will edwards on tue 26 dec 00
Hello,
This thread is getting interesting. June expressed another analogy for ge=
tting
the green. I looked into my folders for several cobalt greens and then
compared the Cobalts that turn out blue as they should and I can't see
anything other than the rutile and generally high boron.
Rutile - *V203/TiO2/*Fe *denotes trace
The variations of sodium or potassium glazes didn't seem to quantify for =
the
change. Could high boron account for the green hue when it hits above a
certain temperature range?
Tom Buck Archived info - "If the glaze is made less acidic =
(less alumino-silicate, more alkali & alkaline earth oxides) its fine =
structure is altered somewhat, and then a shift from blue to green occurs=
=
with coablt oxide colourant." =
In noticing many other related Cobalt theories in the archives most of th=
em
concluded that rutile was/is the culprit. Sodium/TiO2/Cobalt Mmmm. sounds=
like
time for some Alchemy in a few days.
William Edwards
I used to practice Chemistry, but I now prefer Alchemy. You never know wh=
at
your gonna get! Last time I turned a Sow's ear into a silk purse.
____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=3D=
1
Craig Martell on tue 26 dec 00
Hi:
I've seen this color produced with many cobalt bearing glazes. However, to
call this green a cobalt green is only part of the story. I've never seen
a green glaze produced with just cobalt and no other colorant as a
modifier. I've seen green glazes containing cobalt come out of both of my
kilns. The IFB reduction kiln and the hard brick salt kiln. I use
cobalt/rutileslips in the salt and get greens just from that and the
salt. I've used Reitz Green in the reduction kiln with variable
results. It's usually a lovely green but sometimes it's just blue and
other times it's blue and green.
If the green color incobalt glazes is to be understood, I think that a very
systematic approach would yield the best results in the shortest amount of
time. Do some flux blends, lines, triaxials, quadraxials that produce the
green, or are close. Then take the promising flux mixtures and do a
biaxial blend with alumina supplied with clay and silica. Do several of
these 35 glaze biaxials and start accumulating data that point to zones
where this type of color forms and other zones where it does not. I use
Ian Curries Recipe Method for doing this. I can blend the 35 glaze
biaxials in about 3 hrs. That's not bad and you get a lot of good info.
Looking for insight into glaze response by trying random samples is the
famed slow boat to China. I'm not saying that you shouldn't do that if
it's your bent. But there are faster more thorough ways of getting what
you want.
Now station the sea and anchor detail!!, Craig Martell in Oregon
iandol on wed 27 dec 00
My thanks to all the people who have joined in on this tread.
So far the summary seems to suggest that there is a strong relationship =
between the Cobalt Oxide and Titanium Dioxide which can lead to creen =
colouration under the influence of many glaze types.
I will follow this through with my own style of blending, which is as =
systematic as that of Ian Currie. I enjoy weighing and measuring.
Some little bits of information which have come my way are that Cobalt =
oxide CoO is olive green, and that Cobalt oxide Co3O4 may have a spinel =
structure of CoO.Co2O3. Furthermore, at elevated temperatures, possibly =
higher than we get in our kilns, Titanium dioxide starts to react as =
though it were amphoteric and produces Titanates with many of the =
metallic elements.
Best regards to the Alchemists.
Ivor Lewis, Redhill, South Australia
Bonnie Staffel on wed 27 dec 00
Hi Clayarters,
Been lurking this cobalt green dilemma where rutile was used as the
modifier. I put 1% cobalt carb. into Albany Slip. From this I get a
beautiful green with which I paint "majolica style" on top of my basic white
glaze. The opacifier modifies the color to give me the green. My white is
just a standard Neph/Syn glaze with no rutile or other colorant. Just
Zircopax as the opacifier. I fire to Cone 8 - 10 ox., have also used this
in reduction and wood fire, still coming out green.
My take on this is that Albany Slip is heavy in iron and thus changes its
color. If I add more cobalt, then I get blue.
Bonnie Staffel - eyeball deep with snow every day for over a month. Local
Snow Plower doesn't have anymore places to put the scraped snow from my
driveway. Set a live trap to catch an opossum under my house. Well, that
is buried and the critter is in for the duration. Wonder if there are any
mice for it to eat.
Tom Buck on thu 28 dec 00
Ok, Will Edwards, June P., Ivor, et al:
My earlier post that Will extracted from the archive was correct
as far as I went. I stopped short. I should have then said that under
conditions of near alkaline pH (ie, a pH close to neutral of 7, say 6+)
the rutile will induce a yellow hue to transmission colours. But the
cobalt is still there, a bit subdued, to yield a blue. Add together yellow
and blue and you get the green many have noticed. The variation in tone
will come about as the pH of the mix shifts one way or the other (as
someone noted, put in more CaO and the pH shifts towards neutral) and so
the green becomes stronger.
This whole area of acid/alkali balance in a glaze mixture has not
been studied much to my understanding. Any comments from David Hewitt,
Michael B, Gavin, ....????
happy Hogmanay, everybody. Peace. Tom.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
June Perry on fri 29 dec 00
Hi Tom:
You've confirmed my own suspicion about the acid/alkaline balance being one
of the contributing causes of this cobalt green.
A good site to check for the color change due to the altered ratios of silica
and alumina, is the Japanese glaze database site, NIRIN. Especially eye
opening are the photos of some of their celadons which show the same RO
column ingredients and then various alumina/silica ratios. The colors change
quite a bit just with the alteration of these two items.
Warm regards.
June
David Hewitt on fri 29 dec 00
Tom,
Some years back now, Ceramic Review asked Mike Bailey and me to carry
out some tests based on an article by Mike Kusnik. It was a base glaze
to which opacifiers were added in triaxial blend and then different
colouring oxides added. The opacifiers were titanium dioxide and tin
oxide. If you lay out the results for each colouring oxide addition in a
triangular form the left hand side of the triangle has increasing
amounts of titanium dioxide and no tin oxide. The blends with 1% and 2%
cobalt oxide gave green results with 7.5% titanium dioxide. You can see
these results on my web site under 'Opacifier Test Series' sets 'C' and
'D'.
This indicates that for any particular base glaze, you would have to get
the amount of titanium dioxide correct if you want a cobalt green.
The base glaze in these tests was:-
Potash Feldspar 328
China clay 156
Talc 212
Whiting 115
Zinc Oxide 30
Quartz 159
____
1000
Fired to cone 8 in oxidation
With our analysis of our materials this gave an Alkili/Alkaline Earths
ratio of 1/4.98.
We did not do further tests on other base glazes that would have had
different ratios, but I trust that this example is of interest.
David
In message , Tom Buck writes
>Ok, Will Edwards, June P., Ivor, et al:
> My earlier post that Will extracted from the archive was correct
>as far as I went. I stopped short. I should have then said that under
>conditions of near alkaline pH (ie, a pH close to neutral of 7, say 6+)
>the rutile will induce a yellow hue to transmission colours. But the
>cobalt is still there, a bit subdued, to yield a blue. Add together yellow
>and blue and you get the green many have noticed. The variation in tone
>will come about as the pH of the mix shifts one way or the other (as
>someone noted, put in more CaO and the pH shifts towards neutral) and so
>the green becomes stronger.
> This whole area of acid/alkali balance in a glaze mixture has not
>been studied much to my understanding. Any comments from David Hewitt,
>Michael B, Gavin, ....????
> happy Hogmanay, everybody. Peace. Tom.
>Tom Buck ) tel: 905-389-2339
>(westend Lake Ontario, province of Ontario, Canada).
>mailing address: 373 East 43rd Street,
> Hamilton ON L8T 3E1 Canada
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
Craig Martell on thu 30 may 02
Hi:
I've never seen cobalt produce green by itself. The idea of
cobalt-titanium greens has been talked about and these are the two oxides
I've seen produce the green. From a color theory standpoint it's fairly
simple mix blue(cobalt) with yellow(titanium) and you will have green. To
get this to work in a glaze is a bit more difficult, but not too bad.
Cobalt is a fluxing colorant that produces very strong blues even with low
percentages added. Rutile and titanium are fairly refractory colorants and
you need a bit more in the way of percentage to get them to contribute
their needed influence on the cobalt. Alumina /silica ratios come into
play when trying to get the cobalt to not enter so readily into
solution. When cobalt is taken into solution in the glazes and is
dissolved it produces a strong blue that will, in most cases, overpower the
titanium which does not go into solution as easily. As in shino glazes
where the high alumina content inhibits the iron from going into solution,
which would form a celadon, alumina in other glazes has this effect on
cobalt. Look at Reitz Green. This glaze has a relatively small amount of
rutile against the cobalt but it makes a green. The composition of Reitz
Green is somewhat like a shino glaze.
You can also produce green in lower alumina bases by adding a lot more
rutile. Then you could run into problems with blistering. You can also
produce cobalt greens in reduction and oxidation with mixes of cobalt and
chrome. In oxidation you can modify cobalt with chrome and, or, copper.
regards, Craig Martell in Oregon
Alisa Liskin Clausen on fri 31 may 02
When I was using some Dolomite white cone 6 ox. glazes, I painted stripes of
oxides underneath to see how opaque the glaze would be.
I got cobalt greens and was also wondering where the green the coming from.
In fact, there has been a discussion, still in the archives.
Hank told me that when his wife was working on achieving cobalt green she
found that
(abbreviated)
Magnesium is neither required or should be excluded.
Titania is required, ie. Titanium ox. or Rutile.
Make small batches of 1% cobalt plus 4% Rutile
and
1% Coblat plus 4% Titanium
and paint stripes over all the glazes you think will give you a coblat
green.
One base I use for striped effects that gives blues and greens is
WoDo White
50 Neph. Sye.
25 Dolimite
20 Ball Clay
5 Wollastonite
You can mix 1% Coblat ox. plus 5% Rutile itno the base,
and for me, it gives a very green mat glaze with streaking.
good luck, Alisa in Denmark
| |
|
