Empirical glazing!

Of all the individual components of ceramics, glazes and glazing seems to be the one that frustrates people the most. Glazes are fickle, subject to the vagaries of firing, completely unobservable while shut up in the kiln and generally infuriating. For all that though, the best glazes can attire a pot in a garment that enables it to transcend the simple beauties of shape and form.

While most aspects of pottery are skills that can be learnt, glazes themselves are merely recipes. Certainly glazing techniques, including things like decorative brushwork are things that can be built up over time, but glazes themselves are the result of the application of heat to various mixtures of rocks and minerals. There are no physical skills to be learnt, no muscle memory to be acquired. Rather, it is an intellectual discipline, in which the role of each ingredient in the mix must be determined.

Judging by the number of different books, purely on glazes and glazing that are available, it is one of the areas of ceramics that most infuriates people. This is in part because the glazes that are most spectacular are those right on the edge of usability – whether because of their fluidity, or the way in which colourants disperse through the matrix, or the way in which they crystallize during cooling – and thus slight alteration of ingredients one way or another can cause their failure. Not only that but when developing new glazes, it can be hard to find that sweet spot.

In “Chinese Glazes” Nigel Wood declares that the history of Chinese glazes is the exploration of iron oxide as a colourant. By changing only the concentration of iron oxide that was used, and one or two ratios of the components in the base glaze, potters in China were able to produce glazes that ranged from clear sky blue celadons, through deep black tenmokus. Glazes that have been evocatively named: Oil spots, Hare’s fur, Partridge feather. All these glazes were produced, not with a multitude of ingredients, but just by varying the ratios of silica, alumina, calcium, sodium, potassium and of course iron.

It seems to me that many of the books on glazing that are available either pick a few base glazes and thoroughly investigate the effect different colourants have, or they are books that contain multiple recipes, with very little investigation as to why these recipes work as they do, or why changing the base glaze so totally changes the glaze itself. While developing a glaze may seem like medieval alchemy, there is no reason that modern empirical methods cannot be applied to the investigation of glazes. This blog will chronicle my attempts to do precisely that. One of the key points about empirical investigation is that failures, or experiments that yield no obvious result, can be just as informative as those that give spectacular results. I will try and publish those too, along with my speculations.

Over the course of the next few years, I hope that I (perhaps we!), will discover precisely how different components change the properties of the glazes we make.


Change is good, right?


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Choosing a clay …

Before the 20 century, the choice of clay was simple – you used whatever was at hand, whatever clay was within cart’s journey of your pottery. For most potters this often meant using the same clay deposit that their father and grandfather and great-grandfather used. At the end of the eighteenth century, some of the larger potteries, such as those in Stoke-on-Trent or Meissen developed their own special clay bodies from a variety of different raw materials, but the craft potter still dug their own from old river beds and other similar deposits.

These days we are lucky to have available to us many many wonderful clay bodies, but even so potters tend to choose one sometimes two and stick with them for years. One of the reasons for this is keeping everything separate is both hard and annoying. Working with porcelain side by side with an iron rich clay body like terracotta would be a nightmare. Even if you are using porcelain and a white stoneware, you need to keep them separate, since they will have different shrinkage rates (think handles cracking off mugs) as well as a number of other problems. So, for the most part potters stick to one clay at a time.

However every so often potters get tchy feet (or perhaps that should be hands), and decide that they’re just not happy with the clay that they’ve been using for the past 5 (or 10 or 20 years). This generally means the start of a huge long cycle where they try 50lbs of every clay in sight before they settle on one. The really crazy ones (err … I mean dedicated ones) decide that what they really have to do is make up their own body with precisely the recipe they want. The well known potters will collaborate with a pottery supply company to make up their ideal clay. The company will then sell that clay under their name. Kind of like Gibson and famous guitarists.

As you might have guessed by now, I got those itchy hands about 6 months ago. The clay I was using, Highwater’s Phoenix, suddenly seemed bland and boring. It’s a pretty generic white stoneware, easy to throw, but not terribly white, nor interestingly speckly. In addition it has a fair bit of mullite grog in it, althought Highwater describes this as “fine” it does leave a fairly rough surface on the bottoms of mugs and so on. In addition it did not have as much wet strength as I would have liked. Essentially any straight sided cylinders over about 13” would start to bulge at the bottom. As you can probably tell I was a bit fed up. So I started looking around at the different clays.

Initially I thought I would just hop over to Highwater’s Moon White, which has very fine Kyanite added to it, which makes it great for large forms, but even at Cone 10 it has an absorbency of 2.3%. This means that it is not particularly vitrified which can leas to overheating in the microwave and other problems.

I spent quite a bit of time researching different bodies online and trying to figure out what sort of pots I wanted to make. Eventually I realised that the clays I was gravitating towards where the ultra-white stonewares, and that in reality many of these are porcelain wanna-bes, formulated for slightly better throwing.

There are a number of good reasons to choose a true porcelain body, not least of which is its translucency and ability to bring a depth to transparent glazes. However, having said that, the high titanium kaolins used in US domestic porcelains render this translucency almost negligible. Unless one is contemplating doing intricate and colourful decoration where an absolutely white background is a must, there seems to be little point in choosing a high titanium porcelain over a good white stoneware.

There are a number of commercially prepared porcelains that use Grolleg, a low titanium Kaolin (or China Clay as we call it over the pond), from Cornwall, or other low titanium clays from New Zealand, so it was to these I turned my attention.

Probably the best source for online opinions on everything from the state of art education to the best style of lids (to pick a couple of recent topics), is the Clayart listserv, whose archives are based at “potters.org”. Browsing through these archives gave me two leads on great grolleg porcelains. One developed by Tom Coleman is manufactured by Aardvark Clay (and sold through many local distributors), and Tom Turner’s Porcelain sold by Standard Clay. I tried 50lbs of both of these, and Laguna/Miller’s “Frost”.

Although Turner’s porcelain was by far the most translucent, with the exposed areas of clay turning that almost glossy white, it was also the most problematic. Almost all pieces showed plucking – that is small pieces of the base getting stuck to the kiln shelves, which leaves characteristic bites out of the footring. I tried a number of solutions to remedy this – for example coating the footring with a mixture of wax and alumina hydrate – but still did not cure it with the few things I tried. Although I did not make any lidded objects, I would have thought those would also have a problem with sticking together. Hardly surprisingly, considering it’s glassy surface (indicating firing very near its limit) it also showed a tendency to warp and slump slightly.

Coleman Porcelain, was also very translucent, but was slightly less white (in oxidation), and examination of the bare ares showed it wasslightly more refractory. Since Tom Turner has developed his body to be fired in oxidation at Cone 9, while Tom Coleman fires to Cone 10 (or higher) in reduction, these differences are not surprising.

The Laguna/Miller frost, was even less translucent than the Coleman Porcelain, and threw less thinly I did not consider it any further. (On the other hand it is significantly cheaper than either of the other two, and handles have less of a tendency to crack off).

In terms of throwing both the Turner and Coleman Porcelain threw extremely well – in fact I could not detect any more difficulty in throwing it than I did Phoenix. With both of them I could easily throw cylinders more than 12 inches in height from 2kg (4.4 lbs) of clay. I would say that the Coleman porcelain feels more pastic, but the Turner porcelain is more solid and has more tooth. I would say that it throws marginally taller too (maybe 13+ inches versus 12 1/2?). Certainly either of them would be great choices for a porcelain. In the end my choice came down to convenience – Turner porcelain is significantly more difficult for me to get hold of since there are no local distrbutors. It’s also somewhat more expensive, but when you sell mugs for $20-25 it hardly makes a difference. Having used Coleman porcelain solidly for a few months I will say that there are a few problems. Most significantly is that it is significantly less tolerant of differences in wetness when attaching handles. After handles are attached, the mugs MUST be dried very very slowly (4 days or more). When space is at a premum in a small studio, this is very irritating.

The only problem with changing to a porcelain clay? My glazes didn’t work properly anymore. Or rather they looked totally different on the porcelain. And the Mossy Mahogany particularly seemed to run more than usual. In addition, I really wanted to develop more translucent glazes to take advantage of porcelain’s particular properties. I’ve therefore spent the last 6 months or so mostly working on expanding my pallette of glazes.

I’m aiming to bring you a lot more pictures of my glaze tests in future blog posts.

From Left to Right : Coleman Porcelain, Turner Porcelan, Phoenix Stoneware
All with Mossy Mahogany glaze.

… But not as many as the V & A.

The Victoria and Albert museum is essentially a design museum, although the pieces from its older collection are rather eclectic (though fascinating), and perhaps more tenuously linked to design.

The ceramics collection has to be one of the largest on display in the world. Although a good deal of it appears over-decorated Victorian junk, cataloging the rise of the great Stoke-on-Trent potteries such as Wedgewood, Spode, Denby and so on, there is actually a s ignificant amount of other stuff. There are also rooms given over to Near East tin-glazed work, European Majolica, modern European designed ceramics (Royal Copenhagen and so on), modern Studio ceramics, and of course the great progression of work from China.

Just to give an idea of the size here is a floor plan, followed by a picture of one of the ‘open storage’ rooms, to give an idea of the size of each of the rooms




As you can see, with such a large collection they decided to build glass storage shelving n the centre of some of the rooms, not ideal for looking at each individual pot, but gives a great look at the extent of the collection. Around the edges are exhibition cases with the best examples from each series.

Most of the rooms however are light and open with well displayed examples of important types of pottery from around the world. There is of course something of a British bias, especially with regard to 20th century studio pottery. Although they have a significant amount of European studio pottery, they are definitely missing some major modern US potters. While they have dozens of pots by individual Japanse potters (Hamada, Shimaoka for example), and European potters, they only have a single pieces from Warren Mackenzie and Peter Voulkos for example. Even within Europe there is definitely a bias towards Northern European and Scandinavian potters. Despite the central place that Bernard Leach has in the founding of modern British Studio ceramics, I have come to the conclusion that modern British studio ceramics owes a lot more to Lucie Rie and Hans Coper. This is especially true when you compare it to the plethora of Japanese mingei influenced potters in the US, influenced of course by Leach via the towering figure of Warren Mackenzie.

At once end of the gallery, they have a room for rotating exhibits from modern industrial designers. At the moment these are Queensberry Hunt, who, since they were founded in 1966 have produced ceramic designs for companies such as Crate and Barrel in the US, and Habitat in the UK.


I should note that both the Ashmolean and the V&A are free, although the V & A does have large glass cylinders encouraging you to donate $3.


Their entire collection is also available to view online. High resolution pictures may be downloaded free, for private use, upon registration.


Here are some of the pots I found most interesting.

This is one from Cyprus, dated to about 7th Century BC.




One of the great things about Chinese ceramics is the ability to precisely pinpoint when it was made due to the reign marks one the bottom of the pot (although later potters did have a tendency to add earlier reign marks as a mark of respect – not very helpful!). This is a beautiful copper red piece from 1406-1420, during the reign of the Yongle Emperor early(ish) in the Ming dynasty. These early Ming copper reds are extremely rare, but extremely beautiful, and the ability to produce them was lost by 1435. Frankly it makes me wonder if the secret of copper reds at that time, was the work of a single man, and when he died the secret was lost with him.



The Ashmolean has an awful lot of pots …


One of the pleasures of visiting my sister in Oxford is getting to wander around the Ashmolean Museum. Even better, they have a policy of allowing photography, as long as you don’t use a flash or use the photographs for commercial purposes. Since I now have a phone with a pretty good camera on it, I decided to document the pieces of their ceramics collection that interested me.

Although I’ve always known how important ceramics are to archeology and the history of art, today’s visit to the Ashmolean really hammered it home. Unlike many museums, like the Victoria and Albert (which I hope to visit later this week), or the Metropolitan Museum of Art, or the British Museum, it has both an eclectic collection and is organized solely by geography and chronology. As I wandered through galleries on Nubia and Ancient Egypt, various dynasties of China, Islamic Art, Europe through the ages, I realised that every single room had at least a few pots in it. And the older the civilisation, the more likely that the sole remaining artifacts are made of clay. Pottery is probably the most durable art form mankind makes. I can look at a Nubian pot from 3000 B.C., and see it pretty much exactly as those ancient Nubians saw it. No other art form, except perhaps stone monuments, comes close. Clay is a fantastically unique material. Even now, with all our high tech plastics and other materials, it is still the only material that can be molded by the hand, at room temperature, leaving imprints of the artists fingers, and yet be simply and irreversibly fired into an impervious object. It is also remarkable just how much pottery was and is, used in our everyday lives. Even now, I would guess that every house has a number of objects made of clay – even if it is only for a mug of coffee and a bowl of cereal for breakfast. Even only a hundred years ago ceramic objects were perhaps the most important and ubiquitous objects in someone’s house. While none of this was a revelation to me, the fact that every exhibit had such a significant amount of pottery in it somehow made it visually more real.

As a potter, this is both inspiring, and somewhat intimidating. Everything that I make could at some point be found and wondered at by humans or other intelligent creatures in some distant future. And, even as reassembled sherds, it will look pretty much like it did the day it came out of the kiln. Of course I’m not going to let this intimidate me into minutely examining everything I fire, but hopefully it will make me think twice before biscuiting a pot that I know is just not quite right, even if it is a teapot that has taken a fair bit of work to throw and assemble.

Here are some of the more interesting pots that caught my attention.

There’s even something for the serious glaze chemist at the Ashmolean. Nigel Wood’s book “Chinese Glazes”, has a lot more like this.

Most of these pictures are from the “China from 800 AD” galleries.

A small grouping of pots from China. I rather like the small teapot at the back.

An absolutely beautiful example of a hare’s fur glaze.
Note that the curve of the bowl is suddenly altered where the thick roll of glaze collected as it flowed down the pot. This is a deliberate feature of these hare’s fur pots. It also important to see just how thick the glaze coat needs to be for the hare’s fur effect to develop.

The inside of a Hare’s fur bowl.
Absolutely spectacular. Notice how bare the rim of the bowl has become, since the glaze is so fluid in order to develop the long Hare’s fur streaks.

Japanese Cruet made for export to England

A 6th Century Chinese urinal! What more can I say?

A fresh start.

Despite the long hiatus since the last post, both Emily and I (Robert) have been stunned by the consistent number of views and comments this blog has received in the past couple of years.

A lot has happened in the past couple of years. Emily has gone on to greener pastures, and is now back in Tennessee grappling with the far more complicated realm of clinical trials. While Emily did most of the glaze testing, I am hoping to continue in her footsteps and I will continue to post some of the trials and tribulations of a glaze developer.

It did occur to both of us that I should start a new blog, to mark a line that separates our collaboration from my solo pursuit. However this blog is such a wealth of information put together by Emily that it seemed a shame not to continue it. I hope you will bear with me as both the tone and no doubt the content of the posts change.

One last thing. Emily was and is very proud of her glazes and the effort she put into developing them. While she is happy for me to discuss the generalities of how they were developed she has asked me to keep the recipes confidential. I would also say that for the most part these recipes are not particularly helpful as the firing schedule of our electric kiln is fairly unique, and something I looked into fairly extensively before I set it up. The glazes have therefore been developed solely for this firing schedule. Notably, I am firing to just over Cone 9, and I fire down, in order to mimic the slow cooling of gas kilns with thick walls. This means that our recipes will be all but useless for the normal practice (in the U.S. at least), of Cone 6 electric firings.

On the road to a consolidated business space…


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One thing I have mentioned in earlier posts is that pottery has a tendency to take over one’s house.  There are pottery books and magazines, glaze test tiles, not quite right pots, pots to sell, show materials like tents and shelves, business stuff like printers, faxes, filing cabinets, packing materials, boxes, tape, GAH!  I haven’t even mentioned all the actual clay, glazes, kiln, and other stuff you need to actually make the pottery.

Over the last year, this little business has grown in random spurts all over the house.  We had our original office and then the pottery office, the spot where we packed up pottery, the place where we stored the tent and shelves, test tiles in little boxes all over the house, GAH again.

So, just like last year when we had to regroup when we had a breathing spell, we’re doing the same thing now.  I decided to put my organizational skills and Quality Management experience to work.  It’s very useful to audit your work habits and space planning to maximize your productivity.  Folks might not think of doing this for an art business, but it really works for all businesses.  I am happy to say that all pottery paraphernalia is now consolidated in one room.  I am now able to take care of orders in the same room where the computer/printer/files/etc are without running all over the house.

All this just to say that you don’t ever know how to set up a space for a task until you’ve done that task quite a lot.  I’ve also found that this applies to moving into a house and getting comfortable in it.  You just can’t know in a new space how you will “be” in that space until you’ve been there for a while.  So, don’t be afraid of taking stock of your space and asking yourself, “Is this working for me?”  You might find that things can be streamlined into a smaller footprint that facilitates your work habits.

So, why don’t folks just ask?


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The awesome thing about the internet is you can tell what’s going on out there.  I mean, I can see what people are using as search terms to find our blog.  Some things are immensely funny, and I really have to think how I possibly could have used those words during the course of the entire blog.   Other things are fairly obvious.

So, when someone searches for “Tenmoku” and finds my article on Taxonomy of Iron Glazes, or gets to my Glossary of Glaze Colorants,  I am really pleased by that.  I hope that person benefited from the hours of research I did that enabled me to write those articles.  I also hope they give me credit for consolidating the information if they mention it to their friends.  Those kinds of things are a lot of work after all.

Then there are the searches for “Em’s Honan Tenmoku recipe” and “Mossy Mahogany recipe”.  See, I name my glazes the way I do for a reason.  I have worked very, very hard over the last few years to develop the glaze recipes I currently use.  I lost count on how many glaze tests I had done after about 2500.  That’s 2500 in less than 2 years.  I am extremely proud of my glazes, so I give them unique and identifiable names like: Em’s Honan Tenmoku Black, Mossy Mahogany, Midnight Peacock, Spruce Juice, Verdigris Sage, Prussian Peacock, Emily’s Persimmon, Green Leopard, Quartz Clear, Moss Agate, Chalk White, and others.  I develop my glazes from the ground up and take pride in the fact that I have educated myself on glaze chemistry as well as I have.

On one hand, I am flattered that people like my glazes enough to be searching for their recipes by name.  On the other hand, I am dumbfounded that people would spend the time and energy hunting around on the internet instead of just writing to me and asking me directly.  If you appreciate my work, please do me the favor of telling me so.  I take it as a huge compliment.  You can post a comment here, and I’ll respond.

There are exceptions to the “lurkers”, of course.  Just last week a very nice fellow emailed me asking for recipes.  It was a pleasure to talk to him and get to meet a fellow potter.

I come from a science background.  When you do research, you publish papers in peer reviewed journals and get credit for your work.  You’re recognized for the accomplishments you’ve made by others in your field.  Someone can always go back to that publication and find out what you did and how you did it.  The convention is, you give them credit for that work when you build on it.  I find this is lacking in the field of glaze chemistry where studio potters are concerned.  If you are using a recipe someone has given you, then give them credit for it.  There’s no shame in using someone else’s glazes as long as the usage is honest and straightforward.  The converse is also true.  When you’ve busted your bum and worked out a killer glaze formula, you get the privilege of naming it and calling it your own.

So please, just ask.  My recipes are not published anywhere by me, and I’m pretty sure Rob isn’t selling them on the black market.  I look forward to meeting other artists and always enjoy discussing glazes.

Thanks Y’all,