Category Archives: English

Controlling Glaze Application Thickness on Porous Bisqueware.

Controlling Glaze Application Thickness on Porous Bisqueware.

Factors controlling the thickness of a glazecoat on bisque.

  1. Length of time in the glaze
  2. Density of the glaze suspension. That is how much water is there and how much suspended powder.
  3. (Apparent) porosity of the bisque, including how dry it is, how much pore space it has, how quick the pore space absorbs water, and how thick the bisque is.
  4. Rheologic properites.
    • a. flocculation
    • b. surface tension and viscosity
    • c. number of long molecules (might be covered in viscosity)
    • d. The amount of fine particles that can clog surface pores.

Length of time in the glaze

When you dip a piece in your glaze suspension the bisque ware starts to absorb water first making the glaze near the surface a more dense liquid and then turning it solid. So long as the bisque is absorbing water fast enough the glaze coat continues to thicken. As the absorption slows down there reaches a point where the coat of stiff glaze starts to get wetter again and slough off. The thicker the work is, the thicker the glaze can get and the faster it gets thick. In beginning thrown work the base of the pot is often thicker than the top making the glaze thicker near the bottom, just where running has the biggest likelihood of causing an issue.
Dipping the work in water before glazing decreases the availability of pore space for absorbing glaze. Right after you dip it the effect is greater. Because water without glaze absorbs quickly these have to be very fast dips. With work that is thicker near the bottom you can dip the bottom few inches in water before you glaze and if needed pour a little water on the inside bottom and pour it out. I do this with really runny ash glazes so that they will not run too thick on the inside.

How long a pot is in the glaze is perhaps the primary method of control of glaze coat thickness. If you imagine pushing a cylinder in for 5 seconds and then removing it for five seconds, the first part of the pot to enter the glaze will be in the glaze for ten sends and the last for less than a second. If you want an even coat of glaze, you will not have it. I use the words plunge, wait, pull. Don’t go so fast that you create a tidal wave or splash but do not take your time putting the pot in, or taking it out. After you pull it out you usually want to keep it in the same orientation so that you do not get drips down the side of the pot.
If you are doing two different glazes, the amount of time you wait between glazes controls the thickness of the overlap. The longer you wait, the drier the first glaze becomes and the more porousity it has avaialble to dry the second coat of glaze. Being ready with the secoond glaze saves loads of problems. As soon as the high sheen is gone it is usually safe to dip in the second glaze.


More solids in the glaze means that the pot has to absorb less water to make a stiff coat. This speeds up how quickly a coat accumulates. Adding water can work to a point but it also increases the shrinkage of the coat as it dries. With too much water sharp edges of the clay become saturated and get little or no glaze. There are many ways to test the thickness of a glaze coat and to control it. The first measure of control is the density. How much does a given volume weigh? Adjusting that by adding water (it decreases the density of the glaze) is the first thing to do after checking if it is too dense.
Glazes should be stirred immediately before glazing. Some glaze mixtures are particulary sensitive to this. Further, since materials settle out at different rates an unstirred glaze is a different glaze at the top than the bottom. There is a particular watery look to the last part of a pot dipped into an unstirred glaze.


The rheology of the glaze is the next issue to deal with. As you speed the absorption of the water needed to stiffen the coat and as you reduce the water needed to be absorbed you cut down on the space between the particles of glaze. At least this is the theory of Matt Katz, and it makes sense to me. This decreases the amount of air that will be trapped in the melted glaze coat and cut down on pinholes. Adding a deflocculant helps with this as it reduces the amount of water needed to make glaze fluid. Shorter dipping time also helps. Matt also favors low bisques because it increases the force and speed of water absorption decreasing the pore space in the glaze coat.
On the other hand flocculants seem to cut down the amount of thickness variation created by drips flowing off handles or bottoms of pots when they are pulled from the glaze slurry. Since you cannot deflocculate and flocculate at the same time, you have to do what is needed more depending on the glaze.

Fine Particles

Fine particles, especially bentonite, also help to keep drips from setting in thick streams. The fine clays clog the surface pores as the pot is held in the glaze. So once the glaze reaches a certain thickness the rate at which it absorbs water slows down decreasing the impact of drips as you are applying glaze. It is a good reason to add bentonite to most any glaze. Veegum does this too. Glazes with lots of ball clay do not need the addition.
Other additives such as gums, glycols, can slow absorption even further. Some of these materials affect the rheology in multiple ways. They can be deflocculants, or flocculants, they can affect the surface tension or viscosity so test them. Make sure that your kiln is vented regardless and avoid things that you should not have your hands in or are hazardous to burn.

Ways to check glaze thickness

  • Scratch through the applied glaze with a pin tool and look at the thickness of the coat.
  • Look at the glaze coat and see how it covers details,rounds off rims,  and look the thickness at the edge of the coat. This is harder than it seems and takes practice.
  • Make a thickness gauge out of a dial indicator. I am hesitant to give directions as I have not used one.
  • Make a thickness gauge out of a piece of metal with a series of teeth that will scratch into the glaze coat. I believe that I read about this in Cardew’s “Pioneer Pottery” but it could be Leach’s A Potter’s Book”


In order to do this you need some vocabulary, a mental scale of thicknesses. Although if you are using a dial indicator a numeric scale might make sense.

  • Light Wash. A thickness where you see more clay than glaze. The wash is only thick in recesses if anywhere at all. Likely it does not show at all on sharp edges.
  • Heavy Wash. The coat mostly covers the clay but you can see some clay showing through on flat areas of bisque. Usually it is thin on sharp edges.
  • Just Opaque. A little heavier than heavy wash, you cannot see the clay on flat areas at all although edges may show.
  • Photo Paper Thickness
  • Half the thickness of a dime
  • The thickness of a dime
  • Penny
  • Nickel (US or Canadian coin)

Drying of Clay, thoughts, experience, ideas, dynamics, principles.

Understanding the problems of drying thick work.
It would be easy to assume that drying work that is twice as thick takes twice the time. There are many confounding variables in this, and the simple picture is just not true.

It takes only a little heat to heat water up. It takes 1 calorie of heat to heat 1 gram of water 1 degree celcius. Just to get some comparison, scale in this, a kilowatt hour is 860 thousand calories. Just to avoid confusion, a nutritional calorie is 1000 regular calories.

But to evaporate water, to turn it to steam takes 540 calories for each gram. It takes time, or a big heat differential to transfer all of that heat to the water. As the water evaporates it absorbs heat from its surroundings, cooling them. This is why we sweat to cool ourselves. Evaporation of water absorbs heat.

Clay, especially dry clay is a reasonably good insulator. If you think of that 2 inch thick dinosaur as a bit of water surrounded by insulation, an inch of clay on each side, it is going to take some time for enough heat to penetrate the clay to evaporate the water. Remember, just heating it to boiling is not enough to evaporate it, you have to also get 540 more calories per gram to the water.

Below the boiling point of water at normal air pressure you can only evaporate water until the air surrounding it is saturated, until the relative humidity surrounding the water is 100%. So if you heat clay to say 90˚C or 194˚F and the clay is thick, water inside the clay will only evaporate until the air in the pores is saturated with water vapor. It may not all evaporate until there is time for the water vapor to move through the pores and be exchanged with air from outside the clay.

Explosions happen because the pressure inside the clay exceeds the strength of the clay to contain it. This part of the dynamic creates some compounding factors. As the pressure increases, so does the boiling point of water. This property likely contributes to the wide range of temperatures that we see explosions taking place at. Insulating properties of clay also contribute. the outside of a pot may be above normal boiling, but the inside might be colder from insulation and be at a higher pressure.

Fortunately, not everything makes getting clay dry more difficult. There are a few factors that speed things up. The first is that water wicks through the clay and presents itself, at least in part, at the surface of the clay where heat exchange and drying is easy. In order to understand this well you need to understand three terms, capilarity, surface tension, and viscosity.

Viscosity is the rate at which a liquid will flow. Honey and molasses are much more viscous than water. Acetone has a viscosity that is less than water, but most common liquids have viscosities that are higher. Viscosity of water decreases substantively as temperature increases. This increases its ability to move through clay towards the surface as temperature increases.

Surface tension is a nice term. It describes the tension on the surface of a liquid. When water beads up on a waxed surface the beading is because of surface tension. Without surface tension it would spread out. Surface tension is what holds bubbles intact. In mold making and in bubbly glazes a light spritz of alcohol can cause bubbles to burst. This is because even small amounts of alcohol radically lower the surface tension of the water allowing it to spread out and the bubbles to burst. Surface tension of water also decreases quickly with the rise in temperature. This allows the water to spread across surfaces, say clay particles and present more surface area for drying.

Capilarity, the property of water to up thin tubes or pores decreases slightly with increases in temperatures. The decrease is small enough that in most engineering problems the decrease can be ignored. Due to the increase in speed that this happens due to the decrease in viscosity, in our case it is more ignorable.

The loss of viscosity and surface tension presents us with an opportunity. Clay held at a high temperature maintains a more even wetness because water more easily transfers itself from wet to dry areas. Clay, in general, can be dried more quickly with few problems at high temperatures than at low. The phrase “high heat high humidty drying is used in an old text on brickmaking in the Archie Bray Foundation library and is the place I first encountered the concept. A few years later I needed to dry a thick carved mural quickly and dried most of it at 180˚F in a kiln with the lid propped over night, and some on a table with a fan. The ones on the table all cracked, those in the kiln all did not crack. I was convinced.

In this there are other confounding factors. Almost all electric kilns with the doors open tend to have colder floors. Even with zone control, unless there are floor elements this is likely to be the case. This is because cold air is denser than hot air so it settles pushing the lighter hot air out of the way. The more a kiln leaks, the more trouble there is with cold floors. Drying with the door open is an extreme case of a “leak”.

How wet work is changes the amount of time needed to dry below boiling temps significantly. It conspires with thickness to make thick objects often seem impossible to fire successfully. We have all heard the untruth, “You cannot fire thick work”. Having successfully fired kiln pugs as counterweights, I know this to be an untruth.

While I am still a believer that convection leaves bottoms of kilns colder than tops much of the problem with cold kiln bottoms seems to be the shelf near an uninsulated floor adding to the thermal mass . Work loaded on the shelf with the bottom down adds even more to this. It is not a duplicate of the area near the lid of the kiln. Dispersal of heat at low temperature has to be from convection because radiation is not very effective at the low temperatures.   Since none of these factors are very effective with low temperatures or small differences in temperature the added density at the bottom keeps things wet longer. Keeping thick work off the bottom and when possible placing it rim down vastly improves the situation by getting more of the clay higher in the kiln.

Most dispersal of heat at low temperatures in kilns is from convection caused by the differences in density caused by air temperature. The colder air heats at the elements near the bottom. This often leaves a cone of colder area near the bottom of electric kilns. So when you are preheating at 180˚F the bottom of the kiln, especially towards the center can be several tens of degrees colder. The colder it is, the less heat is transferred to the water and the slower it evaporates. Most often it seems that explosions happen in the bottoms of kilns that are fired with some, but not enough care.

Optimal conditions are unachievable. We have to fire in real situations. But if you had a piece of clay that was slightly wet, you could heat it above boiling for a short time. The water near the surface would evaporate quickly, but being near the surface would not create any pressure within the clay. The evaporation would prevent the water further inside the clay from heating as it would be absorbing so much heat to evaporate. After that surface water evaporated you would need to lower the temperature. The question is what temperature to lower it to? Optimally this might be above boiling. We only need to stay beneath the pressure that the clay can withstand. Under perfect circumstances we could even do this with leather hard clay. I believe that under normal circumstances we almost never achieve perfect drying and some water is always expelled from the walls of our clay under pressure.

Kiln pyrometers, even type S are imperfect. Even a few degrees around boiling could likely create problems with explosions. Because of this I usually used large margins. I started at 180˚F (82˚C) moved to 190˚F and as I got surer to 200˚F (93˚C). As I got close to retirement I started to use a slow rise time through boiling and shorten the hold. I believe that fine tuning this would result in quicker firings. Because there are differences in our many clay bodies and firings are mixed, “optimal” will vary even beyond considering thickness.

Sometime when I first started teaching at Texas A&M University Corpus Christi, The Island University, The only university in the US on its own island, surrounded by salt water, I decided that I needed a goal for speed of bisque kilns. How many pieces was it acceptable to explode in a semester? If you fire too slow you waste student time, and some electricity. If you fire too fast you either have not allow thick work or you blow stuff up. I decided that blowing up two pieces a semester was enough. Five was way too many. I also decided that this was true regardless of thickness. I started holding back thick work for special firings.

I dried kilns at 195˚F roughly 90˚C. How long the kiln was held depended on the wetness of the work, and how thick it was. I avoided loading thick work near the floor of the kiln. As things got busier and there were more classes, kilns were loaded less reliably. Work on the bottom started to explode more. I added time, a slow rise and then a short hold at 20˚F above boiling to try and get the bottom of the kiln to not explode. This was effective.

I started to think about the slow rise and the ability of clay to contain some pressure. I think that the optimal technique for getting work dry might be a short hold below boiling to get the work hot throughout and then a slow rise past boiling keeping the rise slow enough that the water remaining can boil without creating too much pressure. I think that this would be worthy of study. Knowlege of optimization of brick drying could likely inform what we do and save us time, money, and carbon.

intendedness vs effortlessness, consideration

It is clear to me that as a media we are making progress at least in some ways. Sure there is more technical know how and horsepower than there was 40 years ago. There is more knowledge and more people doing glaze calculation and substantive kiln innovation. But even the pots themselves are getting better. This can be demonstrated pretty well by looking at handle attachments. I have joked about looking at upper handle attachments pictured in CM from the first issue forward using the criteria of intendedness(1). But I am not so interested in quantitative research in the field. I just want to generate thought. Still it seems that progress here can be demonstrated.

In this regard I have been looking at Simon Levin’s handles on cups. Really the whole cups are wonderous but it is the handles I am most in to. The mimicry of the smooth upper attachment is so well done at the bottom that the effort that goes in is not apparent. There are no signs of any effort. The bottom attachment looks as is it was accomplished the same way the top attachment was, no muss, no fuss. But it wasn’t. Simon has apparently developed the skill  and technique to make the bottom attachments look the same and a lot of effort went into this.

The lack of unresolved details in the bottom attachment meet my definition of intendedness. Every part of the attachment looks like it was considered. The details look like they were all intended. The clean lack of struggle, the lack of unintended marks, makes these lower attachments look effortless and I wonder if the terms “apparent consideration”, “intendedness” and “apparent effortlessness” are not in some ways relating to pots, synonymous.

(1) Intendedness: This is the appearance of intent, rather than intent itself. Something can look like it has intent but if it is actually accidental, or a controlled accident it still has intendedness. The Bauhaus  designers used to say that every aspect of a design needs to be considered. This is an important principle, but in my opinion poorly stated. I say that every aspect of a design needs to look considered or intended. They do not need to be intended. How well something conforms to this ideal, this look of intention or consideration is its intendedness.

Once something looks intentional it is possible or easy and almost automatic to either think you know why something was chosen the way it was or to wonder why. Either of these is a gateway to meaning.


Din Phao, Din Phao: ดินเผาดินเผา

Dankwean Dinpaw , the sales area on the road is hurting. Based on quick appearances……..There has been an incredible building of sales malls. People are still building them despite lots of vacancies. Perhaps, hopefully, people believe that sales will improve. The old large potteries are falling into disrepair. Umdang is closed essentially and the business that they do is via phone sales and visits to sites where they design installations, murals and bas reliefs. The have almost no road sales except perhaps tiles.
Chao Din (people of the earth) seems to still be producing murals, but the bustling stream of visitors and buyers seems to have ended. Its once immaculate display area is getting funky. The fish pond is full of algae and the air and water pumps not working. The koi are oblivious, but as a visitor it is not nearly as nice looking.
The old professors are dead. Ajahn Pit who was always welcoming and nice to me died a few years ago and his daughter and son took over for a few years. I am told that they are now in the US. Like Chao Din the once brilliant and organized display of Ajahn Pit’s Din Phao has seen better days. Eddie McGrath wrote that there is a tendency in restaurants to rather than keep up on maintenance to just “let it go” and sell out to someone who wants to sell cheaper food and build a newer restaurant. The tendency may have some similarities here, but it is not working, Only a few nik-nack shops across the street seem to be doing well with street-side sales. I have seen several places packing work up for sale elsewhere.

Ajahn Wirot from Din-Dam has been dead for some time and his once chaotic display and museum is hard to see among the weeds and behind the distracting buildings. It would surprise me to find out that any sales at all were going on there.

Across the street from Umdang there is a place where tour busses stop because the displays are clean. They sell espresso there for 45 baht and about 100 yards away it is only 25. There apparently are people buying little nicknacks still.

The hand skills on the “traditional” carved surface Dankwean pots have continued to improve. There is truly some incredible carving going on. I hope that the scrafitto workshop really takes hold as this would help create an opportunity for these skills to translated to fired surfaces. That said the painted surfaces look better and better every year I visit.

Up Wind

2014 Prep

I am preparing to go to Thailand. I have lists and even a list of lists. I have packing lists, lists of paperwork to duplicate, lists of people to contact, letters to write, and files to transfer to the laptop. Oy.

But there are other preparations I have to make. They may be more important. I must slow down, remember the Thai manners, the cool heart – jai yen, slow, controlled not too excited. I have to remember to slow and greet people properly, the smile and the ability to let things roll off my back with a smile on my face. It is not just smiling that I need to do, but the smile state of mind.

The idea that desire is the root of suffering, that grasping creates disappointment, is at the heart of this change. It is a part of Buddhist philosophy but, it is so widely accepted and implemented in Thailand , that you have to succumb or find yourself swimming upstream. I no longer can swim upstream for months at a time. I have to smile, go with the flow, allow the troubles, the hurry, frowns, worries, to flow away, to touch perhaps but never stick. I have to learn to “mai pen rai” . To activate the phrase “its not a problem or worry” you have to make it a verb.

I have a huge agenda. It is work. It is too much. It would be good if I could get it all done. It is almost certainly undoable.  An agenda like this can add an off flavor to everything. It can prevent months of work from being productive, too much stress on doneness not enough flex to contemplate, think, digest,,,. I have to start by doing “mai pen rai” by turning off the worries and allowing the future to come. You can only swim upstream so long.

I  have to even stop my little social concerns. Did I fail to slow down and say Sawasdii, did I remember to call them “elder”, was I polite enough. I have to do this because really the first politeness in Thailand is to mai pen rai. It is a necessity like air. When you do this, the little stuff comes easy, and the hard stuff is easier.


Thrift Store Pots

I arrived in Helena with Gail and the boys. If I remember correctly Benny was an infant. I was supposed to be on a quick run to the Rock Hand Hardware Store but guiltily I stopped at a thrift store on the way. I did a quick run through the hardware area. I never buy clothes, well, hardly ever. I walked down one of the isles with pots and turned them over to see if any were made with clay bodies (compositions) from before the 70’s. One ugly little cup with a funky dead form, coil handle, poorly turned footring and bubbled glaze, that was rubbed down with a brick to break the bubbles, was old stoneware. It did not have the typical APGreen brand fireclay look. It was ugly so I set it down.

By the time I got to the end of the isle I was thinking again of the ugly pot. “Whose signature was that?” I went back and turned it over again. Clearly it was signed, “Voulkos” (right).
Peter Voulkos is one of the best known clayers of the 20th century. He made delightful functional pots until he began making abstract sculpture. He began studying pottery at Montana State University in Bozeman under Francis Senska and was a resident artist at the Archie Bray Brickyard. After Berard Leach, Shoji Hamada and Soetsu Yanagi lectured and demonstrated at the Bray (not sure of this it could have been before) the resident artists at the Bray were asked to make “Bray Standard Ware” (I need a source for this). One of the items was a small cup with a little coil handle just like this one. Voulkos, I think, resented having to make these, but made them. In defiance, (again conjecture) he signed his cups.

(Yanagi, Leach, Rudy Autio, Voulkos, Hamada, at the Bray Pottery)

Cup in hand, poker-faced, I paid my 25 cents and left with my cup.

A few months later at the same store I bought a cup by Rosie Wynkoop (left) who had been one of my students in the community classes at the Bray.  It cost a dollar. I think she made it while she was one of my students.

In graduate school one of the off syllabus things we learned was that garage sale and thrift store shopping was a competitive sport. The price tags were left on the pots. One friend was so well known at one thrift store that she received phone calls on the store phone.

On occassion, I invite my students over to my house to view pots. One time while talking about these two drinking vessels a student asked, “Wouldn’t Rosie be upset to find out that her work only cost a dollar?” I answered, “No! She is getting four times the price of Voulkos!”

Burnt Coffee

So, my second daily pot of coffee… I was brewing with the radio on.
Distracted. Boiled over.
Burnt coffee smell of my father melting the percolator.. but without burnt bakelite handle.
I opened the front door… 10 years old. Dad at work. It stunk but was not smoke so I went in.
Aluminum slag. Time for a new coffee pot.
Turned the burner off.
Smart kid.
Nother day.
Came home from school.
Opened front door. Smelled smoke.
Went next door to Sheri’s house.
“Mrs Simons, could I use your phone? ”
Dialed zero. “Operator give me the fire department.”
They came, no smoke.
“So, you decided to see the fire trucks little boy?”
Mother, Fern Katz , pulls up next to the fire truck.
“My son said there was smoke, so there was. Go find it.”
Mom gets extra credit and gold star.
Lint in the dryer caught fire and went out.
No one asked me how much smoke.
The coffee is good. Time to clean the stove top.

Appon’s Thai Food Site


Appon might be an old Thai pronunciation of Apple in Thai. The letter that corresponds with ‘L’ ล (law ling) is only pronounced like an English ‘L’ as the initial consonant in a syllable. At the end it is pronounced like an English ‘n’ in Thai. Consequently ‘hotel” becomes ‘hoten”, and ‘Apple’-‘Appon’.

Regardless, the recipes on her site look to me like the real deal. They are not what you find in most “Thai Restaurants” in the United States. Chicken Feet in red sauce, and Haw Mawk Prik Kai  and Kanom Jiin Nam Ya Tin Kai are on my list to try.


List of Thai Clayers

(needs update)
This is a list of clayers, people involved with clay, in one way or another. If you know others that I should meet on my trip, please let me know via email.

  • Suwanee Natewong
  • Sudarat Kerdmongkol
  • Jum Kerdmongkol
  • Poonarat Pichayapaiboon
  • Bathma Kaew-ngok (Bat) บัทม์
  • Takood Nui
  • Suebpong Powthai มหาวิทยาลัยศิลปากร
  • Surojana Sethabutraสุโรจนา เศรษฐบุตร
  • Atiporn Thongborisut
  • Keofar Kesornsook
  • Lakkana Wongsawat
  • Krisaya Luenganantakul
  • Nino Sarabutra
  • Wasinburee Supanichvoraparch วศินบุรี สุพานิชวรภาชน์
  • Aroon Wattana
  • Vipoo Srivilasa (AU)
  • Pim Sudhikam พิม สุทธิคำ
  • Kritchnun Srirakit กฤชนันท์ ศรีระกิจ
  • Somluk Pantiboon
  • Ittikorn Pornmingmas
  • Torsak Prakhamthong, Vang-Kwang Ceramic Craft Factory
  • Nuttapong Prompongsaton
  • Jariya Kiranantawat
  • Amornthep Mahamart อมรเทพ
  • Chetta Subbumrer
  • Smith Takroodkaew
  • Sayumporn Kasornsuwan
  • Thanin Ton
  • Padungkeat Rattanasri
  • นพอนันต์ บาลิสี (Nop Caermic)
  • Pisarn Boonphoog (sp)
  • Aor Sutthiprapha (SW)
  • I think this is a manufacturer with four potters, Baan Mayo Patani
  • Kritchnun Srirakit Chiang Mai
  • Potter Duke Bangkok’

Hello world!

I have been granted a faculty development leave next year January – May to visit Thai studio ceramists and traditional potteries. I expect to be gone for 4 months. I am not sure of my exact departure date. I have a long list of things I need to do before I leave, some of which I have already started.
One is working on abstract vocabulary, the other is trying to fill out my list of people and places to visit. I expect my time there will be very busy.