2011 Clay Workshop Handbook

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2011 clay workshop handbook

knowledge and techniques for the studio

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Expanding Your Palette in Mid-range Firing by Yoko Sekino-BovéA 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 GroverPorcelain is a great material for creating delicate forms for culinary treasures.

Four Ways to Redby Dave FinkelnburgOne 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 Ironby John BrittIron can be many things, and many colors—many of which are not brown.

Throwing Agatewareby Michelle Erickson and Robert HunterMix it up. Stack and wedge different colored clays together to create thrown forms with dramatic patterns.

2011 Clay Workshop HandbookKnowledge 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 demonstrat-ed 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!

2011 clay workshop handbook | Copyright© 2011, Ceramic Publications Company | www.ceramicartsdaily.org 1

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.


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 every-one. 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 over-whelmingly 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 frustra-tion, 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 ConditionsClay 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.

Expanding Your Palette in Mid-range Firingby Yoko Sekino-Bové


recipes

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N501 TraNspareNT, glossy, aNd CraCkles

Cone 5Ferro Frit 3110 . . . . . . . . . . . . . . . . . . 90 EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 10 100 %

N502 TraNspareNT aNd glossy Cone 5

Gillespie Borate . . . . . . . . . . . . . . . . . 30 F-4 Feldspar . . . . . . . . . . . . . . . . . . . . 46 EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 13Silica . . . . . . . . . . . . . . . . . . . . . . . . . 11 100 %

See chart on page 8 for test results.

N503 opaque, glossy, aNd TexTured

Cone 5Gillespie Borate . . . . . . . . . . . . . . . 52.6 EPK Kaolin . . . . . . . . . . . . . . . . . . . 21.0Silica . . . . . . . . . . . . . . . . . . . . . . . 26.4 100.0 %Add: Zircopax . . . . . . . . . . . . . . . . . 10.0

N504 semi-opaque, semi-saTiN wiTh TexTures

Cone 5Whiting . . . . . . . . . . . . . . . . . . . . . 9.5Ferro Frit 3124 . . . . . . . . . . . . . . . . 44.5 F-4 Feldspar . . . . . . . . . . . . . . . . . . 20.0 Zinc Oxide . . . . . . . . . . . . . . . . . . . 5.5Bentonite* . . . . . . . . . . . . . . . . . . . 7.5EPK Kaolin . . . . . . . . . . . . . . . . . . . 5.0Silica . . . . . . . . . . . . . . . . . . . . . . . 8.0 100.0 %Add: Zircopax . . . . . . . . . . . . . . . . . 9.0

See chart on page 8 for test results.

* Bentonite is typically listed as an addition to recipes, but in larger amounts it contrib-utes 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.

N505 saTiN, opaque wiTh TexTures

Cone 5Dolomite . . . . . . . . . . . . . . . . . . . . . . 12 Gillespie Borate . . . . . . . . . . . . . . . . . 14 Wollastonite . . . . . . . . . . . . . . . . . . . . 10Ferro Frit 3124 . . . . . . . . . . . . . . . . . . 8Cornwall Stone . . . . . . . . . . . . . . . . . 46EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 10 100 %Add: Magnesium Carbonate . . . . . . . 6

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.


0.1% 0.5% 1.0% 5.0% 10.0%

Copper Carbonate

N502CC05 N502CC10 N502CC50 N502CC100

Red Iron Oxide

(regular)

N502ROI05 N502ROI10 N502ROI50 N502ROI100

Cobalt Oxide

N502COX0 N502COX05 N502COX10

Chrome Oxide

N502CH01 N502CH05 N502CH10

Manganese Dioxide

N502MD05 N502MD10 N502MD50 N502MD100

Black Nickel Oxide

N502BN05 N502BN10 N502BN50

Iron Chromate

N502IC05 N502IC10 N502IC50 N502IC100

Rutile (powder)

N502R05 N502R10 N502R5 N502R100

Yellow Ochre

N502Y05 N502Y10 N502Y50 N502Y100

Glaze base N502 with coloring oxides and carbonates

0.1% 0.5% 1.0% 5.0% 10.0%

N504CC05 N504CC10 N504CC50 N504CC100

N504ROI05 N504ROI10 N504ROI50 N504ROI100

N504COX01 N504COX05 N504COX10

N504CH01 N504CH05 N504CH10

N504MD05 N504MD10 N504MD50 N504MD100

N504BN05 N504BN10 N504BN50

N504IC05 N504IC10 N504IC50 N504IC100

N504R05 N504R10 N504R50 N504R100

N504Y05 N504Y10 N504Y50 N504Y100

Glaze base N504 with coloring oxides and carbonates

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.


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 auto-matic 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

The following colorants were tested: black nickel oxide, cobalt oxide, copper carbonate, chrome oxide, iron chromate, man-ganese 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 predict-ability. 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.

Please note that some of the oxides and carbonates in this test ex-ceed 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.

ConclusionThis 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, Pennsylva-nia. She would like to thank the McKnight Foundation and the Northern Clay Center and its supporting staff for making this research possible.

1.0% 5.0% 10.0%

Tin Oxide

N502CT10 N502CT50 N502CT100

Titanium Dioxide

N502CTD10 N502CTD50 N502CTD100

Zircopax

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.

Glaze Base N502 with Opacifiers


when in use, martha grover’s porcelain butter boxes combine two things that were once scarce and are still considered delicacies.

Treasure for Treasure: Lidded Boxby Martha Grover

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 PartsWhen 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


The tools and parts needed to make the lidded container.

Cut the bottom at a bevel and establish four raised feet.

slip and score both parts and use registration marks to realign.

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.

Compress and taper the edge of the bottom slab.

1 2 3

4 5 6

“Thrown” slabTo thin a slab without using a rolling pin, you can start 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 on an absorbent table surface, or canvas covered table.

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.

Repeat this process, rotating the slab as you go to thin it to the desired consistency. These parts and a few simple tools are all that are needed to get started (figure 1).

Assembling the BodyWith the body of the form upside down on a banding wheel, make four indicator marks on the edge that bisect the length

and width of the piece. Use these indicators to cut an undulat-ing 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 impor-tant 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 walls. This makes an impression of the walls on the slab, showing where to slip and score. Make a small register mark on the body 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 slip further into the clay parts. Using the registration marks made 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.

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 (figure 4 ).


gently fold over the bottom slab and join it to the body.

Cut the decorative rim shape using indicator marks above the feet.

smooth and round off the rim, then flute the lip using both hands.

Cut the decorative rim shape at a bevel to soften the transition.

add a slab to the lid and push out a bump on the underside.

attach the handle parts and shape the interior curve.

7 8 9

10 11 12

Joining Slip Recipe1 cup vinegarApproximately 8 squares of cheap toilet paper1 heaping cup of dry clay in chocolate- chip-sized chunksBlunge the toilet paper in the vinegar with an im-mersion 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.

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, be-ing 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 deli-cate, 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 indica-tor 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

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


soup Tureen, 14 in. (36 cm), thrown and altered porcelain, using the same technique.

NoteI 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.

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 LidStart by placing the thrown lid part, wide side down, onto the lid seat of the base. Adjust the shape of the oval if neces-sary to match the base. Then, repeat the steps above to de-fine the shape of the lid (figure 10 ). Note the upward angle of the knife, and attach the slab on top of the lid. Make in-dicator 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).

Attaching the HandleMake 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 fin-gerprints. 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.


Four Ways to Redby 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 Termsstain—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

low-Fire saTiN glaze Cone 04

Ferro Frit 3195 . . . . . . . . . . . . . . . . . 50 Dolomite . . . . . . . . . . . . . . . . . . . . . 30EPK Kaolin . . . . . . . . . . . . . . . . . . . . 20 100 %Add: Encapsulated Mason Stain #6025 Coral Red . . . . . . . . . . . 15

The easiest, most reliable path to red is to use relatively recently developed cadmium inclu-sion stains. These stains also contain selenium combined with sulfur, and they will produce the full range of colors in the red spectrum from yellow through orange to brilliant red. They work in both translucent and opaque glazes, in oxidation and reduction firings, and at all firing temperatures.Historically, cadmium and selenium have produced glamorous red glazes but only at low temperatures. The colorants burned out at higher kiln tempera-tures and the resulting red glazes were pale. The discovery of the encapsulation process (the of melting the colo-rants into a zirconium silicate glass at high temperatures) has now made the many hues of yellow through red reliable at temperatures through cone 10 in both oxidation and reduc-

tion atmospheres. These stains are refractory at pottery temperatures and do not melt much, if at all. However, the manufacturers recommend that the stain not be ball milled. As with lead, cadmium stains can produce food-safe colors. However as with lead, cadmium under certain circumstances can be leached from the fired glaze. A sample of any cadmium stain-tinted glaze used on potential food surfaces should be tested for leaching by a qualified laboratory.Inclusion stains are suitable for use in a wide

variety of base glazes. The amount of stain to use must be determined by testing, because the base glaze and applica-tion thickness will influ-ence the fired results. Reds produced with these stains, while very reliable, tend to be flat

and lack the variation produced when using oxides and/or atmospheric kilns.

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Iron Red

iroN red glaze Cone 10

Bone Ash . . . . . . . . . . . . . . . . . . . 2.91 Pearl Ash (Potassium Carbonate) . 10.68Whiting . . . . . . . . . . . . . . . . . . . . 25.24Custer Feldspar . . . . . . . . . . . . . . 6.80Grolleg Kaolin . . . . . . . . . . . . . . . 35.92Silica . . . . . . . . . . . . . . . . . . . . . . 18.45 100.00 %

Add: Red Iron Oxide (Spanish) . . . 9.71 From Pete Scherzer, CM Sept. 2003

Iron red glazes often have vibrant names like Tomato Red or Ketchup Red, and they are gen-erally 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 (cal-cium 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 vari-able colorant used in glazes. The percentage of iron, particle size, and amount of clay, silica or other contaminants may be dramatically differ-ent from one source of iron oxide to another.

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Copper Red

Copper red #11 glazeCone 10 Reduction

Colemanite . . . . . . . . . . . . . . . . . 10.80 Whiting . . . . . . . . . . . . . . . . . . . . 15.73Kona F-4 (sub Minspar 200) . . . . . 15.57Nepheline Syenite . . . . . . . . . . . . 20.43English China Clay . . . . . . . . . . . . 1.48Silica . . . . . . . . . . . . . . . . . . . . . . 35.99 100.00 %

Add: Tin Oxide . . . . . . . . . . . . . . . 1.72 Copper Carbonate . . . . . . . . 0.42 A vibrant red that may turn blue, green, or purple where thick; runs when thick. From Andy Cantrell, CM May 2000.

Copper reds are achieved between cones 5 and 11 by reducing the copper (either copper oxide or copper carbonate) in the glaze. Only a small quantity of copper is necessary for this; 0.25% copper carbonate is sufficient, though more is often used. The red color is aided by the presence of a limited amount of tin. Iron can also help produce a red color in copper red glazes, but too much iron will lead to muddy reds. The various hues of copper red are influ-enced by the amount of alumina, magne-sium, and boron present in the glaze. High alumina tends to produce cooler reds, as does magnesium, while high boron pro-duces warmer reds. Copper red glazes tend to be somewhat fluid, so glaze runs should be guarded against in glaze application. Boron par-ticularly enhances the fluidity at cone 10. Where copper reds flow off of rims or high points, they tend to turn white. Oxidation copper reds in electric kilns are achieved by mixing a reducing agent, silicon carbide, with the copper in the glaze. Because silicon car-

bide can be a source of glaze blisters and pinholing, its use presents its own set of problems in the studio.

At cone 10, any combination of glaze ingredients that contains, in terms of flux (molar) unity, 0.3 moles of alkalis, 0.7 moles of alkaline earths (preferably most or all as CaO), 0.4 moles of alumina, 3.5 moles of silica, 0.15 moles of B2O3, 1% tin, and 0.5% copper carbonate can produce a fine copper red if properly fired.Understanding and controlling the re-duction atmosphere in a kiln to achieve copper reds is usually by far the most difficult part of working with this family of glazes. The glaze above is best if the kiln is placed in moderate reduction at cone 010 and held there until cone 9 drops. The kiln can then be soaked in oxidation until cone 10 is down. A smoky fire, as used with carbon trap glazes, is never necessary to achieve copper reds.

In fact, a sooty atmosphere in the kiln is likely to produce gray, dingy copper reds due to carbon trapping.

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Chrome-Tin Pink

raspberryCone 6

Whiting . . . . . . . . . . . . . . . . . . . . . 20.0 Nepheline Syenite . . . . . . . . . . . . . 18.0Ferro Frit 3134 . . . . . . . . . . . . . . . . 14.0OM-4 Ball Clay . . . . . . . . . . . . . . . . 18.0Silica . . . . . . . . . . . . . . . . . . . . . . . 30.0 100.0 %Add: Tin Oxide . . . . . . . . . . . . . . . . 7.5 Chrome Oxide . . . . . . . . . . . . . . . . 0.2 This glaze often benefits from a controlled slow cooling. From Mastering Cone 6 Glazes by John Hesselberth and Ron Roy.

Chrome-tin pink glazes are, as their name implies, a combination of chrome and tin that produces somewhat cool reds from a light pink to a deep burgundy. The combination works well from low fire into the cone 6 range, but poorly above cone 9. According to Cullen Parmelee in his book Ceramic Glazes, the glaze chemistry necessary is fairly specific: calcium is the most important flux because it gives the color a greater stability and a more fiery red color while sodium promotes yellow shades. Boron should be limited because it tends to shift the color toward purple. Additionally, if your base glaze contains barium, the color effects will be stronger in the absence of boron. Zinc should be avoided because chrome and zinc can interact to produce brown. High alumina works against the red. Because a glaze can dissolve some of the clay body, changing the alumina and flux content of the glaze, these glazes require careful testing. A good starting point for creating a chrome-tin glaze at cone 6, in terms of flux unity, is from 0.7 to 0.9 moles CaO, from 0.1 to 0.3 moles alkalis, 0.25 to 0.3

moles Al2O3, not more than 0.25 moles B2O3, 2.5 to 3 moles SiO2, up to 7.5% tin oxide, and not more than 0.5% of chrome oxide (0.15% is often enough).A thin application of a chrome-tin glaze will tend toward gray rather than red. Close examination of the glaze with a magnify-ing glass will reveal the red is present in small islands within a matrix of clear glass. This explains why a thicker application will

produce more vibrant red. Chrome-tin pinks can be produced more reliably from commercial stains than from the raw materials, but stains are not required to produce this red.Chrome-tin pinks present special chal-lenges when working at cone 5–6, because chrome (either from chrome green glazes or chrome-tin reds) can vaporize at the peak of the firing and give a pink blush to adjacent ware with white glazes containing tin. Since tin is occasionally used as an opacifier, this is not an uncommon occurrence. This can be especially problematic if working with commercial glazes where the full list of ingredients is not obvious.

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Iron is everywhere in many different forms, but that doesn’t mean it has to be boring—or even brown.

Defining the Termsiron—The fourth most common element in the earth’s crust and 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 (Hi-dashi), 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 ox-ide (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, creat-ing 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.

All About Ironby John Britt

Iron GlazesIt would be impossible to show all iron glazes in this article but highlighting a few will give you a glimpse of the wide variety.

Fake ashCone 6 reduction

Bone Ash . . . . . . . . . . . . . . . . . . . . 5.0 Dolomite . . . . . . . . . . . . . . . . . . . . 24.5Gerstley Borate . . . . . . . . . . . . . . . 10.0Lithium Carbonate . . . . . . . . . . . . . 2.0Strontium Carbonate . . . . . . . . . . . 9.5Ball Clay . . . . . . . . . . . . . . . . . . . . . 21.0Cedar Heights Red Art . . . . . . . . . . 28.0 100.0 %

ChiNese CraCkle (kuaN) Cone 10 reduction

Custer Feldspar . . . . . . . . . . . . . . . . . . 83 Whiting . . . . . . . . . . . . . . . . . . . . . . . 9Silica . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 14Custer Feldspar . . . . . . . . . . . . . . . . . . 20EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 5Silica . . . . . . . . . . . . . . . . . . . . . . . . . 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.

roN roy blaCk Cone 6

Talc . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Whiting . . . . . . . . . . . . . . . . . . . . . . . 6Ferro Frit 3134 . . . . . . . . . . . . . . . . . . 26F-4 Feldspar . . . . . . . . . . . . . . . . . . . . 21EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 17Silica . . . . . . . . . . . . . . . . . . . . . . . . . 27 100 %Add: Cobalt Carbonate . . . . . . . . . . . . 1 Red Iron Oxide . . . . . . . . . . . . . . . 9

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Sources of Iron

*Synthetic and Spanish VarietiesSynthetic 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.

Form Chemical Name

Characteristics most Common use

red iron oxide Fe2O3

ferric iron, Hematite

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 FeOferrous 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

iron Chromate 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

Ferric Chloride/iron Chloride

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

iron sulfate(Copperas)

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 phosphate FePO4 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)

rutile (light, dark, and granular)

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

illmenite (powdered and granular)

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

iron Clays 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 reddish-brown colors

magnetic iron oxide

Fe3O4

MagnetiteIron scale or iron spangles—coarse, hard particles that resist melting and chemical breakdown

Gives speckles in clay bodies and glazes

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.

BackgroundThe earliest English thrown agate is found among the prod-ucts 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 numer-ous 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 3

4

in preparing the clay for throwing, slabs of varied colored earthenware clays are cut and stacked.

The slabs are then stacked in alternating colors. after the initial stacking, the clay is sliced in half and restacked.

The different colored clays are carefully wedged or kneaded together to prepare a ball for throwing.

This ball of clay has been cut in half to show an example of the agate pattern prior to throwing.

5 6Center the prepared clay ball on the wheel. The agate pattern will not be visible on the surface.

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.

Creating a successful variegated appearance also depends on the proportions of clay colors used. Clues for understand-ing 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 dis-similar 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

received as a novelty or as an important scientific discovery.Commercial production of English agateware began in ear-

nest in the second quarter of the eighteenth century. In 1729, Samuel Bell, at the Lower Street Potworks, Newcastle-under-Lyme, was granted a patent to produce “red marbled stone-ware 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 varie-gated surface. These wares typically had a red and off-white clay body even though a black colored clay was used.



7The cylinder is shaped into the final mug form. Note the muddy surface concealing the agate patterning.

a cross section of a fully thrown cylinder shows distinct lines and swirls of the colored clays.

The slip covering the exterior of the thrown mug is scraped away using a metal rib, revealing the agate pattern.

Final finishing of the base and additional design elements, textures or a foot can be done using a rib.

11again using a metal rib, scrape the interior of the mug to remove the slip from the surface and reveal the pattern.

12

8 9

10The form can be supported from the inside and turned on a lathe to refine the agate pattern further.

Archaeological evidence from the excavation of Stafford-shire 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 Rock-ingham 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

general type was produced by North State Pottery of North Carolina in the 1920s and 1930s.

Preparing the ClayThe 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, in-cluding 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 pattern-ing 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 addi-tion to her considerable contemporary work. She produces reproductions for organizations such as Colonial Williams-burg, the National Park Service, Parks Canada, the Mu-seum 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 reproduc-tion 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.


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2011 Clay Workshop Handbook

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Transcript of 2011 Clay Workshop Handbook