2011 clay workshop handbook

Transcription

ceramic arts dail y.org
2011 clay workshop
handbook
knowledge and techniques
for the studio
This special report is brought to you with
the support of Georgies Ceramic and Clay Co.
2011 clay workshop handbook | Copyright© 2011, Ceramic Publications Company | www.ceramicartsdaily.org
i
2011 Clay Workshop Handbook
Knowledge and Techniques for the Studio
Welcome to your ceramic workshop! Whether you’re a wheel thrower, a handbuilder, a glaze-testing geek, or all of the above, we’re
sure you’ll discover an exciting technique or project on these pages to take into your own pottery studio and make your own. Pottery
workshops have become a very popular way to learn about ceramics, because a lot of pottery making techniques can be demonstrated in a relatively short period of time. Presented here on the pages of the 2011 Ceramic Workshop Handbook are some of the most
popular types of techniques and information people go to ceramic workshops to learn. So if you can’t get to a ceramic workshop in
person, the Ceramic Workshop Handbook will bring the pottery workshop into your own clay studio.
Enjoy your workshop!
Expanding Your Palette in Mid-range
Firing
by Yoko Sekino-Bové
A research project that began as a personal exploration ended with a system for testing
glazes that opened up a full view of the possibilities in color, surface, and texture at cone 6
using almost any base glaze recipe.
Treasure for Treasure: Lidded Box
by Martha Grover
Porcelain is a great material for creating delicate forms for culinary treasures.
Four Ways to Red
by Dave Finkelnburg
One of the most difficult colors to achieve in ceramics may not be as tough as you
thought—as long as you choose the right method for your work.
All About Iron
by John Britt
Iron can be many things, and many colors—many of which are not brown.
Throwing Agateware
by Michelle Erickson and Robert Hunter
Mix it up. Stack and wedge different colored clays together to create thrown forms with
dramatic patterns.
Cover (from left to right): Guillermo Cuellar, Shafer, Minnesota. Photo: Dennis Chick; Victoria Christen,
Portland, Oregon; Lisa Grahner. Photo: Gunter Binsack, copyright Kahla/Thüringen Porcelain LLC.
1
Expanding Your Palette in
Mid-range Firing
by Yoko Sekino-Bové
There are so many wonderful books, websites and even software
that feature spectacular glaze formulas; so one may wonder
why this article should be introduced to you. The focus of this
research was to establish a comprehensive visual library for everyone. Rather than just providing the reader with a few promising
glaze formulas, this reference is a guideline. Because it is a guide,
there are some test tiles that do not provide immediate use other
than the suggestion of what to avoid, or the percentages of certain
chemicals that exceed the safe food-serving level, etc., but I
believe that this research will be a good tool for those who wish to
experiment with, and push the boundaries of, mid-range firing.
Many people may be thinking about switching their firing
method from high-fire to mid-range. For instance, students who
recently graduated and lost access to school gas kilns, people
with a day job and those who work in their garage studios, or
production potters who are concerned about fuel conservation
and energy savings. This reference is intended as a tool for those
people to start glaze experimentations at mid-range that can be
accomplished with minimal resources.
There is no guarantee that this chart will work for everyone
everywhere, since the variety between the different resources overwhelmingly affects the results, but by examining a few glazes in
this chart you can speculate and make informed adjustments with
your materials. This is why all the base glazes for this research use
only simple materials that are widely available in the US.
Five years ago, when I was forced to switch to mid-range
oxidation firing with an electric kiln, from gas-fueled reduction
firing at high temperatures, most of my hard-earned knowledge
in high-fire glazes had to be re-examined. Much to my frustration, many earth metal colorants exhibited completely different
behaviors in oxidation firing. Also, problems in adhesion were
prominent compared to high-fire glazes.
The role of oxides and carbonates used for texturing and
opacifying were different as well. But compiling the available
glazes and analyzing them were not enough. I felt there should
be a simple chart with visual results that explained how the
oxides and carbonates behave within this firing range. This
motivated me to write a proposal for glaze mid-range research
to the McKnight Foundation, which generously sponsors a
three-month artist-in-residence program at the Northern Clay
Center in Minneapolis, Minnesota.
Most of the tests presented in these experiments were executed
at the Northern Clay Center from October to December in 2009
using clay and dry materials available at Continental Clay Co.
The rest of the tests were completed after my residency at my
home studio in Washington, Pennsylvania. For those tests, I used
dry materials available from Standard Ceramics Supply Co.
Test Conditions
Clay body: Super White (cone 5–9) a white stoneware body for
mid-range, commercially available from Continental Clay Co.
Bisque firing temperatures: Cone 05 (1910°F, 1043°C), fired
in a manual electric kiln for approximately 10 hours.
2
recipes
N501 Transparent, glossy,
and crackles
Cone 5
Ferro Frit 3110. . . . . . . . . . . . . . . . . . . 90%
EPK Kaolin. . . . . . . . . . . . . . . . . . . . . . 10
100%
N502 Transparent and glossy
Cone 5
Gillespie Borate . . . . . . . . . . . . . . . . . . 30%
F-4 Feldspar. . . . . . . . . . . . . . . . . . . . . 46
EPK Kaolin. . . . . . . . . . . . . . . . . . . . . . 13
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 11
100%
See chart on page 8 for test results.
N503 Opaque, glossy,
and textured
Cone 5
Gillespie Borate . . . . . . . . . . . . . . . . 52.6%
EPK Kaolin. . . . . . . . . . . . . . . . . . . . 21.0
Silica . . . . . . . . . . . . . . . . . . . . . . . . 26.4
100.0%
Add:Zircopax. . . . . . . . . . . . . . . . . 10.0%
N504 Semi-opaque, semi-satin
with textures
Cone 5
Whiting. . . . . . . . . . . . . . . . . . . . . . 9.5 %
Ferro Frit 3124. . . . . . . . . . . . . . . . . 44.5
F-4 Feldspar. . . . . . . . . . . . . . . . . . . 20.0
Zinc Oxide. . . . . . . . . . . . . . . . . . . . 5.5
Bentonite*. . . . . . . . . . . . . . . . . . . . 7.5
EPK Kaolin. . . . . . . . . . . . . . . . . . . . 5.0
Silica . . . . . . . . . . . . . . . . . . . . . . . . 8.0
100.0%
Add:Zircopax. . . . . . . . . . . . . . . . . . 9.0 %
N505 Satin, Opaque
with textures
Cone 5
Dolomite. . . . . . . . . . . . . . . . . . . . . . . 12%
Gillespie Borate . . . . . . . . . . . . . . . . . . 14
Wollastonite. . . . . . . . . . . . . . . . . . . . . 10
Ferro Frit 3124. . . . . . . . . . . . . . . . . . 8
Cornwall Stone . . . . . . . . . . . . . . . . . . 46
EPK Kaolin. . . . . . . . . . . . . . . . . . . . . . 10
100%
Add:Magnesium Carbonate . . . . . . . 6%
See chart on page 8 for test results.
* Bentonite is typically listed as an addition
to recipes, but in larger amounts it contributes appreciably to the amount of alumina
and silica in the recipe and is therefore
included along with the clays in the list of
the main ingredients.
While the test results with all colorant
options are shown for two recipes in
this article, charts showing all of the test
results for all of the recipes listed here are
available at www.ceramicsmonthly.org.
Click the “CM Master Class” link on
the right side of the page to see the
“Expanding your Palette” post and all of
the research.
3
Glaze base N502 with coloring
oxides and carbonates
0.1%
0.5%
1.0%
5.0%
10.0%
N502CC05
N502CC10
N502CC50
N502ROI05
N502ROI10
N502ROI50
N502COX0
N502COX05
N502COX10
N502CH01
N502CH05
N502CH10
N502MD05
N502MD10
N502MD50
N502BN05
N502BN10
N502BN50
N502IC05
N502IC10
N502IC50
N502R05
N502R10
N502Y05
N502Y10
Glaze base N504 with coloring
oxides and carbonates
0.1%
0.5%
1.0%
5.0%
10.0%
N502CC100
N504CC05
N504CC10
N504CC50
N504CC100
N502ROI100
N504ROI05
N504ROI10
N504ROI50
N504ROI100
N504COX01
N504COX05
N504COX10
N504CH01
N504CH05
N504CH10
N504MD05
N504MD10
N504MD50
N504MD100
N504BN05
N504BN10
N504BN50
N502IC100
N504IC05
N504IC10
N504IC50
N504IC100
N502R5
N502R100
N504R05
N504R10
N504R50
N504R100
N502Y50
N502Y100
N504Y05
N504Y10
N504Y50
N504Y100
Copper
Carbonate
Red Iron
Oxide
(regular)
Cobalt
Oxide
Chrome
Oxide
Manganese
Dioxide
N502MD100
Black
Nickel
Oxide
Iron
Chromate
Rutile
(powder)
Yellow
Ochre
The full chart of glazes and colorant combinations tested is available at www.ceramicsmonthly.org.
Just click on the “CM Master Class” link on the right-hand side.
4
Glaze firing temperatures: The coloring metals increment tests
(page 50) were fired to cone 5 (2210°F, 1210°C) in a manual
electric kiln for approximately 8 hours. The opacifier/texture
metals increment tests (page 51) were fired to cone 5 in an automatic electric kiln for 8 hours.
Glaze batch: Each test was 300g, with a tablespoon of epsom
salts added as a flocculant.
Glazing method: Hand dipping. First dip (bottom half): 3
seconds. Second dip (top half) additional 4 seconds on top of the
first layer, total 7 seconds.
Coloring Metals
Increment Chart
Please note that some of the oxides and carbonates in this test exceed the safety standard for use as tableware that comes in contact
with food. Check safety standards before applying a glaze with a
high percentage of metal oxides to food ware and test the finished
ware for leaching.
Test tile numbering system: The glaze name is the first part of
the identification number, followed by an abbreviation or code that
stands for the colorant name. The last part is a two or three digit
number referring to the percentage of colorant added.
So, for example if a test was mixed with glaze base N501, to
which 1 percent cobalt oxide was added, the test tile marking
would be: N501COX10.
Conclusion
The following colorants were tested: black nickel oxide, cobalt
oxide, copper carbonate, chrome oxide, iron chromate, manganese dioxide, red iron oxide, rutile, and yellow ochre. You
should note that tests with cobalt oxide and chrome oxide in
high percentages were not executed due to the color predictability. Other blank tiles on the chart are because either the
predictability or the percentages of oxides are too insignificant
to affect the base glazes.
Depending on firing atmospheres, manganese dioxide
exhibits a wide variety of colors. When fired in a tightly sealed
electric kiln with small peepholes, the glaze color tends toward
brown, compared to purple when fired in a kiln with many
and/or large peepholes.
This group of tests has been a great opportunity for me to
study the characteristics of oxides and carbonates and how
they behave at mid-range temperatures. There are scientific
methods for calculating glazes and proven theories, but there
are many small pieces of information that can only be picked
up when you actually go through the physical experiments. It
is important for us to become familiar with a glaze’s behavior
so that we can better utilize it. Key to that is learning both
the theory and application. It is my hope that these tests will
benefit many potters by helping them to expand their palette
and inspire them to test the possibilities.
Yoko Sekino-Bové is an artist living in Washington, Pennsylvania. She would like to thank the McKnight Foundation and the
Northern Clay Center and its supporting staff for making this
research possible.
Glaze Base N502 with Opacifiers
1.0%
5.0%
10.0%
Tin Oxide
N502CT10
N502CT50
N502CT100
Titanium
Dioxide
N502CTD10
N502CTD50
N502CTD100
N502CZ10
N502CZ50
N502CZ100
Opacifiers were added to glaze base N502 in increments.
The chart at left shows which materials were added for this
purpose, and the percentages tested. All glazes in this test
batch also had 1% copper carbonate added to increase the
visual effect of the chemicals on the glaze.
Note: Some of the oxides and carbonates did not exhibit a
significant visual effect by themselves. However, sometimes
a combination of more than one chemical can change the
glaze characteristics and create spectacular visual effects.
Zircopax
5
Treasure for Treasure:
Lidded Box
by Martha Grover
When in use, Martha Grover’s porcelain butter boxes combine two things that were once scarce and are still considered delicacies.
Butter has always been a staple part of my cooking process and
has a long history as a delicacy as well as a sign of the good life.
From pie crusts and pastries to sauces and atop warm bread, it
enamors the tastes of all. Sources of fat were historically treasured
because of their limited supply. The historical importance of
butter reminds me of porcelain’s historical importance and rarity.
How fitting then, to store one precious good inside another, to
make an elegant but functional porcelain box for this culinary
treasure. Though I created this lidded form for butter, the design
can be adapted for many different uses by changing the height,
width or scale.
I make all of my work from a high-fire Grolleg porcelain.
Pieces begin on the wheel and are thrown without a bottom. I
then alter the shape and attach a slab bottom to finish the piece.
As I combine throwing with soft-slab techniques, the clay needs
to be soft enough to manipulate but firm enough to support itself
and not make fingerprints.
Making the Parts
When working on a lidded form like this, I make all of the
parts at the same time so that when assembly of the piece
begins, everything is at the same consistency—just before the
leather-hard stage.
Both the lid and the body are bottomless cylinders, ribbed free
of throwing lines and excess slurry. The body has straight walls
and a lid seat about ¾ to 1 inch below the rim. The lid’s walls
have a slight taper, wider at the top and narrower at the bottom.
The wider rim of this part sits on the seat inside of the body of
6
1
2
The tools and parts needed to make the
lidded container.
4
Cut the bottom at a bevel and establish
four raised feet.
5
Use a soft brush to smooth and join the
interior seams.
Cut a bottom slab wider than the body.
Make decorative points over the feet.
3
Slip and score both parts and use
registration marks to realign.
6
Compress and taper the edge of the
bottom slab.
and width of the piece. Use these indicators to cut an undulating scallop pattern, establishing four feet on the bottom of the
form (figure 2). The cuts are made at a 45° bevel and curve up to
½ inch from the bottom edge at their apex. The angle will slope
down from the interior edge to the exterior. This bevel is important because it allows the slab bottom to make a gentle curve and
transition more smoothly than if the edge was cut flat.
Make a slab for the bottom that is approximately ¾ inch wider
all the way around than the body. Center the slab on the upside
down body and press it down gently using a rib directly over the
To thin a slab without using a rolling pin, you can start
walls. This makes an impression of the walls on the slab, showing
with a ball of clay, flatten it with the palm of your hand
and then, lifting it with both hands, toss it at an angle
where to slip and score. Make a small register mark on the body
on an absorbent table surface, or canvas covered table.
and the slab so it’s easier to realign the parts. Then, remove the
slab and apply a generous layer of joining slip (see recipe) on
the slab and the body and score both surfaces. I find that I get a
better join when using slip first then scoring because it drives the
Repeat this process, rotating the slab as you go to thin it to the slip further into the clay parts. Using the registration marks made
desired consistency. These parts and a few simple tools are all that earlier, realign the slab on top of the upside down body (figure 3).
Then, rib the slab directly over the body wall to join the parts.
are needed to get started (figure 1).
Working with the body right side up on the banding wheel,
use a moistened, soft, nylon-bristle brush to smooth out the
interior and exterior seams and completely join the two parts
With the body of the form upside down on a banding wheel,
(figure 4 ).
make four indicator marks on the edge that bisect the length
the box; therefore, the exterior diameter should be about ¼ inch
smaller than the interior diameter of the body.
The slabs used to complete the body and lid are hand
“thrown” to ¼ inch thickness and ribbed to compress and
smooth both surfaces.
“Thrown” slab
Assembling the Body
7
7
8
Gently fold over the bottom slab and join
it to the body.
10
9
Cut the decorative rim shape using
indicator marks above the feet.
11
Cut the decorative rim shape at a bevel to
soften the transition.
12
Add a slab to the lid and push out a bump
on the underside.
Next, cut the bottom slab into a shape that accents the scallop
pattern of the feet. Start by making an indicator dot on the slab
directly in front of the middle of each foot. This mark will help
establish where to cut for the decorative trim on the bottom slab.
Trim the slab ¼ inch wider than the body of the form and make
a decorative point at each of the four indicator dots in front of the
feet (figure 5 ). It is important not to cut the slab to its final size
before this step, because it often will stretch and not fit correctly
when attached.
Then, smooth out the edge of the cut slab with a moist sponge
and compress and shape it into a tapered feather edge by applying
slight pressure with a moistened thumb and forefinger (figure 6 ).
Use your index finger to gently and slowly roll the slab over to
join it with the wall of the body. Roll the slab over in stages, being careful not to push it all the way over in one move because it
will buckle and ripple (figure 7 ).
Finally, move on to shape the top lip of the body into a delicate, scalloped edge. Working right side up on the banding wheel,
make new indicator marks on the rim directly above each foot
and four additional marks on the rim bisecting the piece along
its length and width. Cut out the rim scallops using the indicator marks to define the high and low points of the cut (figure
8). These cuts are shallow—no more than 3/8 inch below the
Smooth and round off the rim, then flute
the lip using both hands.
Attach the handle parts and shape the
interior curve.
rim—and should be straight with no bevel. Use your fingers and
a moist sponge to compress the rim and smooth and round out
the lip. Moistening the lip also makes the clay more pliable and
less prone to cracking during the final shaping. Finish the rim
Joining Slip Recipe
1 cup vinegar
Approximately 8 squares of cheap
toilet paper
1 heaping cup of dry clay in chocolatechip-sized chunks
Blunge the toilet paper in the vinegar with an immersion blender to break down the paper fibers.
Add dry clay and stir lightly with a wood tool until
everything is moist. Let the mixture rest overnight.
Stir vigorously with a wood tool the next day. The
mixture will be foamy and the consistency of sour
cream to frosting. Joining slip can be stored almost
indefinitely in a covered jar. Add more vinegar if
the mixture gets too stiff.
8
Soup Tureen, 14 in. (36 cm), thrown and altered porcelain, using the same technique.
by running a moistened index finger along the lip to roll it over
while supporting the outside of the walls with your other hand
(figure 9 ).
Assembling the Lid
Start by placing the thrown lid part, wide side down, onto
the lid seat of the base. Adjust the shape of the oval if necessary to match the base. Then, repeat the steps above to define the shape of the lid (figure 10 ). Note the upward angle
of the knife, and attach the slab on top of the lid. Make indicator marks on the slab, lid, and body to help with proper
alignment later. Repeat the slab attaching process (refer to
steps in figures 3–7). Cut the lid slab to the appropriate size,
including the decorative points, smooth out the cut edges
and fold over the slab to join it to the lid (figure 11).
Note
I finish the lid with a small interior bump made by pressing
on the underside of the lid with my finger (figure 11). I
do this to add visual volume to the lid and to fill up the
negative space under the handle.
Attaching the Handle
Make the handle for the lidded container from four parts—a
large, hourglass-shaped base handle; two accent straps, and
a teardrop shaped ball (see handle parts in figure 1). The
consistency of the handles pulled earlier should be the same
as the body parts—flexible but not so soft as to show fingerprints. Start by attaching the main handle to the lid, and
establish the handle’s curve. Then, place the accent strips
on top of the main handle to establish their location. Cut
off the excess length of the straps so that they are about 3/8
inch longer than the midpoint of the main handle. Smooth
and round out the cut ends of the accent straps and roll each
end up into a curl. Slip, score, and attach the straps one side
at a time to maintain their correct alignment on the main
handle. To complete the handle, insert a small teardrop
shaped ball between the accent curls (figure 12 ).
Dry both parts of the box together until they begin to
change color, then separate the parts and dry the lid on its
side and the base upside down until both are completely dry.
Bisque and glaze fire both parts together.
For information on my glazing technique, see the link to my
CeramicArtsDaily.org article at:
http://ceramicartsdaily.org/tag/martha-grover/.
Special thanks to Joshua David for his help with this article
and the images.
Martha H. Grover received her MFA from the University of
Massachusetts—Dartmouth. She is currently a resident artist
at the Archie Bray Foundation in Helena, Montana. Her
work can be seen online at www.MarthaHGrover.com.
9
Four Ways to Red
by Dave Finkelnburg
Chinese ceramic lore includes the tragic tale of a potter who became so frustrated with his many failures to
produce a red glazed pot for his emperor that he finally threw himself into his kiln. When the kiln cooled and
was opened, so the tale goes, the finest red glazes were found. Modern materials make it considerably easier
to produce red glazes, although challenges remain. Knowing the chemistry and firing requirements of the types
of red glazes will save you from throwing yourself into your kiln.
Defining the
Terms
Stain—Essentially a frit made of
colorant chemicals, compatible fluxes,
and possibly glass formers, which has
been melted, cooled, and pulverized to
a fine powder. Encapsulated Stain (Inclusion
Pigments)—A new generation of stable
stains made by melting metallic colorants
with zirconium silicate, cooling the melt,
and grinding the result to a fine powder.
Because zirconium silicate is refractory,
stains containing it can produce brighter
colors up to cone 10 using pigments that
would otherwise fade at high temperatures.
These colors are safe to use in the studio.
Flux (molar) Unity or Seger Formula—
The chemical composition, commonly of
a fired glaze, expressed as one mole of
total flux to the number of moles of all
other ingredients in the glaze. The term
‘unity molecular formula’ does not specify
whether the flux, alumina, or glass formers
are in unity. Each can be at different times
for different purposes, but flux unity is used
almost universally by ceramic artists.
Selenium/Cadmium Red
The easiest, most reliable path to red is to use tion atmospheres. These stains are refractory at
relatively recently developed cadmium inclu- pottery temperatures and do not melt much, if
sion stains. These stains also contain selenium at all. However, the manufacturers recommend
combined with sulfur, and they will produce that the stain not be ball milled.
the full range of colors in the red spectrum As with lead, cadmium stains can produce foodfrom yellow through orange to brilliant red. safe colors. However as with lead, cadmium
They work in both translucent and opaque under certain circumstances can be leached from
glazes, in oxidation and reduction firings, and the fired glaze. A sample of any cadmium staintinted glaze used on potential food surfaces
at all firing temperatures.
Historically, cadmium and selenium have should be tested for leaching by a qualified
produced glamorous red glazes but only at laboratory.
low temperatures. The colorants burned out Inclusion stains are suitable for use in a wide
variety of base glazes.
at higher kiln temperaLow-Fire Satin glaze
The amount of stain to
tures and the resulting
Cone 04
use must be determined
red glazes were pale.
The discovery of the Ferro Frit 3195. . . . . . . . . . . . . . . . . . 50% by testing, because the
base glaze and applicaencapsulation process Dolomite. . . . . . . . . . . . . . . . . . . . . . 30
tion thickness will influ(the of melting the colo- EPK Kaolin. . . . . . . . . . . . . . . . . . . . . 20
100% ence the fired results.
rants into a zirconium Reds produced with
silicate glass at high Add:Encapsulated Mason Stain
#6025 Coral Red. . . . . . . . . . . . 15% these stains, while very
temperatures) has now reliable, tend to be flat
made the many hues of
yellow through red reliable at temperatures and lack the variation produced when using
through cone 10 in both oxidation and reduc- oxides and/or atmospheric kilns.
Iron Red
Iron red glazes often have vibrant names like
Tomato Red or Ketchup Red, and they are generally warm reds. The true reds are produced
in oxidation around cone 5. By cone 10, they
tend to turn toward orange or persimmon.
High-iron glazes fired in heavy reduction will
turn maroon to black.
Iron reds are mainly iron saturated, which
means they contain between 5 and 10% iron
oxide in the glaze recipe (most recipes use
7% or more). Iron reds with bone ash (calcium phosphate) as a source of phosphorous
(phosphorous in general causes opalescence
and brighter colors) typically contain on the
order of 10%.
Even considering the above specifications,
there is wide variation in iron red recipes.
Traditional persimmon or kaki recipes, for
example, are very high in both alumina and
silica but contain no phosphorous.
The source of iron oxide is important to the
color produced and is possibly the most variable colorant used in glazes. The percentage of
iron, particle size, and amount of clay, silica or
other contaminants may be dramatically different from one source of iron oxide to another.
iron red glaze
Cone 10
Bone Ash. . . . . . . . . . . . . . . . . . . . 2.91%
Pearl Ash (Potassium Carbonate) . . 10.68
Whiting. . . . . . . . . . . . . . . . . . . . . 25.24
Custer Feldspar . . . . . . . . . . . . . . . 6.80
Grolleg Kaolin . . . . . . . . . . . . . . . . 35.92
Silica . . . . . . . . . . . . . . . . . . . . . . . 18.45
100.00%
Add:Red Iron Oxide (Spanish) . . . . 9.71%
From Pete Scherzer, CM Sept. 2003
2011 clay workshop handbook | Copyright© 2011, Ceramic Publications Company | www.ceramicartsdaily.org 10
Copper Red
Copper reds are achieved between cones 5 and 11 by reducing the bide can be a source of glaze blisters and pinholing, its use presents
copper (either copper oxide or copper carbonate) in the glaze. Only its own set of problems in the studio.
At cone 10, any combination of glaze
a small quantity of copper is necessary for
ingredients that contains, in terms of
this; 0.25% copper carbonate is sufficient,
Copper Red #11 Glaze
flux (molar) unity, 0.3 moles of alkalis,
though more is often used. The red color is
Cone 10 Reduction
0.7 moles of alkaline earths (preferably
aided by the presence of a limited amount
Colemanite . . . . . . . . . . . . . . . . . . 10.80%
most or all as CaO), 0.4 moles of alumina,
of tin. Iron can also help produce a red color
Whiting. . . . . . . . . . . . . . . . . . . . 15.73
3.5 moles of silica, 0.15 moles of B2O3,
in copper red glazes, but too much iron will
Kona F-4 (sub Minspar 200). . . . . . 15.57
lead to muddy reds.
1% tin, and 0.5% copper carbonate can
Nepheline Syenite . . . . . . . . . . . . . 20.43
The various hues of copper red are influproduce a fine copper red if properly fired.
English China Clay. . . . . . . . . . . . . 1.48
enced by the amount of alumina, magneUnderstanding and controlling the reSilica . . . . . . . . . . . . . . . . . . . . . . . 35.99
sium, and boron present in the glaze. High
duction atmosphere in a kiln to achieve
100.00%
alumina tends to produce cooler reds, as
copper reds is usually by far the most
does magnesium, while high boron prodifficult part of working with this family
Add: Tin Oxide. . . . . . . . . . . . . . . . 1.72%
duces warmer reds.
of glazes. The glaze above is best if the
Copper Carbonate. . . . . . . . . 0.42%
Copper red glazes tend to be somewhat
kiln is placed in moderate reduction at
A vibrant red that may turn blue, green, or
fluid, so glaze runs should be guarded
cone 010 and held there until cone 9
purple where thick; runs when thick. From
against in glaze application. Boron pardrops. The kiln can then be soaked in
Andy Cantrell, CM May 2000.
ticularly enhances the fluidity at cone 10.
oxidation until cone 10 is down. A smoky
Where copper reds flow off of rims or high
fire, as used with carbon trap glazes, is
points, they tend to turn white.
never necessary to achieve copper reds.
Oxidation copper reds in electric kilns are achieved by mixing a reducing In fact, a sooty atmosphere in the kiln is likely to produce gray, dingy
agent, silicon carbide, with the copper in the glaze. Because silicon car- copper reds due to carbon trapping.
Chrome-Tin Pink
Chrome-tin pink glazes are, as their name implies, a combination moles Al2O3, not more than 0.25 moles B2O3, 2.5 to 3 moles SiO2,
of chrome and tin that produces somewhat cool reds from a light up to 7.5% tin oxide, and not more than 0.5% of chrome oxide
pink to a deep burgundy. The combination works well from low (0.15% is often enough).
fire into the cone 6 range, but poorly above cone 9.
A thin application of a chrome-tin glaze will tend toward gray
According to Cullen Parmelee in his book Ceramic Glazes, the rather than red. Close examination of the glaze with a magnifyglaze chemistry necessary is fairly specific: calcium is the most ing glass will reveal the red is present in small islands within a
important flux because it gives the color a greater stability and matrix of clear glass. This explains why a thicker application will
produce more vibrant red. Chrome-tin
a more fiery red color while sodium
pinks can be produced more reliably
promotes yellow shades. Boron should
from commercial stains than from the
be limited because it tends to shift the
Raspberry
raw materials, but stains are not required
color toward purple. Additionally, if your
Cone 6
to produce this red.
base glaze contains barium, the color
Whiting. . . . . . . . . . . . . . . . . . . . . 20.0%
Chrome-tin pinks present special chaleffects will be stronger in the absence of
Nepheline Syenite . . . . . . . . . . . . . . 18.0
lenges when working at cone 5–6,
boron. Zinc should be avoided because
Ferro Frit 3134. . . . . . . . . . . . . . . . . 14.0
because chrome (either from chrome
chrome and zinc can interact to produce
OM-4 Ball Clay. . . . . . . . . . . . . . . . . 18.0
green glazes or chrome-tin reds) can
brown. High alumina works against the
Silica . . . . . . . . . . . . . . . . . . . . . . . . 30.0
vaporize at the peak of the firing and
red. Because a glaze can dissolve some
100.0%
give a pink blush to adjacent ware with
of the clay body, changing the alumina
Add:Tin Oxide. . . . . . . . . . . . . . . . . 7.5%
white glazes containing tin. Since tin is
and flux content of the glaze, these
Chrome Oxide. . . . . . . . . . . . . . . . . 0.2%
occasionally used as an opacifier, this is
glazes require careful testing.
This glaze often benefits from a controlled
not an uncommon occurrence. This can
A good starting point for creating a
slow cooling. From Mastering Cone 6
be especially problematic if working with
chrome-tin glaze at cone 6, in terms of
Glazes by John Hesselberth and Ron Roy.
commercial glazes where the full list of
flux unity, is from 0.7 to 0.9 moles CaO,
ingredients is not obvious.
from 0.1 to 0.3 moles alkalis, 0.25 to 0.3
All About Iron
by John Britt
Iron is everywhere in many different forms, but that doesn’t mean it has to be boring—or even brown.
Defining the Terms
Iron Glazes
Iron—The fourth most common element in the earth’s crust and
It would be impossible to show all iron glazes in this article but
highlighting a few will give you a glimpse of the wide variety.
the most common element (in terms of mass) on the planet,
comprising 35% of the earth’s core.
Melting Point: 2795°F (1535°C )
Toxicity: Non-toxic
Forms of Iron
Iron oxide is the most common colorant in ceramics. It is so
ubiquitous that it is very difficult to find a material without some
iron—it’s found in almost everything from feldspars to kaolin to
ball clays, earthenware clays, and many colorants. In fact, many
materials require expensive processing to reduce the amount of
iron to acceptable levels.
Iron is a very active metal that combines easily with oxygen. That
means it is very sensitive to oxidation and reduction atmospheres,
producing a wide range of glaze colors and effects from off white,
light blue, blue, blue-green, green, olive, amber, yellow, brown,
russet, tea-dust, black, iron saturate, iron spangles, iron crystalline
(goldstone/tiger’s eye), oil spot, hare’s fur, kaki (orange), leopard
spotted kaki, tan, black seto, pigskin tenmoku, shino, gray (Hidashi), iridescent, silver, gold, etc. Iron also plays a major role in
clay bodies, slips, terra sigillata, and flashing slips.
There are three major forms of iron used in ceramics: red iron oxide (Fe203), black iron oxide (FeO or Fe3O4), and yellow iron oxide
(FeO (OH)). There are different mesh sizes and grades, and each
contains varying degrees of impurities that can make a significant
difference in the results you get.
The most interesting thing about iron is that it can act both as a
refractory and a flux. As red iron oxide, Fe2O3, it is an amphoteric
(refractory/stabilizer) similar in structure to alumina (Al2O3). But
if it is reduced to black iron oxide (FeO) it acts as a flux similar in
structure to calcium oxide (CaO). What this means is that a tenmoku
glaze with 10% red iron oxide will be a stiff black glaze if fired in
oxidation because the iron oxide acts as a refractory. But, if the
same glaze is fired in reduction that 10% Fe2O3 will be reduced
to FeO, changing it to a flux, which will make it a glossy brown/
black glaze that may run.
Another interesting property of iron oxide is that if it is fired in
oxidation it will remain Fe2O3 until it reaches approximately 2250°F
(approximately cone 8) where it will then reduce thermally to Fe3O4
on its way to becoming FeO. The complex iron oxide molecule
simply cannot maintain its state at those temperatures. This results
in the release of an oxygen atom that will bubble to the surface
of the hot glaze and pull a bit of iron with it. When it reaches the
surface the oxygen releases the iron as it leaves the glaze, creating spots with greater concentrations of iron oxide. This is what
creates an oil spot glaze. This reaction can easily be seen through
the spy hole of a kiln or with draw tiles. There is an obvious and
unmistakable bubbling. If heated further, these spots begin to melt
and run down the pot, creating a distinctive “hare’s fur” effect.
Ron Roy Black
Cone 6
Talc . . . . . . . . . . . . . . . . . . . . . . . . . . . 3%
Whiting . . . . . . . . . . . . . . . . . . . . . . . . 6
Ferro Frit 3134 . . . . . . . . . . . . . . . . . . . 26
F-4 Feldspar . . . . . . . . . . . . . . . . . . . . . 21
EPK Kaolin. . . . . . . . . . . . . . . . . . . . . . 17
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 27
100%
Add: Cobalt Carbonate. . . . . . . . . . . . . 1%
Red Iron Oxide. . . . . . . . . . . . . . . . 9%
Fake Ash
Cone 6 reduction
Bone Ash. . . . . . . . . . . . . . . . . . . . . 5.0%
Dolomite. . . . . . . . . . . . . . . . . . . . 24.5
Gerstley Borate . . . . . . . . . . . . . . . . 10.0
Lithium Carbonate. . . . . . . . . . . . . . 2.0
Strontium Carbonate. . . . . . . . . . . . 9.5
Ball Clay. . . . . . . . . . . . . . . . . . . . . . 21.0
Cedar Heights Red Art. . . . . . . . . . . 28.0
100.0%
Chinese Crackle (Kuan)
Cone 10 reduction
Custer Feldspar. . . . . . . . . . . . . . . . . . . 83%
Whiting . . . . . . . . . . . . . . . . . . . . . . . . 9
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 8
100%
Add: Zircopax (optional) . . . . . . . . . . . . 10%
Adding small amounts of red iron oxide to
this feldspathic base and firing in reduction
will result in the following:
Blue Celadon: 0.5%–1.0%
Blue–Green: 1–2%
Olive to Amber: 3–4%
Tenmoku: 5–9%
Iron Saturate: 10–20%
Ketchup Red (Jayne Shatz)
Cone 6 oxidation
Gerstly Borate. . . . . . . . . . . . . . . . . . . . 31%
Talc . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Custer Feldspar. . . . . . . . . . . . . . . . . . . 20
EPK Kaolin . . . . . . . . . . . . . . . . . . . . . . 5
Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 30
100%
Add: Spanish Red Iron Oxide. . . . . . . . 15%
Works best on dark colored stoneware.
If used on a buff clay body, the red is less
intense.
Sources of Iron
Form
Chemical
Name
Red Iron Oxide
Fe2O3
ferric iron,
Hematite
Characteristics
Most Common Use
Most common form of iron and is a finely ground material
that disperses well in glaze slurries, contains 69.9% Fe in
the chemical formula, sold as:
•Natural Red Iron Oxide or Brown 521 (85% purity)
•Spanish Red Iron Oxide* (83–88% purity )
•Synthetic Red Iron Oxide* (High Purity Red Iron or Red
4284) (96–99% purity). Very fine 325 mesh. Sometimes
sold as the brand name Crocus Martis or Iron Precipitate.
Used in glazes, washes, slips, engobes, terra sigillatas,
and clay bodies, used to make celadons, tenmoku,
kaki, iron saturates, etc. (more listed in the text on
page 22)
Normally used from 1–30% in glazes.
Black Iron Oxide
FeO
ferrous
oxide,
wustite
Strongest form of iron, containing 72.3% Fe in the
chemical FeO, sold as:
•Natural Black Iron Oxide (85–95% purity) 100 mesh; is
black in color and has a larger particle size. In glazes it’s
prone to speckling but is easily eliminated by ball milling.
•Synthetic Black Iron Oxide* (99% purity) 325 mesh
Used in glazes, washes, slips, engobes, and terra
sigillatas; used to make celadons, tenmoku, kaki, iron
saturates, etc.
Yellow Iron Oxide
FeO (OH)
ferric oxide
hydrate,
Geothite
Weakest form of iron, containing 62.9% Fe in the chemical
formula, has a high LOI of 12%, sold as:
•Synthetic Iron Oxide* (96% purity) 325 mesh
•Yellow Ochre or Natural Yellow Iron Oxide (35% purity)
contains impurities of calcium carbonate, silica, and
sometimes manganese dioxide
Used in glazes, washes, slips, engobes, terra sigillatas,
and clay bodies; used to make celadons, temmoku,
kaki, iron saturates, etc.; sometimes yellow ochre is
added to porcelain to make “dirty” porcelain (5–9%)
Umber, Burnt
Umber
Calcined Umber which is a high-iron ochre material
containing manganese
Used in glazes, washes, slips, engobes, terra sigillatas
or claybodies to make a range of reddish-brown
colors; darker than sienna and ochre (yellow iron)
Sienna, Burnt
Sienna
Calcined Sienna, which is a high-iron ochre material with
less manganese than umber
Used to make browns in glazes, washes, slips,
engobes, terra sigillatas or clay bodies
Cr2FeO4
Contains chrome and iron oxide (ferric chromate); toxic—
absorption, inhalation, and ingestion
Used to make dark colors in glazes, slips, engobes or
clay bodies; can give gray, brown, and black; can give
pink halos over tin white glazes
FeCl3
Water soluble metal salt; toxic—corrosive/caustic, affects
liver, inhalation and ingestion
Used in low-fire techniques, like pit firing, aluminum
foil saggars, horse hair and raku techniques; also
used in water coloring on porcelain techniques
FeSO4
Water soluble metal salt, soluble form of iron, (aka
Crocus Martis)
Salt used in water coloring on porcelain, raku, and
low-fire soda
Iron Chromate
Ferric Chloride/
Iron Chloride
Iron Sulfate
(Copperas)
Iron Phosphate
FePO4
Rutile (light, dark,
TiO2
Most common natural ore of titanium, containing various
impurities including iron ( up to 15%)
Used in glazes, washes, slips, engobes, and terra
sigillatas to give yellows, tans, greens, blues, and
milky, streaky, mottled textures; also used to produce
crystalline glaze effects
FeTiO3
Naturally occurring ore containing iron and titanium, higher
in iron than rutile (when 25% or more iron is present)
Commonly used to produce speckles in glazes or
clay bodies
e.g., Redart, Albany slip, Alberta Slip, Barnard Slip (aka
Blackbird Slip), Michigan slip, Lizella, laterite, and other
assorted earthenware clays
Used in glazes, slip glazes, slips, engobes, terra
sigillatas, and claybodies to make a range of reddishbrown colors
Iron scale or iron spangles—coarse, hard particles that
resist melting and chemical breakdown
Gives speckles in clay bodies and glazes
and granular)
Illmenite (powdered
and granular)
Iron Clays
Magnetic Iron
Oxide
Fe3O4
Magnetite
Rarely used but can be used to develop iron red
colors; sometimes used instead of bone ash as a
source of phosphate without the calcium in synthetic
bone ash (TCP or tri-calcium phosphate)
*Synthetic and Spanish Varieties
Synthetic Red Iron is produced by calcining black iron oxide particles in an
oxidation atmosphere. They are then jet milled, which produces “micronized”
red iron oxide particles that are approximately 325 mesh. This type of red
iron is very heat stable (up to 1832°F (1000°C). This differs from black iron
oxide, which changes color at 365°F (180°C) from black to brown to red as it
oxidizes. The color of red iron oxide changes from light pinkish to red to dark
purplish red as the particle size increases.
Spanish red iron oxide is bacterially ingested iron oxide that is micronized.
The Tierga mines in Spain found that their iron sulfide was inadequate for
steel making (which accounts for 95% of the iron market). After some time
a worker noticed that the iron in a pool of rain water turned a brighter shade
of red after it was heated. This turned out to be caused by a bacterium that
uses iron sulfide as an energy source. The bacterium changes the state of the
iron, which is then put into evaporative ponds where it forms green crystals.
These are then roasted to produce Spanish red iron oxide.
Thrown Agateware
by Michelle Erickson and Robert Hunter
Factory of John Dwight, England (Fulham), 1670-1859. Covered Tankard, ca.
1685-1690. Stoneware with salt glaze, 10½ inches (27 cm) in height. Courtesy
the Nelson-Atkins Museum of Art, Kansas City, Missouri. Gift of Frank P.
Burnap. Photograph by Jamison Miller.
Beginning in the late seventeenth century, a prepared mixture
of various colored clays that mimics the variegated appearance
of agate stone was used to fashion a class of English ceramics
recognized as agateware. There are two types of agateware, one
is wheel thrown and the other is made using hand-building
techniques, and is classified as laid agateware. We’re focusing
on thrown agateware today, but for an expanded explanation
of laid agateware, see the November/December 2009 issue of
PMI, pp. 17-21.
Background
The earliest English thrown agate is found among the products of John Dwight (even though he himself referred to it as
marbled rather than agateware). Considered by many as the
father of English pottery, Dwight is well known in the annals
of ceramic history for his innovations. He conducted numerous ceramic experiments beginning in the 1670s, delved into
the mysteries of porcelain, and recorded his recipe to produce
“marbled” stoneware.
Dwight’s notes reveal two important material considerations
in making an agate body: clay color and clay compatibility.
To create the illusion of agate striations, different colored
clays must be obtained naturally or by modification, adding
pigments or coloring agents. The tone of a natural clay can
also be altered by sieving to remove impurities such as iron
and sand.
1
2
In preparing the clay for throwing, slabs of
varied colored earthenware clays are cut
and stacked.
3
The slabs are then stacked in alternating
colors. After the initial stacking, the clay is
sliced in half and restacked.
5
4
This ball of clay has been cut in half to
show an example of the agate pattern
prior to throwing.
6
Center the prepared clay ball on the wheel.
The agate pattern will not be visible on
the surface.
Creating a successful variegated appearance also depends
on the proportions of clay colors used. Clues for understanding problems related to combining multiple clays are also
contained in Dwight’s formula. These include shrinkage rates,
firing temperatures, density, plasticity, elasticity, and strength.
All of these properties must be considered when mixing dissimilar clay bodies.
Today, only a few of Dwight’s “marbled” or agatewares
survive. In addition, some agateware fragments have been
recovered from archaeological excavations of his pottery site
in Fulham, England. These fragments include examples of a
tankard, a gorge (or mug), a cappuchine (coffee cup), and a
teapot. It is hard to judge whether Dwight’s agateware was
The different colored clays are carefully
wedged or kneaded together to prepare a
ball for throwing.
Open the clay ball and pull up quickly into
a low cylinder. Try to do this in one pull as
to not overwork the clay.
received as a novelty or as an important scientific discovery.
Commercial production of English agateware began in earnest in the second quarter of the eighteenth century. In 1729,
Samuel Bell, at the Lower Street Potworks, Newcastle-underLyme, was granted a patent to produce “red marbled stoneware with mineral earth found within this kingdom which
being firmly united by fire will make it capable of receiving a
gloss so beautiful as to imitate if not compare with ruby.” Like
Dwight’s agate, the Bell products were thrown on the wheel.
After being initially formed, the wares were turned on the
lathe to thin the body and concurrently create a clean variegated surface. These wares typically had a red and off-white
clay body even though a black colored clay was used.
7
8
The cylinder is shaped into the final mug
form. Note the muddy surface concealing
the agate patterning.
10
9
A cross section of a fully thrown cylinder
shows distinct lines and swirls of the
colored clays.
12
11
Final finishing of the base and additional
design elements, textures or a foot can be
done using a rib.
Again using a metal rib, scrape the interior
of the mug to remove the slip from the
surface and reveal the pattern.
Archaeological evidence from the excavation of Staffordshire factory sites, including that of John Bell and of John
Astbury at Shelton Farm, shows that many potters made this
type of thrown agate. Tea wares were the most common forms
made, although mugs and bowls were also produced. Thrown
agate reached the height of its popularity in the 1750s and
continued in production into the early 1770s.
A number of nineteenth-century American potteries used
agate clays to manufacture doorknobs covered with a Rockingham glaze. In the twentieth century, agateware makes
a brief appearance in the arts and crafts movement, most
notably in the “Mission Swirl” line of the Niloak Pottery
Company in Benton, Arkansas. Another thrown agate of this
The slip covering the exterior of the
thrown mug is scraped away using a metal
rib, revealing the agate pattern.
The form can be supported from the inside
and turned on a lathe to refine the agate
pattern further.
general type was produced by North State Pottery of North
Carolina in the 1920s and 1930s.
Preparing the Clay
The key step in creating an agateware body can be summed
up in three words: preparation, preparation, preparation! For
creating thrown agate, the initial selecting and mixing of clays
is critical. For the piece demonstrated here, three colored
earthenware clays were selected: a white clay, a red iron clay,
and an iron manganese body (figure 1 ).
Tip: You could also start experimenting by using only
one light colored clay body and adding commercial stains or
oxides to create three different colors. This can reduce some
“The key step in
creating an agateware
body can be summed up in
three words: preparation,
preparation, preparation!
For creating thrown agate,
the initial selecting and
mixing of clays is critical.”
of the problems that arise when using different clay bodies, including different shrinkage rates, firing temperatures or other
compatibility issues, though some stains can be incompatible
with one another as well, so testing is necessary.
To begin, clay slabs are built up, alternating the colors
(figure 2 ). This stack is then wedged or folded to form a ball
that can then be thrown on the wheel (figure 3 ). Care has to
be taken during wedging to ensure the clays are mixed without
overly distorting or blurring the resultant agate pattern. For
demonstration purposes, the wedged ball is cut to show the
pattern prior to throwing (figure 4 ). The degree of success in
an agate pattern comes from the initial wedging process as
well as the throwing.
Creating Wheel-Thrown
Agateware
The prepared clay ball is then centered on the wheel (figure
5 ). Once centered, the clay ball is opened and pulled quickly
into a cylinder (figures 6 and 7 ). During throwing, the surface
of the clay body becomes smeared so that the agate patterning is obscured. Care has to be exercised during throwing so
as not to overwork the clay; otherwise, the pattern becomes
muddled. This test piece was cut in half to show how the
agate patterning shifts as the clay moves in response to the
pressures exerted when throwing. The different colors remain
distinct (figure 8 ). Since nearly all Staffordshire thrown agate
was subsequently trimmed on the lathe, the veining usually
appears sharp and crisp. Later smearing and smudging can oc-
cur at the attachment points of handles or spouts as evidenced
on some antiques.
Most potters don’t have a lathe, but the pattern can be
revealed by using a metal rib (figure 9 ) to scrape away the slip
and outer layer of clay from the surface. Using a shaped rib or
the metal rib again defines the foot or base while maintaining
a crisp pattern (figure 10 ). Lastly, scraping the interior of the
form reveals the agate pattern on the inside (figure 11 ). If you
have access to a lathe, the form can be supported from the
inside on a chuck and the pattern further refined and revealed
(figure 12 ).
Michelle Erickson has over 20 years experience in working
with 17th- and 18th-century reproduction pottery in addition to her considerable contemporary work. She produces
reproductions for organizations such as Colonial Williamsburg, the National Park Service, Parks Canada, the Museum of Early Southern Decorative Arts, the Philadelphia
Museum of Art and Historic Deerfield museum. She is a
partner in the business, PERIOD DESIGNS in Yorktown,
Virginia, an innovative firm specializing in the reproduction of 17th- and 18th-century decorative art.
Robert Hunter is a professional archaeologist and editor
of the annual journal, Ceramics in America, published
by the Chipstone Foundation of Milwaukee, Wisconsin.
He is a partner in the business PERIOD DESIGNS. Mr.
Hunter lectures widely and has written for a variety of
ceramic publications including Ceramic Review, Studio
Potter, Ceramics: Art and Perception, Keramik Techni,
and ANTIQUES.
... see what’s hatching this
summer when you bring
Georgies ^6 Glazes into
your garden !!!
Visit our website for
technique details.
www.georgies.com
800.999.2529

2011 clay workshop handbook

Transcription

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