Project Badgermascus – Interlude 1 – Sciencing How to Patina on Brass


So, the blade came out of the etch just spiffing, and giving it dark bluish-grey patina with tannic acid made it look really cool and mean, almost tacticool. But it is, of course, no longer shiny. Which made me think a lot about how to proceed from now on.

As you know, this is my first time working with damascus. Up until now, I have worked either with carbon mono steel or stainless mono steel, either with mirror or satin finish. Making the fittings on such steels from new brass, aluminium or steel is perfectly OK and does not detract from the blade. But making fittings shiny on this blade would feel, as we say in Czech, “jako pěst na oko” (like a fist punch in the eye). I do not have any spare damascus or mokume gane to go with it, nor the means and knowledge to make them, so what can I do? And the title, of course, gives away what I have decided to try – to make the fittings out of brass and make a patina on them.

So I went to my personal library, took out my favorite book “Chemistry for everybody” (published in CZ in 1990) and looked up the recipes in there. Then I looked a bit around the internet too. And then I went and bought a lot of pre-made commercial solutions for the job… NOT.

I looked up which chemicals that I already have could kinda-sorta emulate what the book says should be used for copper, brass and bronze and then I have of course performed a series of experiments to try whether I can make my own solutions. And the results are pleasing. And because there are a lot of pictures in this, the rest is below the fold.

These are the chemicals that I have used that proved to have some effect:

Citric acid  – instead of acidic acid, because it is easier to make a really concentrated solution of it.

FeCl3 – Ferric chloride – 1:10 dilution of the commercial printed circuit etching solution

CuSO4 – Copper sulfate – readily available here, I do not know why, it just is.

Ca(NO3)2 – Calcium nitrate – fertilizer

CaNCN – Calcium cyanamide – fertilizer

NaCO3 – Natrium carbonate – washing soda

K2SO4 – Potassium sulfate – fertilizer

And here you can see my highly equipped chemical lab:

© Charly, all rights reserved. Click for full size.

The only professional chemist thing in that picture is the fifty-something years old ethanol burner. I had to quickly improvise the wire stand for the small stainless bowl so I can make hot baths. I will keep it, it is a temporary permanent solution now.

I did not bother with making any precise solutions at this moment, I only wanted to check what works, I might – just might – bother with some precise shit later. So I just gave a few teaspoons in each designated beaker, approx 250 ml water on top and that was it. I also did not bother with mixing ratios except with “splash of this, then this”. I have also made no measure of time, but I have deliberately left the piece in the solution for several minutes each time to get the strongest result.

To document the results, I wrote the recipe for the solution on a piece of paper, I put on it a piece of brass polished with 120 grit next to the aged piece for comparison and then took a picture. So here go the results and a bit of talk to them.

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CaNCN has proved to be a very useful chemical. At room temperature, it has made a  mostly brown color.

Chemically this is less stinky ammonia-aging of brass (it still stinks, but I have a weak sense of smell so I smelt nearly nothing), because CaNCN decomposes into ammonia in water (very, very, very slowly).

© Charly, all rights reserved. Click for full size.

Heated CaNCN bath has made the brown darker and with a bit of blue-lilac tint to it. Sometimes the blotchiness on some pictures is due to me just polishing the piece with 120 grit after each test and not bothering to de-grease it -as I think it is here – but sometimes it is due to how the solution works (see further).

© Charly, all rights reserved. Click for full size.

CaNCN with citric acid at RT made a nice, regular golden-brown patina, slightly lighter than just CaNCN alone.

© Charly, all rights reserved. Click for full size.

Heating up the solution of CaNCN with citric acid gave the patina again blue-lilac-purple-ish tint.

© Charly, all rights reserved. Click for full size.

Adding Na2CO3 to the CaNCN precipitated a lot of CaCO3 out of the solution, and since I did not bother with skimming the solution first, it shows. The streaks here are where I brushed the precipitate from the brass piece with a bamboo BBQ skewer. It is slightly reddish-lilac-brown, and darker tone at RT than previous solutions.

© Charly, all rights reserved. Click for full size.

And again, heating the solution made the color darker, in places almost black. This time there was also some green tint to it in places, but that might just be an accident.

All in all, I can achieve a great variety of results with just this one chemical alone or just one other component, but I wanted to see whether I can get some other colors the book mentioned that it did not give me – namely green and black.

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My first attempt at green consisted of me trying whether I can substitute the NaNO2 in the book recipe with Ca(NO3)2. It turns out I cannot. I only got this light brown.

© Charly, all rights reserved. Click for full size.

When heated up, I got some hint of green in there, but mostly it is again just dark brown, a result very similar to what I have got several times over already.

CaNO3 is of course a mistake, should be Ca(NO3)2. In my defense, I was tired at this stage already. © Charly, all rights reserved. Click for full size.

This one was a bit complicated. I cannot simply buy NaNO2, because despite its wide availability in the 90s (as a food additive!), nowadays it is not sold any more because it is toxic and a proven carcinogen. The nearest chemical with similar properties is NaNO3, but that I cannot simply buy too, because it can be made into nasty thingies that go kablooey.

So I have first mixed Ca(NO3)2 with Na2CO3. That precipitated CaCO3 out of the solution which I let settle for a while and then I skimmed the solution, which was by now mostly NaNO3. However, some colloidal precipitate remained, thus the blotchiness.

To the solution of NaNO3 I have added the CuSO4. And I got a kinda greenish-turquoise color, as the book said I would get with NaNO2 and CuSO4. So I call that a success. Therefore for this solution I will probably bother with calculating the proper ratios and prepare a clean one in a bottle, with the precipitate filtered out. It is a bloody nuisance not being able to buy some stuff easily. I could possibly still order it online as a technical chemical, but gob knows whether I would not end up on some watchlist or something plus it would cost several times more.

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Heated up this gave me again dark brown, but this time without the bluish-lilac-purple tint. This solution seems to work better cold, but might be worth heating up for plain dark brown. More research will be needed.

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When I added CaNCN to this solution, I got nearly nothing at room temperature, but at the high temperatures, I have got very nice and regular light bluish – grey color.

© Charly, all rights reserved. Click for full size.

But I still did not get anywhere near the black color. So I tried a mixture of FeCl3, CuSO4, and citric acid. The reasoning was that it will dissolve the zinc and expose the copper. The reasoning was correct.

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When heated up, the reaction was even stronger and the piece of brass quickly got the red color of unpolished pure copper. It has also cleaned the unpolished backside, where the patina from all previous experiments has accumulated.

(I got nearly the same effect with another solution both cold and hot, namely: Ca(NO3)2 + K2SO4 -> precipitates gypsum, remains KNO3 solution to which I added CuSO4 and citric acid)

© Charly, all rights reserved. Click for full size.

I followed on this now nearly pure copper layer with a separate solution of CaNCN, which did give it a lot of grey tint.

© Charly, all rights reserved. Click for full size.

When heated up, that grey tint got a lot darker, but not quite black yet. To get proper black will require more experiments, but for now, I am satisfied with the results I got, because for this project a complete black is not required anyway.

This whole thing took me the better part of two whole days. Now to decide which color I want to pursue and I can start to make the fittings. At least if it goes wahoonie-shaped, it won’t destroy the blade, I will just have to make new fittings as long as it takes to get the result I want.

What I have decided to do next could, however, still destroy the blade…

Comments

  1. kestrel says

    “What I have decided to do next could, however, still destroy the blade…” he said, ominously…

    This is super interesting because as a jeweler I often have to put a patina on metal. The one I use all the time in my work is called “Midas Black Max” and if there are any jewelry supply places near you, you might be able to get it there. On the bottle it says it contains tellurium compounds and hydrochloric acid, and advises the user not to swallow it or get it in your eyes… yeah, no kidding. The cool thing about it is that at room temperature it turns sterling silver nearly jet black almost instantly. (Well -- it takes a few seconds but hey.) It will also patina gold.

    Just for fun I polished off a piece of copper, brass and jeweler’s brass (an alloy of zinc and copper). All three turn black almost right away at room temperature.

    It will be exciting to see what you do with your blade, and here is hoping it does not get destroyed!

  2. says

    jako pěst na oko” (like a fist punch in the eye)

    Funny that we have the very same saying. Passt wie die Faust aufs Auge.
    Those are very cool results.

  3. says

    @kestrel, tellurium makes sense, it is in the same periodic group as sulfur, so that it creates a similar patina on metals as sulfur is not that surprising. And I assume it has the benefit of not stinking as much as sulfur compounds with similar properties do, plus it is probably a bit more reactive.

    Thank you for the tip, but 1) this specific compound is not sold here at a casual glance and 2) I might go for a commercial compound, but only after I have had a lot of fun failing to develop my own concoction.

    I looked at the prices and compositions and my eyes popped. A few ml tiny bottle of patining compound costs on casual glance 10-20,-€, whereas several kg-weighing bag of fertilizer or a bottle of vinegar etc. cost one-tenth of that. Plus they often contain really poisonous and dangerous chemicals, whereas the surplus of yesterday’s work could be safely diluted and dumped onto the lawn since that was the intended purpose for most of it anyway.

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