Judy Smith on tue 30 nov 10
I did several glaze tests this week and none of them came out as expected.
I used Ron Roy's cone 6 oxidation firing schedule. What am I doing wrong?
Here are the recipes for 3 of the glazes. The comments inside the brackets
tell what the test tiles look like instead of their intended colors.
Thanks,
Judy Smith
*Jennie Purple cone 6 ox semigloss* - (came out denim colored instead of
purple)
18 Nepheline Syenite
14 Frit Ferro 3134
20 Whiting
18 Ball Clay - Old Mine #4
30 Silica
Add:
0.2 Chromium Oxide
3.8 Tin Oxide
0.6 Cobalt Carbonate
*Cranberry Red cone 6 ox semigloss* - (came out gray with brown speckles
instead of red)
18 Nepheline Syenite
14 Frit=3D96Ferro 3134
20 Whiting
18 Ball Clay=3D96Old Mine #4
30 Silica
Add:
3.8 Tin Oxide
0.2 Chromium Oxide
*Raspberry cone 6 ox semigloss* - (came out green instead of raspberry red)
21 Gerstley Borate=3D961999
16 Nepheline Syenite
11 Kaolin=3D96EPK
20 Whiting
32 Silica
Add:
5 Tin Oxide
7.5 Chromium Oxide
Neon-Cat on tue 30 nov 10
Chrome-tin glazes are a good example of the benefits of forming
crystals on the way up given sufficient temperature, not during a
firing schedule that relies on slow-cooling to achieve effects.
In these type of chrome-tin glazes it is important to make both
malayaite (calcium tin silicate, CaSnSiO5) and cassiterite (SnO2)
crystalline phases. Malayaite forms best between 1200 C =3D96 1600 C in
most standard recipes using a basic raw glaze composition of
Cr2O3-SnO2-CaO-SiO2, with 1240 and 1260 C known to be good areas for
crystallization that we can easily reach. Once formed, malayaite and
chromium ions and/or cobalt ions (as chromosphere agents) combine by
slow diffusion processes to form the colored compounds Ca(Cr,Sn)SiO5
and CaSnSiO5 - Co. [Violet color can also be produced at higher
temperatures when chromium ions become incorporated in the cassiterite
structure
(Sn,Cr)O2]. Early on in such glazes (1100-1200 C), the only major
phases present will be anorthite, cassiterite, and quartz, depending
on the glaze recipe. Malayaite develops in sufficient abundance
later.
So if you are using this Mastering Glazes Cone 6 schedule
(http://digitalfire.us/4sight/firingschedule/mastering_glazes_cone_6_firing=
=3D
_schedule_83.html)
you probably have not made enough malayaite. If you=3D92re only going to
1200 C or so, try an hour or more hold at peak and see if you don=3D92t
begin to develop more color. Don=3D92t worry too much about slow cooling =
=3D96
it is the nature of these glazes to form the necessary crystalline
phases as temperature increases. In fact, cassiterite and malayaite
crystallinity increase almost linearly with an increase in
temperature. A calcium-tin oxide phase, a precursor to malayaite, and
malayaite itself can sometimes begin forming in wee amounts in some
glazes around 1000 C if the amount of tin is great and a mineralizer
is present (e.g., a 2-3% H3BO3 equivalent or another =3D96 mineralizers
increase malayaite production while increasing the solubility of the
Cr ions) so I=3D92d go ahead and leave step three as it is unless you need
to go a little faster.
The denim color in your first example is just the result you=3D92d get
from a cobalt-colored glaze in which none of the special crystals of
malayaite (and cassiterite) formed to combine with the chrome and
cobalt.
The gray color in your second glaze example is simply the color of the
glaze without the formation of the chrome-tin pigment. The glaze
without chrome would probably look white to white-gray. Barium, if
enough is available to the glaze melt from a treated clay body, will
also dirty chrome-tin glaze colors. To reduce speckling sieve your
chrome and/or glaze a little finer.
A greenish color can result from 1) too much malayaite and too little
tin (not likely at your firing temperature and the MC6G schedule, or
2) too much Cr2O3 is used and the glaze network dissolution limit is
exceeded causing Cr2O3 segregation, or, 3) as in your third recipe
example, insufficient malayaite has formed and the Cr2O3 remains
segregated (uncombined).
Happy glazing, Judy!
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
Bryan Johnson on tue 30 nov 10
Hi
In thinking about this some more and looking at recipes;
1) With a glaze like Jenny's I've had the denim where it was thin.
2) It looks like the Raspberry is a variation of MC^6 Raspberry. The
original has 0.2%Chromium oxide and 7.5 % Tin oxide.
I'd also consider ball milling, or blending the chrome and tin before
adding to the glaze.
Bryan Johnson
Bryan Johnson on tue 30 nov 10
> *Raspberry cone 6 ox semigloss* - (came out green instead of raspberry re=
d)
> 21 Gerstley Borate-1999
> 16 Nepheline Syenite
> 11 Kaolin-EPK
> 20 Whiting
> 32 Silica
> Add:
> 5 Tin Oxide
> 7.5 Chromium Oxide
This looks like a slipped decimal, 0.75 would be more likely.
Bryan
>
Stephani Stephenson on wed 1 dec 10
I have always had the complete opposite problem. Seems like whenever I ha=
=3D
ve
any smidgeon of chrome fuming anywhere in the kiln , and any tin in a
glaze, I get raspberry everywhere! i
Just a a couple of thoughts. Total shots in the dark as I am not familia=
=3D
r
with these glazes.
My thought is that the application might be a factor . Does thickness m=
=3D
atter?
Sometimes when a glaze is too thin, the desired interaction just doesn't
occur. Are there any thick and thin spots, any spots at all where you not=
=3D
ice
something happening? Careful detailed observation can give you a clue
sometimes. or, possibly is excessive thickness, rather than thinness a
possible purple killer?.
Second, I wonder if the clay body plays a role. Is there some zinc or ot=
=3D
her
material somewhere in the body that could be affecting the glaze? clay
bodies can absolutely affected fired glaze results including coloration=
=3D
.=3D20
just a few thoughts to toss around.
best wishes in solving the problem
Stephani Stephenson
Neon-Cat on wed 1 dec 10
There is little excuse for not understanding chrome-tin glazes these
days =3DE2=3D80=3D93 there are literally hundreds of articles written by in=
dustry
experts that show clearly that both cassiterite and malayaite are
necessary crystalline phases and that malayaite is a high temperature
phase =3DE2=3D80=3D93 we will only get it, generally, from 1200 C on up. Si=
nce th=3D
e
1980=3DE2=3D80=3D99s when new test equipment became available ceramic scien=
ce and
technology took giant steps toward understanding what is going on in
our clay body and glaze systems. Pigment makers especially, were
looking to get away from cadmium reds so you much time, money,
intellect, and experimentation was done and continues to be done on
the tin-chrome system. If we were pigment makers we would fire at
1300-1400 C so that potters and ceramic artists would have dependable
red and pink-red pigments with very long firing ranges. Rhodes, as
Ivor mentions in a separate but related thread, is a great reference =3DE2=
=3D80=3D
=3D93
I really like his books and respect him greatly because he never
guessed, assumed, tried to dazzle with bs, or take us beyond the
knowledge that was available when he was writing. My copy of =3DE2=3D80=3D9=
CClay =3D
and
Glazes for the Potter=3DE2=3D80=3D9D (Revised edition) has copyright dates =
of 195=3D
7,
1973, and 1974. A lot has happened since then.
Minerals, natural and synthetic, form in preferential reaction
sequences =3DE2=3D80=3D93 some we will never see if we don=3DE2=3D80=3D99t =
achieve cert=3D
ain
temperatures. Malayaite, crucial for chrome-tin pigment or color made
during firing in a glaze, is one. Once it begins forming at proper
temperatures, it becomes the preferred crystalline phase at that
temperature. It is the product of reactions within the glaze then at
that temperature. Being a crystalline phase it does not enter into the
melt matrix and need not recrystallize-out as Ron suggests. It=3DE2=3D80=3D=
99s
there. Slow cooling may help lower temperature phase minerals
reappear, something we may or may not want. But slow cooling will
never produce some crystalline phases unless we first produce them.
All the groovy kiln gods in the world can=3DE2=3D80=3D99t make this happen.=
The
initial stages of slow cooling might help keep temperatures
sufficiently high to promote malayaite production but the later stages
of slow cooling are not necessary. If we buy a chrome-tin based
pigment to color our glaze with, one of its nice features is that it
is durable and dependable over a wide range of firing schedules =3DE2=3D80=
=3D93 o=3D
nce
made for us, it will be there for us.
Here (below) are three articles I came across the other day before
this thread came up. I post them now just because they are recent
examples of the hundreds of articles out there on chrome-tin (ceramic
science types love to share and show off what they=3DE2=3D80=3D99ve learned=
and
discovered). They now even make chrome-tin pigments out of wastes and
sludges that contain various forms of chromium and cobalt =3DE2=3D80=3D93 =
it is =3D
not
hard because the science has been a given for so many years.
We are always dealing with systems in disequilibrium and we should
always think in terms of effective temperatures (heat plus time minus
any temperature lag). One firing schedule cannot satisfy all potters
or all glazes. So we learn to adjust =3DE2=3D80=3D93 adjustment should alwa=
ys be =3D
in
the realm of the real.
In this holiday season I am reminded of Santa Claus. Lots of us really
believed in him when we were young. Then came a time when we outgrew
the entire myth. Learning the truth, however startling it might have
first seemed at the time, was no big deal, and in fact served to
elevate us a little as somewhat =3DE2=3D80=3D9Cenlightened=3DE2=3D80=3D9D c=
hildren amon=3D
g other
children. We did not kill off a real Santa Claus or hurt our fellow
beings with our new way of thinking when we stopped believing in the
jolly old elf. It simple was the way it was and is. I am happy to
allow any of you your Santa Claus clay myths but do try to allow
others our way of thinking. Clay science and technology are unfolding
to us in ways that are not and cannot be like they always were.
The development of a Chromium pink glaze coloring
Hyun Soo Lee and Byung Ha Lee
Journal of Ceramic Processing Research. Vol. 9, No. 3, pp. 286~291 (2008)
http://jcpr.kbs-lab.co.kr/english/journal/JCPR/vol9num3/286-291.pdf
Synthesis of Sphene (CaSnSiO5)-Pink Pigments with CrCl3
Hyun-Soo Lee=3DE2=3D80=3DA0 and Byung-Ha Lee
Journal of the Korean Ceramic Society, Vol. 46, No. 4, pp. 405~412, 2009
http://kcers.or.kr/storage/journal/K/46_4/6601/articlefile/article.pdf
CrCl3 is Chromium(III) chloride, a slightly soluble, violet-colored
solid (some hydrated forms can appear green and are soluble to varying
degrees).
This article from 2002-2003 deals with oxidation states and other
really geeky stuff for those so inclined to go further:
http://www.sincaf-icmse.es/elementos/articulos/AM_2003_51_2371_Lopez_navarr=
=3D
ete.pdf
As a practical studio artist, I=3DE2=3D80=3D99d rather read about what=3DE2=
=3D80=3D99s =3D
possible or
not and share a little of what I=3DE2=3D80=3D99ve learned rather than have =
any of=3D
us
spend months and years testing and re-inventing wheels. Information is
readily available from so many reliable sources and, what=3DE2=3D80=3D99s m=
ore, i=3D
t=3DE2=3D80=3D99s
there to help us.
Prior to my first post I did check the third glaze with questionable
amounts of chrome -- potters do report getting the proper color with
the recipe given by Judy.
Cheers!
Marian Gooding
Neon-Cat Ceramics
Larry Kruzan on wed 1 dec 10
Dittos - 7.5 would sure give a green. A deep bluegrass color???
Larry Kruzan
Lost Creek Pottery
www.lostcreekpottery.com
-----Original Message-----
From: Clayart [mailto:Clayart@LSV.CERAMICS.ORG] On Behalf Of Bryan Johnson
Sent: Tuesday, November 30, 2010 5:19 PM
To: Clayart@LSV.CERAMICS.ORG
Subject: Re: Purple glaze test come out denim blue
> *Raspberry cone 6 ox semigloss* - (came out green instead of raspberry
red)
> 21 Gerstley Borate-1999
> 16 Nepheline Syenite
> 11 Kaolin-EPK
> 20 Whiting
> 32 Silica
> Add:
> 5 Tin Oxide
> 7.5 Chromium Oxide
This looks like a slipped decimal, 0.75 would be more likely.
Bryan
>
=3D=3D=3D=3D=3D=3D=3D
Email scanned by PC Tools - No viruses or spyware found.
(Email Guard: 7.0.0.18, Virus/Spyware Database: 6.16410)
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=3D=3D=3D=3D=3D=3D=3D
Ron Roy on wed 1 dec 10
Hi Judy,
Sounds like they they need a slower cooling.
You can try one or two in a bisque firing to see if that is the case.
You don't want to fire too high as remelting the crystals means you =3D20
have to give them more time to reform.
If that is really 7.5 chrome in the last one it will never be red.
RR
Quoting Judy Smith :
> I did several glaze tests this week and none of them came out as expected=
.
> I used Ron Roy's cone 6 oxidation firing schedule. What am I doing wrong=
?
> Here are the recipes for 3 of the glazes. The comments inside the bracke=
t=3D
s
> tell what the test tiles look like instead of their intended colors.
>
> Thanks,
> Judy Smith
>
> *Jennie Purple cone 6 ox semigloss* - (came out denim colored instead of
> purple)
> 18 Nepheline Syenite
> 14 Frit Ferro 3134
> 20 Whiting
> 18 Ball Clay - Old Mine #4
> 30 Silica
> Add:
> 0.2 Chromium Oxide
> 3.8 Tin Oxide
> 0.6 Cobalt Carbonate
>
> *Cranberry Red cone 6 ox semigloss* - (came out gray with brown speckles
> instead of red)
> 18 Nepheline Syenite
> 14 Frit=3DE2=3D80=3D93Ferro 3134
> 20 Whiting
> 18 Ball Clay=3DE2=3D80=3D93Old Mine #4
> 30 Silica
> Add:
> 3.8 Tin Oxide
> 0.2 Chromium Oxide
>
> *Raspberry cone 6 ox semigloss* - (came out green instead of raspberry re=
d=3D
)
> 21 Gerstley Borate=3DE2=3D80=3D931999
> 16 Nepheline Syenite
> 11 Kaolin=3DE2=3D80=3D93EPK
> 20 Whiting
> 32 Silica
> Add:
> 5 Tin Oxide
> 7.5 Chromium Oxide
>
ivor and olive lewis on thu 2 dec 10
Dear Judy Smith,
Given the scientific information proffered, then, apart from that slip with
the decimal point and given the example illustrated by Ron Roy and John
Hesselberth it would appear that you are not making any errors.
Daniel Rhodes in "Clay and Glazes for the Potter" makes this remark. "Chrom=
e
Oxide is the most versatile colouring Oxide, a veritable turncoat. It will
produce red, yellow, pink, brown or green depending on the kind of glaze
used and the temperature of firing" The rest of his statement elaborates
the nature of the relationships that can enhance or retard colour
development (pp 209-10)
My solutions for the development of violet or mauve hues are to use mixture=
s
of Copper Carbonate (or oxide) with Cobalt oxide (or carbonate) then fire i=
n
a reducing atmosphere or rely on manufactured glaze stains.
Regards,
Ivor Lewis,
REDHILL,
South Australia
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