search  current discussion  categories  materials - copper 

copper reds / why ray bathes with molten salts

updated fri 30 oct 98


Jeff Lawrence on thu 29 oct 98

Hello Clayart,

Charge up the dendrites on the other side of the brain, gang!

Ray is currently abusing blends I ran of varying amounts of black, red
copper oxide and copper carbonate. These I ball-milled for 2 hours and
found that
(1) the copper coloration was much stronger, I'm assuming because of better
dispersion and
2) the glaze sealed over much earlier (by about 200F or so) and made all
tests in the hot part of the kiln forest green!
The reds in the bottom were very consistent but livery -- as if I'd used
too much copper. Today's tests will ball-mill for shorter time with less

Here's Ray:

>Subject: Copper thoughts
>Date: Tue, 27 Oct 1998 13:57:22 -0700
>Here is why I am interested in molten salts. Hope it makes sense.

>The earliest method used to analyze ceramics for elemental
>composition involved putting them into solution in molten
>carbonates. You can't do it with just one carbonate, because
>the melting point is too high for anybody but a potter (I can't
>do it with a hot plate or simple burner). You need mixtures.
>Some molten-salt systems are the following.
>Eutectic Temp. ((C) Components Composition (mole %)
>86 LiNO3-NH4NO3-NH4Cl 25.8-66.7-7.5
>120 LiNO3-NaNO3-KNO3 30-17-53
>142 KNO3-NaNO3-NaNO2 44.2-6.9-48.9
>188 KCl-TlCl-AgCl 7-37-56
>357 KCl-LiCl-NaCl 24-43-33
>397 Li2CO3-Na2CO3-K2CO3 43.5-31.5-25.0
>512 Li2SO4-K2SO4-Na2SO4 78-8.5-13.5
>Nitrates can provide oxidation, nitrite provides both reduction
>and oxidation (depending on the thing it is reacting with), and
>carbonates are extremely strong bases. Silica is an acid, and
>it reacts with strong bases to form soluble salts. After you
>fuse a silicaceous material with carbonate, you can just
>dissolve the "glass" in water. But the glass shows you some
>things about composition by itself --- just like a borax bead
>The point here is that we know that oxidation of the red gives
>a green/blue, somewhat cruddy color. The hypothesis has been
>that Cu0 causes the color. If that is true, reduction of the
>color should not show a change. You should not be able to
>reduce the red too much, if it is already the copper metal.
>The only effects would be to deposit too much carbon or to
>boil off copper (as carbonyl, etc.).
>I haven't been able to find oxidation potentials for copper
>compounds in glass, but the potentials for them in water are
>well known. They will almost for sure be in the same order
>in any vitreous medium. Anything below +0.158 volts will
>reduce Cu+2 to Cu+1. Anything below +0.522 volts will reduce
>Cu+1 to the metal. And anything below +0.3402 volts will
>reduce Cu+2 to the metal. Incidentally, the nitrate-nitrite
>couple has a voltage near zero (0). You might think that any
>reducing system should produce the metal, but that probably
>isn't the case. If Cu2O goes into solution in the vitreous
>glaze very fast, it will be out of rapid contact with CO (etc.).
>And it used to be a pain for copper producers that Cu2O is
>very soluble in copper metal (the metal didn't work right).
>They used to stir molten copper with green wood sticks to
>reduce the oxide. That caused bubbles of CO2 to form in the
>metal, so they called it "blister copper."
>I think it would be a considerable help to find out what is
>actually causing the color. Then you can shoot for it
>If the color is Cu2O in solution and/or suspension as a colloid,
>adequate reduction of the system should give Cu0, and that will
>certainly change the color. And --- mild oxidation (anything
>above +0.158 volts) will switch the Cu2O into cruddy green, but
>it won't touch the copper metal. Time for some molten-salt
>experiments on the microscope?
>I hope we don't have an "all-of-the-above" problem here.
>Good luck.
Jeff Lawrence
Sun Dagger Design
Rt. 1 Box 394L
Espanola, NM 87532
vox/fax 505-753-5913