Paul Lewing on mon 5 mar 01
Marjory Kline wrote:
> 1, Colorants as fluxes. A base glaze I've been using behaves differently
> depending on the colorant. With copper I get a satin matt; with cobalt, a
> dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
> can function as a flux at high temperatures (^10?), but I usually fire to
> ^6. Can it flux at that temperature? What about the other colorants?
Copper, cobalt and chrome are mild fluxes. Manganese is refractory.
Iron is a flux in reduction and refractory in oxidation. Your
observation about iron being a flux at high temperature is based on the
fact that most people who do high-fire do reduction, but most people who
do mid-range fire oxidation. It's the atmosphere that makes the
difference, not the temperature.
>
> 2. What happens in the melt when one glaze is layered over another?
> Examples: a. You don't like the glaze, so you apply another glaze and
> refire at the same temperature. Does the base glaze stay where it is and
> not combine with the outer glaze? b. What if you layer one glaze over
> another on a bisqued pot, and then fire. Does the base glaze combine with
> the outer glaze? c. What if you glaze a piece of greenware, fire it to
> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
> melt into the outer glaze, or does the outer glaze act as a shield to the
> base glaze?
What effect you will get from layering glazes is almost impossible to
predict. Which glaze is on top, relative thickness, firing temp, rate
of heat increase and decrease, degree of reduction, and the chemistry of
each glaze all affect the result. In general, glazes that are
dissimilar flow and interact with each other more than glazes that are
similar. When dissimilar glazes meet, each added oxide introduces a new
eutectic. And glazes with very different Si:Al ratios will flow into
each other quite dramatically.
Another thing that affects not so much the surface or color of overlaps,
but the texture, is the amount of outgassing from each glaze,
particualrly the bottom one. Glazes which contain things like the
carbonate of any oxide (whiting, dolomite, barium carb, strontium carb)
will boil a lot as they release the gas. This results in a mottled
texture if it boils through another glaze.
>
> 3.
Sorry, can't help you with #3.
Paul Lewing, Seattle
Marjory Kline on mon 5 mar 01
I know you are all getting ready to depart for NCECA, but before you go,
could you respond to the following so I can get some sleep:
1, Colorants as fluxes. A base glaze I've been using behaves differently
depending on the colorant. With copper I get a satin matt; with cobalt, a
dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
can function as a flux at high temperatures (^10?), but I usually fire to
^6. Can it flux at that temperature? What about the other colorants?
2. What happens in the melt when one glaze is layered over another?
Examples: a. You don't like the glaze, so you apply another glaze and
refire at the same temperature. Does the base glaze stay where it is and
not combine with the outer glaze? b. What if you layer one glaze over
another on a bisqued pot, and then fire. Does the base glaze combine with
the outer glaze? c. What if you glaze a piece of greenware, fire it to
^06, say, then add a second glaze, and fire to ^6. Does the base glaze
melt into the outer glaze, or does the outer glaze act as a shield to the
base glaze?
3. Recently on Clayart, someone mentioned getting a celadon by refiring
Floating Blue to a lower temperature (^04?). How does the glaze change?
The fact that the color changes seems to imply that the chemical makeup of
the glaze changes. Would it be less stable or have no effect at all?
I would welcome your comments, even if it's just to refer me to some book.
Thanks very much,
Marjory
Cindy Strnad on mon 5 mar 01
Hi, Marjory.
Well, I'll give it a shot. There are others here who know a lot more than I
do about this subject, and I welcome any correction.
Based on my experience of the way these colorants have influenced the
behaviors of my glazes: iron will flux a glaze at ^6. I'm not so sure about
copper or cobalt. Based on your own experience of the copper glaze showing
satin matte and the cobalt glaze showing dry (this is in the same glaze,
right?), I'd say copper fluxes more than cobalt at ^6. At least in that
particular glaze.
When glazes are layered over one another, they will intermingle to some
extent. How much will depend on the glazes, firing schedule, phase of the
moon, whether Jupiter is in the 3rd house, and so on. I hypothesize that
flowing glossy glazes would tend to mingle more than still matte glazes.
I refire Opal Blue to varying temperatures and get different shades of
turquoise and light blue, but I don't know why this happens. You can also
refire an iron red glaze to bring out the red color. I don't think this is a
chemical change, unless it's increased oxidation. I suspect it's more of a
mechanical change.
I hope this will at least give you some additional fuel to think on. I'll be
following your answers, too.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
John Hesselberth on mon 5 mar 01
Hi Marjory,
You questions are very complex and I don't know that they have definitive
answers. Here are some of my thoughts.
Marjory Kline wrote:
>
>1, Colorants as fluxes. A base glaze I've been using behaves differently
>depending on the colorant. With copper I get a satin matt; with cobalt, a
>dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
>can function as a flux at high temperatures (^10?), but I usually fire to
>^6. Can it flux at that temperature? What about the other colorants?
There is no question that colorants affect the gloss of the finished
glaze. I'm not convinced this is always a fluxing affect. I think in
the case of copper it my be changing the physics of the situation by, for
example, causing some of the other materials in the glaze to be more
soluble. In a true (or maybe I should say well-formulated) matte, the
mattness comes from some of the oxides recrystallizing on cooling. The
copper may retard this effect. If it were a fluxing affect you would
expect the actual maturity point of the glaze to be changed and I don't
think there is much evidence this happens with copper as a colorant--but
change the gloss it does. I have seen a semimatte base glaze (containing
added rutile) go glossy with the addition of 5% copper carbonate.
>
>2. What happens in the melt when one glaze is layered over another?
>Examples: a. You don't like the glaze, so you apply another glaze and
>refire at the same temperature. Does the base glaze stay where it is and
>not combine with the outer glaze? b. What if you layer one glaze over
>another on a bisqued pot, and then fire. Does the base glaze combine with
>the outer glaze? c. What if you glaze a piece of greenware, fire it to
>^06, say, then add a second glaze, and fire to ^6. Does the base glaze
>melt into the outer glaze, or does the outer glaze act as a shield to the
>base glaze?
There is some intermingling of the two glazes at the interface, but not
much. You may remember some time ago I layered a stable glaze on top of
an unstable glaze (both dipped at the same time) and saw a 10x reduction
in leaching rate. This tells me the stable glaze was still pretty much
intact on top of the unstable glaze.
>
>3. Recently on Clayart, someone mentioned getting a celadon by refiring
>Floating Blue to a lower temperature (^04?). How does the glaze change?
>The fact that the color changes seems to imply that the chemical makeup of
>the glaze changes. Would it be less stable or have no effect at all?
I haven't a clue on this one, but I will make the point that color we see
is a combination of chemical and physical effects. In this case the
physics may be giving the effect. It could be a change in the number or
size of the crystals or what materials are in crystalline form vs.
dissolved in the melt.
Regards,
John
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
Chris Schafale on mon 5 mar 01
Hi Marjory,
You ask some excellent and interesting questions. I'm not a glaze
guru, but here's my take.
> 1, Colorants as fluxes.
I've had the same experience as you with cobalt and copper. Iron
is supposed to be refractory in oxidation, but my experience with
some glazes at least, was that it made glazes glossier if I used
more than say, 4%.
> 2. What happens in the melt when one glaze is layered over another?
> Examples: a. You don't like the glaze, so you apply another glaze and
> refire at the same temperature. Does the base glaze stay where it is and
> not combine with the outer glaze?
This seems to depend a lot on what the glazes are. Midrange
oxidation glazes seem more apt to just sit there, relative to high-fire
reduction glazes, though there are many exceptions. Nicely
balanced, safe, within-limits glazes seem especially apt to be
irritatingly stable in this regard, much to the dismay of cone 6
potters everywhere.
b. What if you layer one glaze over
> another on a bisqued pot, and then fire. Does the base glaze combine with
> the outer glaze?
See above. Some glazes not only don't mix, they seem to
positively repel each other. Glazes can have slightly different
melting points, depending on their make-up, even if they all are
nominally cone 6 glazes. Fluxes become active at different
temperatures, so the balance of oxides in each glaze becomes
important.
c. What if you glaze a piece of greenware, fire it to
> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
> melt into the outer glaze, or does the outer glaze act as a shield to the
> base glaze?
Haven't tried this one, but I would think that the comments above
will apply. Since both glazes are getting heated to cone 6 in the
course of the firing, both should melt. Whether they melt together
or blend at all will depend on their composition.
>
> 3. Recently on Clayart, someone mentioned getting a celadon by refiring
> Floating Blue to a lower temperature (^04?). How does the glaze change?
> The fact that the color changes seems to imply that the chemical makeup of
> the glaze changes. Would it be less stable or have no effect at all?
The Floating Blue color change is a weird phenomenon. When I
refired an FB piece to 06, I got a khaki-tan color. Then I refired to
cone 5 and the blue returned. This seems to have something to do
with crystals that form at lower temperatures and then melt when
the kiln gets hotter. My best understanding is that in FB, the
crystals are calcium borate. If there are enough of these crystals,
they somehow obscure the blue. If the glaze is fired to cone 5 and
cools quickly enough, the crystals melt and don't reform and the
glaze is blue. However, if you fire FB with a long, slow cool-down,
you will also get color shifts, from greenish to light blue to khaki,
depending how much you prolong the cooling.
This crystal formation is the same reason that some saturated iron
glazes get redder when you refire them to bisque temperatures and
cool them slowly -- they need the maximum amount of time in the
crystal zone to develop the color.
So, there you are. Let's see what the real gurus have to say.
Chris
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Tom Buck on tue 6 mar 01
MK:
see below
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
On Mon, 5 Mar 2001, Marjory Kline wrote:
> I know you are all getting ready to depart for NCECA, but before you go,
> could you respond to the following so I can get some sleep:
>
> 1, Colorants as fluxes. A base glaze I've been using behaves differently
> depending on the colorant. With copper I get a satin matt; with cobalt, a
> dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
> can function as a flux at high temperatures (^10?), but I usually fire to
> ^6. Can it flux at that temperature? What about the other colorants?
Most colourants act as fluxes, ones which I prefer to call secondary
fluxes since they seldom are included in the Seger Formula when it is
calculated. Copper oxide is a good flux, and if enough is present will
change a dry matt to a satin matt or even a gloss. Cobalt is used in low
amounts 1-2% usually, so it does not much affect the maturity (or
otherwise) of a glaze. Iron oxide black (Iron(II) oxide) becomes Iron(III)
oxide (red oxide) below 1000 C (1800 F), and stays that way unless
moderate to heavy redcution is carried out in the kiln; if this happens
then Iron(II) oxide is formed again and it is an active flux in the
mid-fire and high-fire ranges.
But if you fire in oxidation to Cone 6 look for the iron oxide not
to do anything except provide colour, either by going into solution (if
present in 5% or less) and causing the glaze to show brown, or if iron
oxide (now present as red iron oxide) is present in excess then the glaze
may show the red colour of Iron(III) oxide crstals on the surface of the
cooled glaze (see Randy's Red recipe).
> 2. What happens in the melt when one glaze is layered over
another?
> Examples: a. You don't like the glaze, so you apply another glaze and
> refire at the same temperature. Does the base glaze stay where it is and
> not combine with the outer glaze? b. What if you layer one glaze over
> another on a bisqued pot, and then fire. Does the base glaze combine with
> the outer glaze? c. What if you glaze a piece of greenware, fire it to
> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
> melt into the outer glaze, or does the outer glaze act as a shield to the
> base glaze?
Answer: a) No, they both combine if you fire to full maturity. b) If you
place a dark colour over a light base, the dark will persist although
diluted somewhat. but if there are two completely separate layers placed
on the biscuit, you end up with whatever the combination provides, and the
relative thickness of each coat will have an affect. c) 1) the 2nd coat
will be hard to apply to a sealed biscuit surface. 2) if brought to C6
maturity, the underglaze if fluid may cause the mixture to run off the pot
but otherwise the two glazes will intermingle with unpredictable results
(at first firing, but you'd know what would likely happen thereafter).
> 3. Recently on Clayart, someone mentioned getting a celadon by refiring
> Floating Blue to a lower temperature (^04?). How does the glaze change?
> The fact that the color changes seems to imply that the chemical makeup of
> the glaze changes. Would it be less stable or have no effect at all?
The reheating to C04 (1060 C, 1940 F) brings about a change but it is not
specifically a chemical change, it is rather more of a physical change;
the pattern of colourant oxides will shift somewhat when the glaze's
temperature is brought above the gelling point (glass transformation
point), and the components are allow to move and find a new arrangement
(not guaranteed, but feasible). When cooled again this new arrangement
will receive and transmit light differently. Hence, the colour of the pot
changes.
The intricate details of glazes involves some rather peculiar
physical chemistry, a great deal of which has been explored and catalogued
by scientific reasearchers over the past century. However, with the
development of glaze calculation software for the typical studio computer,
much of the notions of glaze formation can be simplified and made useful
by approximations that serve to allow the careful potter to make good
glazes for his work.
Tom Buck, March 5, 2001
Lili Krakowski on tue 6 mar 01
Some colorants are fluxes; some are somewhat refractory; copper and MnO2
tend to fuse things, some like chrome and nickel are refractory.
According to some red iron oxide does nothing black iron oxide fluxes. As
colorants are
used in such small quantities the influence is rarely something to get
worked up over.
As to layering glaze: think lasagna.
Some layers (glazes) will bubble actively, sometimes really BUBBLING
through the top layer. Others will not. If you put a slip or a quite
refractory glaze over a clear well fluxed one (that should end up shiny)
you may get one type of show through, and another effect if you reverse
them. As someone said in a post today you frit a glaze when you fire it;
i.e. you burn off the burnoffable. In refiring you get the effect from
that.
The only solution is 1. BUY A STRONG MAGNIFYING GLASS to study your glaze
results with. 2. Make a test tile, and wearing your safety goggles, smash
it and look at the cross sections.
The rest is practice. Glaze work is like detective work. A combo of
knowledge, instinct developed by experience, and patience.
Bonne chance
On Mon, 5 Mar 2001, Marjory Kline wrote:
> I know you are all getting ready to depart for NCECA, but before you go,
> could you respond to the following so I can get some sleep:
>
> 1, Colorants as fluxes. A base glaze I've been using behaves differently
> depending on the colorant. With copper I get a satin matt; with cobalt, a
> dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
> can function as a flux at high temperatures (^10?), but I usually fire to
> ^6. Can it flux at that temperature? What about the other colorants?
>
> 2. What happens in the melt when one glaze is layered over another?
> Examples: a. You don't like the glaze, so you apply another glaze and
> refire at the same temperature. Does the base glaze stay where it is and
> not combine with the outer glaze? b. What if you layer one glaze over
> another on a bisqued pot, and then fire. Does the base glaze combine with
> the outer glaze? c. What if you glaze a piece of greenware, fire it to
> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
> melt into the outer glaze, or does the outer glaze act as a shield to the
> base glaze?
>
> 3. Recently on Clayart, someone mentioned getting a celadon by refiring
> Floating Blue to a lower temperature (^04?). How does the glaze change?
> The fact that the color changes seems to imply that the chemical makeup of
> the glaze changes. Would it be less stable or have no effect at all?
>
> I would welcome your comments, even if it's just to refer me to some book.
>
> Thanks very much,
>
> Marjory
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
Lili Krakowski
______________________________________________________________________________
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Lili Krakowski on tue 6 mar 01
Some colorants are fluxes; some are somewhat refractory; copper and MnO2
tend to fuse things, some like chrome and nickel are refractory.
According to some red iron oxide does nothing black iron oxide fluxes. As
colorants are
used in such small quantities the influence is rarely something to get
worked up over.
As to layering glaze: think lasagna.
Some layers (glazes) will bubble actively, sometimes really BUBBLING
through the top layer. Others will not. If you put a slip or a quite
refractory glaze over a clear well fluxed one (that should end up shiny)
you may get one type of show through, and another effect if you reverse
them. As someone said in a post today you frit a glaze when you fire it;
i.e. you burn off the burnoffable. In refiring you get the effect from
that.
The only solution is 1. BUY A STRONG MAGNIFYING GLASS to study your glaze
results with. 2. Make a test tile, and wearing your safety goggles, smash
it and look at the cross sections.
The rest is practice. Glaze work is like detective work. A combo of
knowledge, instinct developed by experience, and patience.
Bonne chance
On Mon, 5 Mar 2001, Marjory Kline wrote:
> I know you are all getting ready to depart for NCECA, but before you go,
> could you respond to the following so I can get some sleep:
>
> 1, Colorants as fluxes. A base glaze I've been using behaves differently
> depending on the colorant. With copper I get a satin matt; with cobalt, a
> dry Wedgewood-like matt. I understand from Hamer & Hamer that iron oxide
> can function as a flux at high temperatures (^10?), but I usually fire to
> ^6. Can it flux at that temperature? What about the other colorants?
>
> 2. What happens in the melt when one glaze is layered over another?
> Examples: a. You don't like the glaze, so you apply another glaze and
> refire at the same temperature. Does the base glaze stay where it is and
> not combine with the outer glaze? b. What if you layer one glaze over
> another on a bisqued pot, and then fire. Does the base glaze combine with
> the outer glaze? c. What if you glaze a piece of greenware, fire it to
> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
> melt into the outer glaze, or does the outer glaze act as a shield to the
> base glaze?
>
> 3. Recently on Clayart, someone mentioned getting a celadon by refiring
> Floating Blue to a lower temperature (^04?). How does the glaze change?
> The fact that the color changes seems to imply that the chemical makeup of
> the glaze changes. Would it be less stable or have no effect at all?
>
> I would welcome your comments, even if it's just to refer me to some book.
>
> Thanks very much,
>
> Marjory
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
Lili Krakowski
Paul Taylor on wed 7 mar 01
Marjory Kline wrote.
>> 2. What happens in the melt when one glaze is layered over
> another?
>> Examples: a. You don't like the glaze, so you apply another glaze and
>> refire at the same temperature. Does the base glaze stay where it is and
>> not combine with the outer glaze? b. What if you layer one glaze over
>> another on a bisqued pot, and then fire. Does the base glaze combine with
>> the outer glaze? c. What if you glaze a piece of greenware, fire it to
>> ^06, say, then add a second glaze, and fire to ^6. Does the base glaze
>> melt into the outer glaze, or does the outer glaze act as a shield to the
>> base glaze?
Others have answered the other questions including the floating blue .
If you have a crystal stage in any glaze the firing schedule will change the
glaze quite radically so If some one gives you a matt glaze of any sort get
a rough idea of the firing cycle including the cooling rate. Its all to do
with the forming of nuclei and forming crystals and all the fancy
calculators be they swamies or gurus can not help you much because there are
too many variables.
Which means that lots of glazes are still imperical and take skill and
testing to get their interesting effects which by the time you are finished
will be yours and yours alone in spite of the fact that you are using a well
known recipe.
Of course firing experiments are expensive and unless you are set up for
them almost impossible. Small adjustments to the crystal former in any
glaze can adjust it to your Kiln firing schedule. I have asked if there are
any formulaic rules to help with the adjustments but the large crystal
glazes are the only ones there are any certainty in - and again not much. If
any thing has tin or titanium (rutile) you can expect to have to tweak it a
little unless you hit upon the right firing schedule but - not to worry you
- most potters have a similar schedule. so most glazes fire nearly the same.
Some old glaze recipes - from the days of longer firings - may need a
little tweak by adding a bit more nuclei former like titanium
(rutile/ilmenite) especially since the iron titanites (rutile, ilmenite) can
be very variable. Of course adjustments for crystallization can be made with
the calcium for calcium mats. And you could look at the silica alumina
balance but glaze adjustment to the last two can get a bit confusing and the
changes may have bad effects for glaze stability.
While we are on the titanium bit. When titanium does form the crystal
former in a glaze it picks up the color from a slip or under glaze very
well. Its a bit difficult to control but gives very lively effects.
The more interesting part of your question is the layered glaze. Glaze
chemistry is so influenced by industry and the tight standards set by the
american FDA that one starts to wonder if glaze making is now beyond the
studio potter unless he is prepared for chemistry. Chemistry is the
preferred language on this list because it does allow for accurate
information.
But like so many languages they come with their own propagandas that can
be rigid and limiting. One gets the feeling that the chemistry is the only
consideration in a glaze.
Putting two glazes on a pot is a very exiting and creative thing to do
and separates the artist from the Technicians. The only thing to worry about
is to put the less viscose glaze on the top so you do not get crawling .
However there is a subscriber who makes a feature out of the crawling so
even that rule has its exceptions and I expect others as well.
The ancients used to double dip or spray most of those fat celadons and
thoes glazes still have not been equaled - although I have not given up yet.
The reason they have not been equaled is because the chemistry of these
glazes are only the minor part of the glaze : its application, firing,
particle size and the design of pot ( the way the glazed is framed with in
the form) are just as important.
So the answer to your question is as always "depends" some base gazes are
designed to react with the glaze on top to form leopard skins Etc . Others
are put there to show off either the top or the bottom glaze better.
If you are having trouble getting two glazes to go on evenly and with out
cracking putting one on before the biscuit is a good idea the glazes will
still combine.
lamenating a glaze with a thin layer of a less viscose glaze on top will
prevent the crawling of a thick underneath glaze . If I feel that a thick
glaze on a big pot may crawl, at say the rim, I put a less viscose glaze of
the same color on top - just a light spray.
Also lamenating glaze layers may allow you to get a better color response
from a matt glaze. This I am experimenting with myself
-- Regards Paul Talyor.
http://www.anu.ie/westportpottery
Even dead things change they just change more slowly.
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