zoomer
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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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Mr. Wizard
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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.
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zoomer
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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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Mephisto
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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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zoomer
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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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Mephisto
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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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chemoleo
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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.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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zoomer
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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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zoomer
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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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Mr. Wizard
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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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zoomer
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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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Mr. Wizard
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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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zoomer
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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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Mr. Wizard
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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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zoomer
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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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12AX7
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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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zoomer
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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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12AX7
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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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zoomer
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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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