New Technical Challenges

January 3rd, all too soon, I leave to teach a 2-week class at Curaumilla Art Center in Chile with high-fire potter Suze Lindsay. I’ve known Suze since she came to Louisiana State University as a grad student, and appreciate her sense of decoration and boundless good humor. We only overlapped for one year before I left the L.S.U. faculty to move to Florida and marry Leland Shaw (then an Architecture Professor at U.F. and a part-time potter). Ever since that time, we’ve felt we didn’t get enough time working together, and have wanted to work in studio together. The Curaumilla program is a great opportunity to come together with other potters who want to talk about form and surface in a beautiful place for 2 weeks.

At Curaumilla, the trail has been blazed by wood-fire potters like Doug Casebeer and Randy Johnston, and Chris Gustin and Ron Meyers have also taught there. Suze and I wanted to expand the glaze palette to include more hue and value contrasts and more variety in color. The Curaumilla people dry-glaze and once-fire with wood. Hmmm. This should be a challenge. I think I’ll have to pot more thickly for my pots to survive dry-glazing.

The glazes we’ve been looking at are common cone 10 reduction glazes, like Emily Purple, shinos, saturated irons like Ohata Kaki. The research in Frank and Janet Hamer’s book The Potter’s Dictionary of Materials and Techniques, and Robert Fournier’s Illustrated Dictionary of Practical Pottery say for once-fire dry glazing, clay content in glazes should be 10-20% (for leatherhard about 50%). Our glazes often has less clay, occasionally more. It was fun for me to be able to use glaze calculation to change the clay content in the raw glaze but keep the glaze oxides in the fired glaze the same. I’m just enough of a materials and glaze calc nerd to get myself in trouble, but I enjoy the informaton and the challenge.

Richard Burkett, (potter, sculptor, San Diego State U faculty, materials guru, computer nerd, and musician) wrote HyperGlaze, originally a Mac-only program. It now runs on either PC or Mac platforms. It’s a lovely program, with a good-looking user interface and helpful support programs.

Although I’ve used HG, I find I use a PC-only program, GlazChem, by Bob Wilt, most often. Perhaps it’s from familiarity, but I find the layout (while not beautiful) very functional. I have some glazes that can be imported into GlazChem posted on my website handouts page and a link to the patch that will let you use the help files in Windows 7. Note, when you install GlazChem, you have to right-click and do RUN AS ADMININSTRATOR, and ditto the first time you run it. GlazChem will let you try the program before you buy, and the cost is a nominal $35.00.

With the help of GlazChem, I re-calculated some of my favorite U.F. cone 10 shop glazes to have about 10% clay. They don’t have frit in Curaumilla, and I read in my research materials that wood ash, although a raw material, is frit-like. Good solution. Cheap. They use it in China often. Ash will deflocculate the glaze and it should be flocculated with Epsom Salts to fix this before glazing. Ceramic Arts Daily video about doing this here.

Wishing everyone happy holidays. It’s a positive and optimistic challenge to create things. Below, some high-fire decorated works I’ve done. On the top 2, glazing method ws something I learned from Bill Brouillard: dip in Emily Purple, wax a design. Let the wax dry a bit, then wash off under the faucet all that is not waxed. Dry the piece overnight. Dip in a second glaze. Use a sponge to remove dots of glaze from wax. The Emily Purple is a magnesium-fluxed matt that will turn cobalt to purple. Where it overlaps with the celadon, the magnesium is diluted enough to make it shiny and back to blue. I love the blue line margin that happens.

Arbuckle stoneware bowl: Emily Purple next to Choy Celadon

Arbuckle stoneware bowl: Emily Purple next to Choy Celadon


Arbuckle stoneware bowl: while, Emily Purple, Pete’s copper red


Teapot glazed in shino with in-glaze decoration of colorants + flux done like majolica.

More Majolica Decorating Color chat

David Gamble, lowfire clay artist, Skutt kiln expert, did an article a while back. I don’t remember where – someone out there remember? Tell us! – about the various products for decorating on majolica glaze. I asked David to recap his experiences for us. He said:

When I did all my tests I found GDC’s seemed the best – Spectrum was 2nd in my use & assessment and they had lots of color – I heard Color Robbia had some but folks I know said they did not like them & I never tested them – Mayco says that their Stroke & Coats work & yes they melt on top of whites – but I found that they were not as stiff & moved/bleed at the edges – so exact lines were out of the question with them.

 All your tests with velvets on white glazes are still good with me — though not sure about food surfaces – they are so soft they would have to melt completely into the surface to be dinnerware safe. Of course you already know all of this.

Before there were GDCs, I tested the AMACO Velvet Underglazes on top of majolica. The results of this are posted on my website Handouts page. Some melted very well and were usable for majolica, some were very refractory and pig-skinned (so matt they left a crinkled, stony surface on top of majolica.)

David was in touch with AMACO, and checked on the availability of GDC colors at this point. He reported:
They are out of pink in 2oz but have pints of pink left — red & turquoise are low in pints around 10 each .  Seems to be lots of 2 oz jars left in all except pink colors.

So folks should get them know while they still have a great selection at AMACO’s retail store – Brickyard Pottery- Cheri is the manager you can call 317 244 5230  or Amaco’s 800 – 374 1600  and ask for Cheri at BYP .

In my studio world, pink is safe, and so is brown. I probably have enough brown for the rest of my studio lifetime. I’d probably run out of the bright yellow (mixed with turquoise 3-4:1 it’s a great Chartreuse), Light Red, Pearl Grey, and Avocado first.  At one point, I did make it a project to use brown, just to see if I could. I do like certain browns – nice ochre-tan-saddle colors, and reddish, rusty browns. Below, a bowl from 2003 during the self-imposed brown challenge. Summer flowers on the front, winter leaves on the back. Perhaps the theme presaged my later immersion in George R.R. Martin’s Game of Thrones series and the house of Stark motto: winter is coming.

Arbuckle majolica bowl, 2003, Now and Then

AMACO GDC Majolica Colors Sail into the Sunset

AMACO has announced that it’s GDC line of color for majolica decoration will be discontinued in December. It may be available through specific suppliers through the following year, but production will cease. These have been my go-to colors, supplemented here and there, for a number of years, so I’m sad to see them go. It was not a profitable line for AMACO – we use these in small amounts and there were 36 colors. There was at one point discussion about making fewer, more inter-mixable colors, but the final decision was that they were focusing elsewhere in the product line. My thanks to AMACO and its good people for the conversation and support over the years I’ve used the GDCs. I’ve appreciated the product line and my relationship with the company’s people.

Before the GDCs, I mixed my own colors. This began with Gerstley borate as a flux, mixed with oxides or stains. The GB kept things in suspension, made brushing easy, and gave a firm surface when dry to wax over. BUT, the GB gave a very tiny reticulation of miniscule white snowflakes in melting that pastelled the colors. Even before the announcements of the GB mine closing, I was aware that people using frit for a flux had brighter, denser color on majolica. Switching to frit + color or stain revealed that this mix alone was very powdery when dry, and made a real mess if handled, or waxed over – smearing, smudging. Pete Pinnell, the Mister Rogers of clay and glaze materials, advised adding a bit of bentonite, and that did take care of the problems. The colors brushed better and were firm enough to wax over when dry. I’ve been experimenting with adding glycerin or CMC gum for added brushability, but too much of that makes the mix gloppy and interferes with crisp line quality. Still looking for the sweet spot. the mixes I was using:

  • Oxides that melt well at low fire (copper, cobalt, manganese, iron) are mixed by volume (say, teaspoons) 1 colorant + 1 frit (I used 3124 because it’s in the glaze I use) = 1/2 bentonite.
  • More refractory (resistant to melting) oxides (chrome, rutile, nickel) or stains are mixed by volume 1 part colorant + 3-4 parts frit + 1/2 bentonite.

Bentonite “bloats” in water – like trying to mix cinnamon or cocoa powder with liquids – so mix dry with other materials and then add water. Screen if lumpy. There are some nice, small test sieves that fit in a pint container available at the ceramic suppliers.

I have some tests in mind with varying the bentonite and CMC, and trying Veegum-Cer, which the commercial people use in their colors. The last iteration of the GDCs seemed over-gummed and a bit gloppy for good line quality to me, so it’s a needed push out of the nest to do my own testing.

There are other commercial products. I was just given a sample of Spectrum Majolica colors. I fire to 03, and at 03, some of the colors are broken and seem over-fluxed. See image below. The top 3 rows are the Spectrum colors, the last block to the right in row 3 is a test of chrome chloride and water (a soluble colorant toxic raw), and the bottom row is mixes of the Spectrum colors I was curious about, as I had open squares on my test tile.

Mayco has product for this as well. More later on this. My testing life is s-l-o-w as we are in the busy last part of the semester at school, and I’m also being treated with i.v. antibiotics for Lyme disease. I hope people who have experience with other products will comment.

Test of Spectrum Majolica colors on Arbuckle majolica glaze

Test of Spectrum Majolica colors on Arbuckle majolica glaze. Fired to junior cone 04 at 3 o’clock in a visual cone.

Fired Results of Testing for Treasure

The iron and manganese glaze show featuring Clean Creek products has been bumped back to February to allow some of the participants more time. I’ve done my first round, and photographed some of the results. I still have plans to work on some pieces with the dark cordovan colored-glaze, and I have a lurid-looking cup that isn’t photographed yet, but here are some of the results in fired works. The oval servers are about 15.5″ L and 11 W, 2.5 high. Bring on the pasta. The rectangular trays are the size of a butter dish in length. Click on the thumbnails for bigger images and descriptions. See my June post for glaze info. My best shots at making brown look interesting.


Working with drop molds with Chris Pickett

The web site is the 21st-century’s business card. UF Ceramics MFA alumnus Chris Pickett has a snappy new web site designed by UF Graphic Design student Marisa Falcigno. Marisa worked as a graphic designer before deciding to return to school for MFA study. She’s been a great consultant and designer for many people affiliated with Ceramics, and assisted Masters Art Education alum Jeni Hansen with her K-12 ceramics resource site for teachers, Oh Happy Clay!

Chris’s new site includes a fun process page that shows how Chris handbuilds his work using drop molds made from foam insulation. It’s a variation on forming slabs that uses shallow cut-outs to slump soft slabs into curved forms. I’ve seen people use plywood forms, but the stiff foam insulation sheets are easy to cut and light-weight. It’s a flexible method of adding volume in shapes to slab-building. Ceramics Monthly had a nice article about Chris’s methods in Dec. 2011. Chris is inspired by inflatable toys, mid-20th century modern furniture designers, and the seduction of touch.

Chris Pickett drop-molded tray, cone 6

Feldspar changes

Somone just asked me about a spar substitution. My experience is that for many applications, spar substitutions within the categories of soda spars or pot (potassium) spars have some leeway and will often endure substitutions fairly easily. Unless they don’t. The “pot” or “soda” spar refers to the dominant flux in the feldspar. I suggest testing, and I’d test the easiest fix first – substitute a spar you have, and see if it changes things in an unacceptable way. If it does, try the things suggested below. At home I’m still slogging through my big bucket of old majolica glaze. At workshops, I’ve had people use various substitutions, and other than some alternate soda spars deflocculating the raw glaze, it’s been fine. If you need to review floc/deflocc, see the  Ceramic Arts Daily video demo on this.

feldspar Idealized formula KNaO• Al2O3• 6SiO2

In the idealized (simplified) feldspar formula above, KNaO refers to any combination of sodium or potassium. K2O is potassium oxide – 2 potassium for every oxygen. Ditto Na2O. So KNaO maintains the 2 units of flux for every oxygen, but is shorthand for “any combination of” because they often operate interchangably and it’s easier to calculate if you overlook the distinction because if often doesn’t matter. An actual feldspar, say F4, will have traces of other oxides in it, and not be quite that simple. These small inclusions are often overlooked in calculation of glazes because they often are insignificant. Since sodium and potassium have different atomic weights, the weights of soda spar and pot spar differ in idealized form. If you plug in two K instead of KNaO, the weight is 556 units vs. 524 for sodium. So, you might get a weight difference if you jump pot vs. soda categories. It would be conservative to stay within K or Na family of the spar you’re substituting. Wikipedia on feldspar. Image from Wikipedia.

Info below thanks to NM Clay in 2010.  If you go to Brant’s blog post below, you can download the datasheets for the spars.

Soda Feldspar – Kona F4 There is a supply change in the Soda Feldspar Kona F4. It is not available from the mine any longer due to a fire and mine closure. We will have Minspar 200 available. They are both very close to the F4 in composition. A chemical analysis of the materials is below.

 Minspar 200 Feldspar  Kona F-4 Feldspar   Sodium Feldspar
CaO 1.50 CaO 1.70
~~~ MgO 0.05
K2O 4.10 K2O 4.80
Na2O 6.50 Na2O 6.90
Al2O3 18.50 Al2O3 19.60
SiO2 68.60 SiO2 66.80
Fe2O3 0.06 Fe2O3 0.04
Volatiles % LOI 0.30 Volatiles %    LOI 0.20

Minspar 200 was Formerly NC-4 Feldspar.

G200 update From Jeff Zamek Article from Ceramics Technical (November 1, 2009)

“G-200 felspar, mined by Imerys North America Ceramics, has been discontinued. While there are still stocks in potters’ studios and ceramics supply bins it will eventually join Albany Slip, Gerstley borate, Kingman felspar, Oxford felspar, and other materials that have gone out of production. G-200’s demise is based on economic considerations that potters do not control. Originally, G-200, as potters know it, was blended at the processing plant by using 70% Minspar 200, a sodium based felspar, and 30% HP G-200, a potassium based felspar. The blended felspars then became G-200 and were shipped throughout the United States.

The increasing costs of shipping Minspar 200 from Spruce Pine, NC and G-200 HP, mined in Siloam, GA, to the processing plant in Monticello, GA, a trip of over 200 miles, made this situation uneconomical. Potters are now faced with either using Custer feldspar in place of or finding a new potassium based felspar.”

Thank you Mr. Zamek I couldn’t have said it better or at all!

Laguna Clay will be blending G-200 so it will be available. G-200HP is the new product and according to Jon Pacini from Laguna the major difference is slightly more fluxing power, similar to (the also defunct) Kingman Feldspar of old.

Posted by Brant from New Mexico Clay Inc on his blog.



Blinded by Science

There are a couple of guys from Alfred who have a forward clay company, Matt and Matt and Dave's clay imageDave’s Clay. I haven’t met then face-to-face, but they’re fun, lively guys, at least on the internet. For those of you who wonder about the materials WHY in ceramics, they have a Science page on their clay web site. Included are their presentations from NCECA, a free Unity Molecular Formula calculator for Excel, and other goodies.

More testing

There’s a glaze that has been used by sculpture people for a fleshy surface. Lately Drew Avakian at UF has been looking into it for his constructed pottery. I like the waxy look, an tried it with iron. at 5% iron, it has dramatic thin-to-thick changes from rust to creamy, showing dark slip and breaking a bit on texture. Still testing the cordovan color to see if I can reproduce it on a piece. I think the tin could be lowered and still get results. Tin is expen$ive. Without the tin (and zirconium opacifier in its place), the color doesn’t quite look the same, I suspect. The waxy look and mottling is accentuated by the tin, and the iron color might be subtly different. But I don’t think it has to be 10%. I’m going to drop to 8%, and maybe later see if I can go to 5. The original glaze name is about using rutile to tint the glaze, and counts on there being some chrome impurities in the rutile to pink up the high-tin glaze. If doesn’t show on the test tiles, but on small pieces, thick glaze over the green copper + chrome slip does turn kind of mottled burgundy. It would take at least 5% tin if you care about chrome-tin pinking like that. I want the nice iron browns, waxy surface, and mottling/breaking.

Flash-oPink test tiles

Test tiles for the Flash-o-Pink glaze base, w/tin, and increments of Clear Creek yellow iron.

Testing for treasure

Clean Creek Products is a company that markets materials reclaimed from coal mining. They’ve had a booth at NCECA to showcase their iron and manganese. There is a show coming up that features these materials, and I’ve been working with their iron and manganese to try to get something pretty in a glaze. On top of most non-lead majolica glazes, iron is just so very brown. Not amber-golden, not rich rust, but just brown. I rarely use brown in my majolica color palette, so I thought I’d take this as a nudge to do some fun slip work with paper stencils and look for glazes to go over that that are rust or golden.

circles are unity amounts of oxides for Meyers Clear 04 compared to grey bars of normal limits

UF has some clear glazes for 04 use – Deb’s Clear and (Ron) Meyers Clear. Both have issues. The Meyers Clear is the main one. If it gets thick, you get evil small craters that won’t heal, and just get worse if re-fired. I think it’s too viscous. The alumina is high, in addition to having boron. I’ve tested some similar formulas with lower alumina. The  raw Meyers is also very high in frit. In addition to cost, high-frit glazes have to be applied unusually thinly for a good result – hard for beginners to judge.

The recipe for the Meyers Clear:
Frit 3124     89.8
EPK            10.2  plus 2 Bentonite.

When I look at the glaze under a 30x little microscope (Radio Shack has some that are cheap – $9.00 last I bought one), the glaze is filled with trapped bubbles. The up side is that it doesn’t run or even move, but it also in unforgiving about being too thick, and it’s very easy to get it that way.

My new version with less alumina and a bit more KNaO, and a test even less Al. Results were counterintuitive. The Less Alumina version of M4 has MORE bubbles that causes milkiness thick. Hmmm. Difference in clay color may be from different batches of test tiles. Using up old stuff. Slip colors very similar. Somehow, the bucket labeled WHITE SLIP is coming out pale yellow. Need to re-mix. Out of touch w/studio during the school year. Nice to get back to daily contact. M4 bleeds the black slip ever so slightly when thick. Maybe I can live w/that. I know just enough about glaze materials to know I don’t know enough. The M4 version looks better than the original Meyers Clear to me, but it will take testing on real thiings to see how it works.

Unity formula info from Glazchem:
Meyers Clear original
KNaO  0.302   Al2O3  0.392    SiO2  2.826
MgO 0.005       B2O3  0.541
CaO  0.693

Alumina:Silica ratio is  1.00:7.21 Neutral:Acid ratio is    1.00:3.03 Alk:Neut:Acid ratio is 1.00:0.93:2.83

Expansion: 74.8 x 10e-7 per degree C Oxides causing abnormal expansion:   B2O3

Clear M4
KNaO  0.336 Al2O3  0.326    SiO2  2.786
CaO 0.658     B2O3  0.545
MgO  0.006

Alumina:Silica ratio is  1.00:8.55 Neutral:Acid ratio is    1.00:3.21 Alk:Neut:Acid ratio is 1.00:0.87:2.80

Expansion: 77.5 x 10e-7 per degree C Oxides causing abnormal expansion:   B2O3

M4 Less Alumina
KNaO  0.337   Al2O3  0.247    SiO2  2.507
MgO 0.005    B2O3  0.544
CaO  0.658

Alumina:Silica ratio is  1.00:10.14 Neutral:Acid ratio is    1.00:3.18 Alk:Neut:Acid ratio is 1.00:0.79:2.51

Expansion: 80.5 x 10e-7 per degree C Oxides causing abnormal expansion:   B2O3

Glaze tests M4

Left M4 less alumina version, Right M4 glaze test

Limit formula chart for Clear M4

Limits M4 less alumina

Limit chart from GlazChem of M4 Less Alumina test