Talbott on wed 18 dec 96
I thought that some of you might benefit ..
Tom Buck replied...
MT: Copper carbonate is not CuCO3; the article of commerce is
CuCO3.Cu[OH]2, aka copper carbonate basic. Hence, the hydroxyl groups are
a major aid in dispersing the fine powder throughout the glaze slurry
without it clumping and thereby causing specks in the fired glaze. Copper
metal powder, unless it is extremely fine powder (sub-micron sized
particles), will not disperse easily, and unless you "gel" your glaze
slurry, the copper metal particles will settle out rapidly. Cu metal has a
density of 9 g/mL vs 3.7-4.0 g/mL for the carbonate. If you have access to
copper metal dust/powder, then I suggest you try to place the Cu metal
under or over a clear glaze by using a strong adhesive (white glue?) to
hold it in place. Then spray on the clear glaze if the coppr metal is on
first; or dip glaze the clear then spray on the metal/glue mix. Usually,
what happens in firing is that the copper metal is fully oxidized to
copper oxide black, CuO, which then will need reducing back to copper
oxide red, Cu2O.
Cheers TomB Hamilton ON Canada URL
Tom---Thanks for your reply...So when one buys copper carb from a pottery
supply dealer they are not buying CuCO3...they are in reality purchasing
copper carbonate mono copper (II) hydroxide. How difficult is it for
suppliers to properly label the contents of your purchase? (not blaming
you)... How much difference would it make in the final product if the
copper carb and the tin oxide were ball milled? Someone posted me on this
topic and said that the Chinese used sea shells as a source of calcium.
What is your opinion on the contents of those shells as to the effect on
the glaze? Would you object if your msg was posted to clayart?
Marshall T. Let me reply to you questions in reverse order.
1) If you judge the information on copper metal and carbonate basic is of
value to other potters, then by all means post the comment to the full list.
(I feel my reference books are supplying correct data).
2) Sea shells. Well if you mean oyster shells, then you are dealing
with calcium carbonate (limestone), 93-97%, some MgCO3, 1%, a bit of silica,
0.5 to 2%, a bit of CaSO4, 0.3-0.4%, and trace amounts of many other mineral
substances. The same would apply to most other marine life shells but there
are some tiny shells that contain a significant amount of phosphorus, as
calcium phosphate; and "diatomaceous earth" is mostly silica.
3) If you have access to a ball-mill, and you wish to avoid
colour-specking, then it is always a good practice to mill the glaze slurry
for 4-6 hours, possibly longer.
The oxides of copper, cobalt, chromium and nickel are quite dificult
to disperse in a glaze to achieve a non-specking fired result. The same is
true for the iron oxides, black and red, although the specking is less
noticeable in most glaze mixes. So, when non-specking is essential,
ball-milling is the best practice.
As I noted in my earlier post, the commericially-available
carbonates of copper and cobalt are an oxide/hydroxide mix (the molecular
structure/crystal contains 1 mole each of oxide and hydroxide). Consequently,
these compounds wet more easily and therefore the powder disperses more
readily. However, tin oxide is hydrophobic like the oxides cited above, so
it does not disperse easily. So, if you can ball-mill a copper carbonate/tin
oxide glaze, then please do so, otherwise you may find you may have to pass
the glaze through a 60-80 mesh screen at least three times, perhaps four or
five times, before the white specks of tin oxide disappear in the fired glaze.
4) Do suppliers know that their copper and cobalt carbonates are not
true carbonates? Probably yes. But most shops feel that potters do not need
or wish to know the analysis of these colourant materials. After all, there
is an approx. 9% difference in the amount of the metal (as elemental form)
between pure carbonate and carbonate basic -- if you bought 1 kg of copper
carbonate basic you would have 575 grams of copper (metal); if you thought
you had bought copper carbonate CuCO3, you would believe there was 514 grams
of copper metal present, or 8.9% less than would be in the carbonate basic.
This difference, when it shows up as oxide (green) in most fired glazes, is
really not noticeable, especially when weighing errors can reach 5%. But
it maybe important in reduction glazes seeking a certain shade of copper red.
The technical matters presented here are largely in the domain of
"physical chemistry" a branch of Chemistry seldom presented to students in
programs leading to an arts degree. May you now be better informed.
Cheers TomB Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm
Tom...Thank you for for your help and expertise... Marshall
Celia & Marshall Talbott
Pottery By Celia
P.O. Box 4116
Naples, Maine 04055-4116
(207)693-6100 voice and fax