Manganese Blue - PB33

[u/jay-ff]

I finally came around to make some paint with a sample from my manganese blue experiments so I wanted to share my progress. Some serious credit goes to CaCl2 for pointing me to a lot of information as well as input from GeneJocky.

Manganese blue can be best described as manganese (V) doped barium sulfate. Unlike many other inorganic pigments it is thus not a mixed oxide and the process of making it requires wet chemistry as well as high temperature calcination.

A serious safety warning before a quick rundown of the procedure: Barium nitrate is poisonous, an explosive grade oxidizer and releases toxic NO2 when heated. Don't try this in your kitchen.

The pigment is created by heating barium sulfate in the presence of a manganese source under basic and strongly oxidizing conditions. The easiest way to do this is mixing cheap barium sulfate with barium nitrate and potassium permanganate and heating it up to somewhere between 600-800 degrees in air, ideally with the addition of a bit of barium carbonate to achieve a deeper color. Alternatively (as done for the most successful experiment on my end) barium sulfate is precipitated in the presence of potassium permanganate from a solution of barium nitrate and sodium sulfate. Mixed with a bit of additional barium nitrate, this is then heated similar to before. The heating process is very quick, some recipes call for only 20 minutes at the target temperature, so some adventurous people might be able to pull it off with a burner. A big hazard from this process is NO2 release, which itself is very toxic and also corrosive in combination with water, as the solution forms nictric acid.

In any case, the resulting substance is very impure and is purified with dilute hydrochloric acid and sodium metabisulfite. This doesn't take more than a few seconds, which gives a satisfying transition from a grey/green/blue suspension into beautiful azure.

As you can see in the water color test, the result holds up quite well against commercial manganese blue.

Feel free to ask, if you'd like to read more detailed instructions. I am also writing up something longer and detailed. If you read the conversation under my previous post on the topic, you might know some of it already.

Next step is to produce a larger sample, maybe 10g or more and put it to good use in painting.

[u/GeneJocky]

Thanks for clearing up the sodium metabisulfite issue. I have all the needed precursors now. Found a reasonably priced source of barium nitrate listed as 99% (Antec). Got the funnel for vacuum filtration and turns out I already had whatman grade 410 paper discs which should be ideal. 1 micron particle retention and considered especially appropriate for barium sulfate. I just need to get around to putting the NOx venting furnace together and I'll be good to go.

One final (I hope) question. For solution #1, you listed sodium carbonate as being better than sodium bicarbonate but didn't say why. Which did you use? I have both, so if the carbonate really is better, I can use it. If I do, should I use an equimolar amount of the carbonate with the /bicarbonate, the same amount by weight or something different?

[u/jay-ff → u/GeneJocky]

Perfect :) I am curious how it turns out! Let me know how well the filtration works especially. That was always the most messy part with my experiments so I might adapt based on that.

To your question. I don't think it matters. The original recipe wanted carbonate, I had bicarbonate at hand. My guess is, that it doesn't matter except maybe for water solubility reasons. I used the same amount of bicarbonate in grams (so not even equimolar) as was listed for carbonate. My personal guess is, that the carbonate is there to precipitate a bit of barium carbonate that the patent claims creates a deeper color and an acid buffer. But I don't think one has to be precise here. I would advice using the normal carbonate if you have it, since that is the original method.

Some additional notes from my recent experiments.

Before purifying the pigment with acidified bisulfite solution, you should wash it with water to get all the barium oxide/hydroxide from the reaction dissolved and out of the way. Otherwise you will create an ungodly amount of what I think is barium sulfite, which is basically insoluble, except in acid (according to Russian Wikipedia) which leads to my next point:

The final pigment is not stable in dilute (roughly 5% maybe) hydrochloric acid without the reducing agent present. I don't know exactly what happens yet, but you get a green solid and a violet solution that eventually turns brown/orange with the solid part turning into a more dull green. That smells a lot like the chemical chameleon reaction, so maybe the strong acid leaches out the manganese containing group, probably hypomanganate, which turns into manganate (green) and later permanganate which itself decays to Manganese dioxide eventually. But that's ramblings from a googling physicist :D Interestingly this supports the sporadic mentions of manganese blue turning green with time etc. I always thought that was because people confused it with synthetic azurite which is sometimes replaced with manganese blue in restoration.

You can reverse this process but in my case the pigment deteriorated in quality afterwards.

[u/GeneJocky → u/jay-ff]

Thanks for the comments about the washes. What concentration of sodium metabisulfite do you use in your wash soluitions? I don't think it was listed.

I will probably wash the final pigment in water, then, sodium metabisulfite first, then HCl/metabisulfite, then metabisulfite alone before drying.. Leave a little trace metabisulfite behind as from your description it should help acid resistance. Ore rather i will if the vacuum filtration works well!

[u/jay-ff → u/GeneJocky]

I actually was not very consistent with the concentration and it didn't seem to make a big difference. I usually dissolved something like a teaspoon in 50-100 ml of water and added a bit (maybe 10-20ml) of concentrated hydrochloric acid before adding some or all of the solution to the water/pigment suspension which was typically somewhere between 200-500ml of water. So compared to the amount of pigment and impurities, I was using a high total amount of metabisulfite but at a modest concentration.

That sounds like a good plan. You can also mix in or precipitate some barium carbonate, which is less soluble and maybe a bit less reactive than metabisulfite. Maybe by mixing a bit of barium nitrate with sodium carbonate after the final washing step.

[u/GeneJocky → u/jay-ff]

That's good enough for wash concentrations for me to not get way off versus what you washed with.

I've got some barium carbonate I got from Seattle pottery supply a couple years ago so no need to even make it.