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Author: Subject: Azurite synthesis?
zoomer
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[*] posted on 22-3-2005 at 12:15
Azurite synthesis?


(Apologies in advance for the long post and simple-minded chemistry understanding; my high school classes are *many* years behind me.) I am doing some work with historical paints and pigments, and I am trying to make some blue paint from the mineral azurite. Azurite blue was a favorite with artists everywhere up through the Renaissance. (Unfortunately azurite oxidizes to green malachite over time, that’s why the skies in Michelangelo’s paintings are all green now.) The original paint recipe is simple -- just grind up some azurite (not too fine or it turns grey) and add it to a medium. But today natural azurite is a very expensive material, so I am hoping to synthesize it myself for the lowest possible cost.

I’ve done some research and found that there is a synthetic pigment called “bice” that is supposed to be azurite [Cu3(CO3)2(OH)2] or something very close. It is made from ammonia and copper nitrate. However, many artists claim that bice paints are slightly greener than azurite paints. I can’t tell if the bice process accidentally includes some malachite [Cu2(CO3)(OH)2] or some other contaminant, or if somehow it creates something slightly different than true azurite. Also, the mineral data sites I’ve visited (e.g. webmineral.com) say that both azurite and malachite occur naturally when sulfides of copper ores contact CO2 + water (i.e. carbolic acid). Based on these two data points I think I’m looking for a recipe that does not include copper nitrate.

I did find a recipe for malachite that creates tetraammine copper(II) sulfate from ammonia and copper *sulfate*, then adds carbolic acid + heat (60C) to form a malachite precipitate. This sounds like a rough approximation of the natural process. I’m wondering if some variation of this recipe might result in azurite. (I can provide additional process detail here if needed, I didn’t want this post to go much longer.)

Does anyone have any thoughts or suggestions for an azurite recipe based on this (or any other) information?

Thank you!
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[*] posted on 22-3-2005 at 13:07


If it's just the blue color you want, try some copper phosphate, easily made from Tri Sodium Phosphate and Copper Sulfate. Artificial turquoise can be made from it as well, in combination with Aluminum Phosphate. The stuff forms a precipitate, and is easily washed by repeated mixing with distilled water and shaking, decanting off the sodium sulfate, and repeating. The sample in the picture was allowed to dry in a plastic bottle, covered by a paper towel.

copperphosphate.jpg - 41kB




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[*] posted on 22-3-2005 at 13:55


Mr. Wizard,

Cool stuff about the turquoise, thanks! For my own curiosity I’d like to hear more. However, I do need actual azurite for historical reasons. Any guidance there is most appreciated!

Thanks!

Z

[Edited on 22-3-2005 by zoomer]
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[*] posted on 22-3-2005 at 15:21


In my opinion the preparation won't be a problem. Long-time stabilisation of the product might be more a problem. As you know azurite is metastable and will convert to malachite by the time. Temperature, acids-traces and maybe even water seem to catalyze a conversion. Therefore I recommend fast working while preparing the pigment. After the precipitation of azurite (see quote) it would be good to cool the solution down, filter off the azurite, wash it with ethanol and let it dry on air.

The particle size of the pigment is important for its colour. Different particles could be separated with an egg yolk solution. A certain Michael Price worked on the improvement of synthetic azurite powder for artistic purposes. I think this article will be interesting for you.

According to Ullmann's Encyclopedia, azurite is prepared by adding copper(II) sulfate solution, to a concentrated solution of sodium carbonate at 45-65 °C. Lower temperatures may be sufficient too.
Quote:
[color=darkgreen]...
Production. Two grades of copper(II) carbonate are available commercially, the light and the dense. The light grade is a fluffy product of high surface area. It is precipitated by adding a copper(II) salt solution, usually copper(II) sulfate solution, to a concentrated solution of sodium carbonate at 45-65 °C. Azurite is formed initially, and complete conversion to malachite usually occurs within two hours. The conversion is accelerated by the addition of malachite nuclei to the reactor.
A dark green, dense product results when a copper(II) salt solution is added to a solution of sodium hydrogen carbonate at 45-65 °C; conversion to malachite requires about one hour in this case. The density is maximized if the reactor is washed with acid prior to the precipitation to prevent premature nucleation on malachite nuclei. (A less dense product would be produced if malachite nuclei are added to the slurry of azurite.) Solutions of copper(II) salt and sodium carbonate can also be added simultaneously at a pH of 6.5 - 7.0 and a temperature between 45 and 65 °C; conversion to malachite is usually complete within one hour.
When a solution of copper(II) ammonium carbonate is boiled, ammonia and carbon dioxide are expelled from the solution, and a deep green, dense copper(II) carbonate precipitates (38).

(38) W. Kunda, H. Veltman, D. Evans, Copper Met. Proc. Extr. Met. Div. Symp. 1970, 27-69[/color]
~Mephisto



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[*] posted on 22-3-2005 at 16:04


Mephisto,

Great stuff! How did you discover the article by Price? I've been google'ing this for weeks.

You're right, this method would need quick action to stablize, but it could be what I'm looking for. I will try this as soon as I can.

Thanks!

Z
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[*] posted on 22-3-2005 at 16:39


I've found it, because I've searched for azurite in the catalogue of Kremer-Pigments. And there was the important note:
Quote:
One of the unsolved mysteries of painting technology is why azurite used in oil sometimes does not turn to green. Azurite contains copper, which is notorious for changing its color in acidic environments, such as linseed oil. Now a customer of ours - Michael Price - has developed a process to preserve the subtle blue hue of azurite in oil by coating the particles with protein. As a by-product of this process, the pigment particles are sorted according to purity and size - yielding many new and vibrant shades that can be used in oil painting. Scientific analysis supports the use of a protein coat in historical copper paint pigments - the IRPA in Brussels found protein coatings on most of the pigments used by the brothers Van Eyck.
~Mephisto



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[*] posted on 22-3-2005 at 17:19


I found it interesting that it mentioned proteins (which egg yolk has plenty of) - it really does seem to be a good reagent for preserving colours, and the original compound (didn't this use to be the 'reagent' of choice for many centuries?). I wonder what the chemistry is that causes greater stability, as opposed to linseed oil. Oil obviously is degraded by lipases, while protein is degraded by proteases. So where's the difference? Presumably intermolecular crosslinking and complexing, which is something that natural oils cant do.

Upon reading the first post, my first thought was 'Kremer Pigmente'! Here we go, Mephisto was first!

Anyway, zoomer, most of us probably prepared 'azurite' before, if it is just about mixing the right quantities of copper sulphate and sodium carbonate. It is a nicely turquoise product that isn't too dense, and takes some time filtering.
I have not observed conversion to anything else.

How different are the copper pigments you can buy from pottery suppliers to the pigment you desire yourself? I guess it's hard to tell without comparing the two directly next to each other.




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[*] posted on 22-3-2005 at 18:44


Chemoleo,

Thanks for the insight! To clarify, I am helping an artist acquaintance who teaches, among other things, historical restoration techniques. In some situations the customer requires authentic materials to be used, instead of the better modern substitutes. I volunteered to help her find another source for the expensive azurite, but I was having little luck, hence my post

Egg mediums have been used since ancient times, and were widely used until refined oils became more prevalent in the sixteenth century. But oils did not replace egg mediums completely (and are still used today), which may partly explain why some paintings from the same period have oxidized and others haven’t. But I’m no chemist... :-)

I did manage to make some powder, but it has the characteristic grey tone of too-finely-powdered azurite. I believe I need some slightly larger crystals that I can then grind down a little, but still leave the brilliant blue that the Masters preferred. Any help here would be appreciated. The article from Price should be a big help in the final paint preparation.

I was expecting just a chemical formula from this post, but all this additional information has made this thread fascinating! If there are any more observations or suggestions, please post them, it’s all been very helpful!

Thanks!

Z
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[*] posted on 28-3-2005 at 19:36
Azurite followup


Several people kindly responded to my previous post with info about using copper sulfate and sodium carbonate to synthesize azurite (Cu3(CO3)2(OH)2, copper carbonate hydroxide). For safety sake, I have a few quick follow-up questions -- what happens to the sulfur and sodium in this reaction? Do I need to take precautions beyond normal chemical handling? Also, can someone supply the equations?

Thank you!

Z
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[*] posted on 28-3-2005 at 20:15


Zoomer; The Copper and the Sodium will switch anions , that is the Carbonate will precipitate out of the solution with the Copper, leaving the Sodium ions (Na+) and the Sulfate anions (SO4)-- behind in solution; two Sodiums for every one Sulfate, to balance their charges. The solution, and precipitate will just require normal chemical handling.



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[*] posted on 29-3-2005 at 07:04


Mr. Wizard,

Thanks for the info. I'm curious, what would happen if I used carbonic acid (H2CO3) instead of sodium carbonate?

Thanks!

Z
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[*] posted on 29-3-2005 at 08:45


Zoomer, as you might guess, I've simplified the process, both to save time and hide my lack of knowledge ;-). Using "Club Soda", or carbonated water, the H2CO3 solution will NOT work to any large degree. When these ions are in solution there is a constant "swapping" of partners. By using H2CO3, a weak acid, you are trying to displace the --SO4 making a stronger acid H2SO4, and CuCO3, a stronger base than CuSO4. Thus ending up with a stronger acid, mixed with a stronger base. These reactions do take place at a very low rate, but are like a rock falling uphill. In general, unless on of the reactants has a very low solubility, which 'tilts' the balance in it's favor, most reactions end up with weaker acids, and weaker bases. That's my understanding of it, but give it a try, what can it hurt?

You can use H2CO3 , with atmospheric O2, to directly attack copper metal though. It is a slow process, but it does proceed, and is what makes the green on statues, Verdigris (green of Greece, French?, from it being found on old bronze Grecian statues).

[Edited on 30-3-2005 by Mr. Wizard]




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[*] posted on 29-3-2005 at 13:03


Mr. Wizard, thanks for the clarification. That’s interesting, because all the mineralogy texts I’ve read say that azurite forms naturally when copper sulfates, leached from sulfide ores, are acted on by CO2 dissolved in rainwater that percolates down through the soil. But I have never found a source that validates that, or describes the process in any more detail. And as you point out, that reaction is unlikely because of the relative strengths of the reactants. Unless there is some natural, additional mechanism that could absorb the sulfate anions? I’m curious now if that mineralogy description is just an assumption that has become “fact” by retelling over the years.

I’ll give it a try and let you know what happens.

Thanks!

Z
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[*] posted on 29-3-2005 at 20:28


No doubt copper sulfides will be oxidized by air to form sulfates, sometimes with the help of bacteria, and water. I'm not sure what role CO2 would play in the process. The resulting sulfates would react with limestone and the like to form Copper Carbonates. You can demonstrate the process by washing the white crud from your car battery terminals (Ferrous Sulfate) onto your concrete driveway, and witnessing the resultant brown iron stain. Water running from mines having sulfide ores are often so acid from this oxidation process that they are considered a toxic waste source, and will pollute rivers downstream. Maybe someone can tell us what role CO2 plays in the formation of the sulfate from the sulfide.



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[*] posted on 30-3-2005 at 07:27


Oops, sorry, I wasn't very clear. The sulfide ores weather just as you describe into copper sulfate, that then accumulates in underground chambers of all sizes. Later, rainwater with CO2 dissolved from the air and soil enters the chamber and contacts the copper sulfate. [Magic happens], and the complex carbonate minerals azurite [Cu3(CO3)2(OH)2] and malachite [Cu2(CO3)(OH)2] result. (Not at the same time though, apparently the difference is caused by the amount of CO2 present.) All of this is pretty well documented, it’s the [magic happens] part that I can’t find any info on. And as you point out, just mixing the two together doesn’t do anything. (At your suggestion I tried it and you’re right, you always wind up with copper sulfate in the end.) So what happens? Where does that pesky sulfate anion go?

If it helps any, areas where azurite forms are always arid, geologically stable areas of igneous rock. Many heavier metals such as tin, lead, silver, etc. and especially iron are present in chemically but not economically important amounts. There is an abundance of silicates. The lighter metals such as calcium, beryllium, etc., are rarely found near azurite, although aluminum sometimes is.

Thanks!

Z
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[*] posted on 30-3-2005 at 13:00


It might be that the sulfate ions are leeched away, possibly taking, for instance, iron along for the ride. Ultimately, copper carbonate is left behind, while the more soluble sulfates seep further.

Eh, lots of weird things happen when you have a few million years to do it. I seem to recall limestone is altered to dolomite and magnesite through ancient seas, presumably the MgCl2 content is exchanging ions with the CaCO3. How that is favorable (CaCO3 is less soluble than MgCO3), I don't know!

Tim
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[*] posted on 31-3-2005 at 09:39


12AX7, that's what I was thinking, too. But as I understand it from another thread, wouldn't iron steal the carbonate and leave only copper sulfate?
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[*] posted on 8-4-2005 at 09:53


Hey, I found something on blue copper precipitates:

http://67.15.145.24/~sciencem/talk/viewthread.php?tid=1605

May or may not be true hydrated basic copper carbonate, but it's worth a shot.

Tim
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[*] posted on 10-4-2005 at 07:16


All this time I've been searching this forum under "azurite," should have included "copper carbonate." Duh! Thanks for the pointer, 12A7X!!
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