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Why blacken brass when you can just paint it? There are two answers - blackened brass looks more like iron and there's no fear of filling in fine details as you can with brushed-on paint. Of course, you can use spray paint to hold detail, but using a spray can or setting up a sprayer can be a lot of work and can be messy. Blackening is simple and easy.

Supplies

You'll need two chemicals - white vinegar and brass blackener. I used to recommend muriatic acid but it can be dangerous, so now I just use white vinegar and it seems to work just as well.

Brass blackening chemicalsThe brass blackener I have been using for several years is Birchwood Casey Brass Black. It's available at gun shops and online. There used to be a good product called Blacken-It, but the company is out of business and the product is no longer available. Brass Black is meant to be used on brass, copper, and bronze only. I've had mixed results on solder and it will not work on cast metal parts.

There's another product called Jax Pewter Black that I tried recently and it seems to work well also, but requires a different technique than Casey's. According to the bottle, Jax will blacken pewter, lead, brass, bronze, copper, tin-lead alloys and solder. I have only found Jax online. Jax will blacken cast metal.

I suggest using either blackener full strength. I have tried a 50/50 dilution (with tap water) but have not had good results. I store the blackener and vinegar in small plastic containers with tight-fitting lids. You will use both of them over and over rather than throwing them out with each use, so you want something that will be good for storage.

Technique

Before you blacken the part, it must be clean! I do this by filing or sanding it and then using a wire brush on a Dremel tool. Try to get it as clean as possible. Parts that aren't clean will not blacken properly. Did I mention - the part must be clean? The part must be clean!! Brass can sometimes have a coating of varnish or even plastic. This coating must be removed or the part will not blacken.

Technique for Casey's Brass Black:
When the part is clean and ready to blacken, I start by dunking it into the vinegar. You can let it sit in there as long as you want. I'm usually not very patient when I'm trying to work with a blackened part, so I let mine sit about 10 minutes. That seems to be enough time but longer won't hurt. I have tried less time and gotten mixed results where some spots don't blacken, so 10 minutes is the minimum. I stir the part around a few times just to make sure the vinegar gets to all of it. This is especially important if you're doing multiple parts at the same time - you don't want parts sticking together. I use a wooden stick to stir the parts. When you think it's been in the vinegar long enough (or you happen to remember it) remove the part from the vinegar and rinse well with tap water. A small strainer can be helpful unless you would enjoy watching your laboriously made part sliding down the drain.

Then I toss the part into the brass blackener and watch it. You'll want to stir it around while it's in there (I use a different wooden stick for this). Again - especially important if you're doing multiple parts. It shouldn't take too long to blacken nicely. When it looks good and black, remove the part and rinse well with water. You may not want to use metal tweezers - they can turn black too - unless you keep a pair just for this operation. Allow the part to dry on a paper towel. When the part is dry, you will likely notice a black residue on it. Buff this off with an old t-shirt or other soft cloth.

Technique for Jax Pewter Black:
Follow the same instructions as above for soaking the part in vinegar. I have used the Jax immediately after sanding and wire brushing and sometimes there can be little specks that don't blacken. Soaking in vinegar seems to mitigate that possibility.

Once the part has soaked long enough and is then rinsed in water, paint the Jax on with a paint brush. DO NOT immerse the part in the Jax! If you do, a brown coating will build up on the part and it will flake off when you rinse it. I don't know why it makes a difference, but believe me - painting it on is the only way to do it. I sometimes find rubbing it around on the part with the brush helps to even out the color. The part will turn black very quickly and then you can rinse it in water and dry on a paper towel. Buff if desired.

One of the advantages of Jax is that it's almost clear whereas Casey's is blue. If you need to blacken something that is adjacent to wood or a painted surface, Casey's can stain. Jax seems not to cause any discoloration but it's worth testing first. Jax will also blacken pot metal (cast metal parts that are common in many kits). Casey's will not.

Brass parts before blackeningIn these two images, you can see the brass parts before and after blackening. Note that I soldered the ring closed using lead-free silver solder (Stay Brite from Micro Mark). I deliberately didn't do a great job of soldering. I wanted to see if the solder would blacken so I left a lot of excess solder on the part. (The solder was much brighter than it appears in the photo.) As shown in the photo below, this small bit of solder blackened just fine and I used Casey's for this test. I have had some problems blackening solder with Casey's in the past so your mileage may vary. Jax is probably better for solder but I don't have enough experience with it yet to say for sure. I do know that Jax will turn clean solder black as night so I expect this is what I will use in the future.

The bright highlights on the blackened parts in this photo are not indicative of incomplete blackening but simply highlights from the lamp used to light the part for photographing it. Although it's hard to tell here, the parts have a slight sheen to them. If you don't like the sheen, spray them with a clear acrylic matte coating.

I'd say these parts look pretty much like black iron - wouldn't you?

If you happen to scratch the part down to brass while you're installing it, you can sometimes touch it up with a Sharpie (black permanent marker). If you've sprayed it with the matte coating, it's less likely to scratch.