dusted with the metal (pigment-sized powder) and burnished, which leaves it looking like polished metal. It's surprisingly durable. A variation on the theme is systems used for fingernail decoration. Those use a UV resin dusted with a metallic-looking powder (not sure if it is even metal, but it looks realistic).

• Electroplating: You can electroplate brass at home with readily available supplies, and no dangerous chemicals. There are a number of variations on how to do it (search "brass plating at home"). Here's a video to give you an idea of what's involved: How to Electroplate Copper and Brass at Home (it covers both brass and copper plating, done with essentially the same method).

  For some metals, there's a way to plate without a solution bath and power source. You basically rub a paste on the item. I've never explored whether that's available for brass, but it might be worth checking out.

• Heat plating: I just came across this option. It involves getting the item pretty hot (demonstrated with a steel item), then you brush it with a brass brush and the item picks up some of the brass. Here's a video of the technique: Brass Plating at Home - Brass Brushing. I don't know whether both the brass and the item require the high temperature to transfer the metal (with steel, you heat it to 600°C, which is well below brass's melting point of about 930°C). So, for example, whether you could get a lower melting point metal hot but below its melting point, and it would pick up some brass with this method. But this is a simple, cheap method to try (just be careful not to get the dome hot enough to melt, and if it is made of a soft metal, the brass brush could scratch it).