Feet of Clay

When firing a wood kiln, or salt or soda, one of the challenges is preventing the feet of the vessels from sticking to the shelves. In any of these firings the atmosphere in the kiln is full of glass making materials, either as fine particles of ash or gaseous sodium trying to flux any free silica in the clay to a liquid glaze. In fact, that is our objective, and by allowing this flow of glass making materials to wash over our vessels we invite nature to glaze and decorate our work for us. It is difficult to be selective, however, about where those gases or particles flow without sacrificing the spontaneity of the glaze effect on the whole vessel. If the liquid glass flows down to the foot of the vessel, then when it cools the foot will be stuck to the kiln shelf by solid glass. This is difficult to remove without chipping the piece or damaging the kiln shelf, and such losses can become quite expensive.

There are several ways of addressing this challenge and I employ two methods in my kiln, depending on where the pots are to be placed. The first is to try to prevent the pots from sticking. My first step in this process is to design my feet so that there is a minimum amount of surface area in contact with the kiln shelf. The foot ring itself is trimmed with a chamfer on each edge so that only a thinly angled line touches the shelf. This also creates a slight overhang to help prevent fly ash from getting under the foot. When the vessel is removed from the kiln the foot ring can be touched up with some sand paper and "Viola!", a nice smooth foot ring. When I am waxing the feet of some vessels before glazing, on plates for example, I use a special wax mixture of candle wax, kerosene and a handful of alumina powder. The wax burns off in the kiln, leaving a very fine residue of alumina between the vessel and the kiln shelf. Alumina (Al2O3) is a highly refractory material, the melting point of which is about 2040C, and so it remains as a dry powder even at glaze firing temperatures, and will serve the same purpose as flour on the work bench when you are making pastry or bread, preventing sticking. Following the same train of thought, Alumina is used as a coating on kiln shelves and kiln furniture to prevent sticking, just as flour is used on the inside of a cake tin. On its own, however, alumina floats around as a powder and can stick to glaze surfaces making them matte and unattractive, so a coating mixture containing some clay material as a fixative is necessary. The mixture which I use is Alumina 2: Kaolin 1 proportionately by weight, mixed with enough water to make is a pourable creamy consistency. I put this into a watering can and pour it over the surface of the kiln shelves, then put them out in the sun to dry. After every firing I scrape any accumulated ash or soda deposits off the kiln shelves, so that they are ready to be coated again for the next firing. It is important to use a dust mask and eye protection during this process! The materials are all non toxic, but inhaling dust can cause lung disease, and chips of broken glass in the eyes should be avoided at all costs!

My other approach to this challenge is to accept that a wood or soda firing is what it is. There is going to be ash and glass dripping of the pots, and it's going to leave marks. If you are trying to avoid that and it happens by accident it can leave nasty scars on the pots. If, however, you accept it and allow it to happen as a part of the total design, those marks become cicatrices, like tribal scars, and a beautiful expression of the firing process in the finished work.

I do not use wad mix directly on the clay, as that goes in the nasty scar basket. Instead I use a ball of fire clay, rolled in alumina, stuffed inside a sea shell. A set of three, or sometimes more, of these will form a stand on which the vessel can be placed, raising it off the kiln shelf. When the kiln is fired, the salt in the shell will volatilize off leaving flashing marks on the clay. The Calcium Carbonate (CaCO3) in the shell will lose Carbon Dioxide (CO2) and become Calcium Oxide (CaO), still retaining its original structure between the vessel and the fire clay. Any glaze, ash or soda will flow into the shell structure. When the vessel is removed from the kiln and placed in a bucket of water, the Calcium Oxide (Quick lime) will rehydrate to become Calcium Hydroxide (Ca(OH)2 or Slaked Lime), and will just wash off the vessel leaving a fossil like cicatrix where it was in contact with the surface. The slaked lime goes on the compost heap. A touch up of the sharp edges left on the glaze deposit with some sand paper, and Robert is your fathers brother! This is the method I use at the fire face, where the forces of nature are most energetic and capricious. I described this process and my feelings about it in my "Art for Eternity" entry, and I invite you to read that by all means. The greatest challenge for a potter, however, is knowing when to take control, and knowing when to surrender it. It is this balance, this dance with the elements, that creates such beauty, that gives great joy to the user of these vessels, and makes being a potter such a fulfilling career.

Legal standards for functional ceramics in Japan

You may remember by blog entry "Safety First" , concerning the food safety standards for functional pottery in Japan. I have been asked by some of you for more information about this, so I have read the notification in the original Japanese and the following is my brief interpretation of it in so far as it applies to functional ceramics. The law applies to Glass, Ceramics, Enamel and other food vessels and packaging, so I won't deal with the full scope of the law. I am, after all, Euan the Potter.

According to notification #416 of the Japanese Ministry of Health, Welfare and Labour, as of the 1st of August 2009, it is required for all manufacturers and importers of ceramics for food to conform to the standards set by the International Standards Organisation for levels of heavy metals, specifically Lead and Cadmium.

These safety levels are based on the amount of Lead and Cadmium which can leech into a liquid from the vessel under set test conditions. These levels and conditions are set out in the second table on THIS LINK.

The levels differ slightly for vessels which are used to heat food and those which are not. However, Sake bottles (tokuri), Chawan Mushi Chawan (vessels for savoury steamed custard) and other vessels which can be used to heat or reheat food by any method not exceeding 100 degrees centigrade, where heating food is not the main function of the vessel, will be treated the same as vessels which are not for heating food.

The notification does not state the method by which these standards will be enforced, but department stores in particular are asking for certification of safety in Japanese before vessels can be sold. This is probably in line with the Public Liability Laws which were introduced a few years ago making the manufacturer and retailer responsible for injury to customers due to faulty or dangerous products. I believe that it is required by customs for imported ware, and as I was required to provide this for Mitsukoshi, and my pots are made in Japan, it seems to be required for domestic sales as well. I have only been asked by department stores, however, so I am unsure of the extent to which certification is required.

It only applies to vessels for food. In particular it is important with low fired lead glazes and on glaze enamels, but it applies to all food vessels. Within Japan testing can be done in a variety of places, but I recommend the Tajimi Ceramic Institute. Their home page outlines their testing methods and costs. One representative sample of work should be sent for testing, and they return it. I do not know whether they accept work for testing from overseas.

So, there you have it. If you are making vessels for food, Japanese law requires that they be food safe according to ISO standards. That sounds fair to me.

Black is Black

My friend and fellow deshi of Shimaoka sensei asked me an interesting question the other day.



"How do you keep your black glaze black in a Soda Firing?"



There are indeed lots of ways of getting a black glaze, but you have to decide what kind of black you want. Many black glazes use an overcharge of several colouring oxides to give a dense black. In my case, I am looking for the warm translucency one finds in lacquer ware. Lacquer is in fact brown, but layer upon layer make it so dense it appears black.





In order to achieve a similar brown based black I turned to the Tenmoku style iron glaze. The base glaze is clear, using only four ingredients; Feldspar, Silica, Whiting, Kaolin. To this I add FeO, or Black Iron Oxide.



Using Black Iron Oxide gives you a head start, as the Iron is already reduced. Using Red Iron Oxide, Fe2O3, will require more work to get rid of the unnecessary oxygen.



There are a plethora of glaze recipes available which are excellent Tenmoku in a gas kiln.


Within any plant there are minerals which have been drawn from the ground, and when burnt these minerals either become gaseous and volatise off into the atmosphere or remain as a solid residue, ash. When firing a wood kiln the gaseous minerals, like sodium, will flow through the kiln with the flame and exhaust gases, combining where it contacts with free silica in the clay or glaze surface to form a soda glass glaze. Ash is blown into the kiln with the flame as a powder, settling on the pottery and glaze, adding a coating of a variety of minerals depending on the plant. At a high enough temperature, usually about 1300 centigrade, these minerals, silica and fluxes, will melt and form a glass coating all on their own. The colour and surface will be affected also by what minerals are contained in the vessel surface and mix into the glaze matrix. At the end of my wood firing I also add some Soda Ash , NaCO3, which becomes a gaseous flux inside the kiln in the same way as the natural vapours from the wood.


So, in a wood and soda firing the glaze gets watered down. The ash adds an extra wash of relatively clear glaze and the gaseous flux Sodium makes the glaze melt even more. You end up with a variety of oatmeal and honey glazes. You can see how the "hidasuki" markings from the "Igusa" straw have left golden veins in the black glaze on this plate. Too much straw and it all turns to golden toffee. That can be nice, but it's not black, and it might not be what you want.



So, you need to make the glaze harder, so that it is a bit short of flux and a bit resistant to soda, raising its melting point. By reducing the Feldspar, for example, it raises the melting point, but also raises the ratio of Iron in the glaze. Iron is resistant to soda. Effectively you adjust the glaze recipe to make allowance for the extra flux which is floating around as a gas in the kiln atmosphere.


Glaze application is also important, as no matter how good your glaze is, if it is too thin the ash and soda are going to win. I apply my glaze at a viscosity of 60 by the hydrometer. Tenmoku is very sensitive to variations in texture underneath, so also be aware that the finished glaze surface and colour are only as good as the finished surface of the vessel.

It has taken me years to get my clay body, glaze recipe, glaze application and firing right, and all of those variables affect the finished result. The same glaze will be different in a different firing or on different clay, so it is necessary to experiment with your own materials and firing. In the end it becomes another inimitable expression of your own aesthetic. Hopefully these principles will help you along the way.

Safety First

I have always made it a point to use only natural and safe materials when producing my work. There are no toxic materials in my clay, glazes or firing process. No Lead, No Barium, nothing toxic. Particularly considering that my studio is next to my children's bedroom!

I am careful to reduce dust and use a dust mask wherever necessary (special thanks to Phil Rogers for sending me a Dust Mask from the UK that didn't bend my nose over like the Japanese ones do!), and I use eye and ear protection where appropriate.

I have therefore always known that my products were safe for the consumer. This has always been a primary concern for me.

However, in order to sell my work at Mitsukoshi Department store I had to have my work tested by a public authority to prove it was safe for use with food. This is now necessary for all ceramics imported into Japan, and will apparently become standard practice by law for local production as well in the near future.

So, I sent my work to the Tajimi Ceramic Institute for testing and received this certificate proving that my work is perfectly safe for use with food. You no longer have to take my word for it!

ART FOR ETERNITY; Discovering "Nontemporary Art"

Twelve million years ago Japan was the bed of a cold and shallow sea. Volcanic action and the moving of continents pushed this archipelago up out of the sea, built mountain ranges and valleys. Rain and erosion resculpted the landscape, filling the valleys with clay. Vegetation created rich soil and over ages these islands became what they are today. Throughout Japan, however, there still remains a layer of sedimentary rock, a strata that was once sea bed, and the evidence of its history can still be found there. 　　

　　

　　

"Ichikai", the name of the town in which we live, means "City of Shells". In this area there are some of the best fossil deposits in Japan, and the other day we went excavating with some people from the prefectural museum of natural history. 　　　

　　

　　

　

　　

　　

　　

　　

　　

　　

　　

　　　

　　

　　

The children all participated, and we found a variety of different shells. 　　　

　　

　　

　　

　　

　　

　　

　　

　　

　　

　　

　　

　　

　　

　

　　

Time and pressure had changed the structure of the primordial mud, forcing the separate particles to meld together, embedding these beautiful forms and pattern into the rock for all eternity.　　

　

　　　　

As a potter, I too am trying to transform mud into stone, melting the particles together to form a new material which bears the patterns and forms which I have consciously created. By firing the kiln to 1300 Celsius I achieve in human time scales what nature does in geological time over millions of years. There is a limit to what I can do though, and I trust the forces nature to take my work beyond my limitations.　　　

　　

　　

To prevent the pots from being stuck in the kiln by liquid glass from the molten, running ash I will sometimes set the pots on sea shells. I fill them with clay to prevent them from collapsing, then place the pots on top of them.In the firing some carbon dioxide will burn off from the calcium carbonate of the shells, leaving the shape of the shells intact as calcium oxide. 　　

　　

　　

Even if ash runs down and sticks on the shells, the pots can be taken from the kiln with the shells attached. They can then be put into water and the calcium oxide will dissolve into sludge, leaving only the shell marks on the foot of the vessel, in this case tea bowls.
　　

　　

Sea shells also contain a small amount of salt, which will turn into sodium gas during the firing, giving orange flashing on the inside of the foot. The spiral left by the trimming tool on the base of the pots resonates with the spirals of sea shells, not as a conscious representation but as a natural consequence of the forming process. 　　

　　

　　　

It is the beauty which springs from the natural process that imbues these works with their intrinsic charm. This beauty, this art, is not pretentious nor contrived, like much representational art, nor is it bound to a specific set of aesthetics or social mores as is contemporary art and fashion. This beauty is relevant to any one who loves beauty regardless of culture or creed, in any age, and will last for all of eternity. This is "Nontemporary Art".

Opening the Box

No matter how cleverly you stack the kiln, or how well you have mastered the firing, the fact remains that the results from a wood kiln are in the hands of nature. You will never know for sure how the pots will come out until they do, actually, come out. It is a matter of faith, of trusting the process of nature. And if you have guided the firing well, the kiln will reward you with exceptional pots, far beyond the limits of your own ability.







My firing on Saturday took 13 hours, so it was fairly fast, but I gave it a very heavy reduction, including a reduced cooling. When I opened the kiln this morning I was surprised by a slightly metallic lustre on the surface of flashing and blue clouding in the black glaze. The pink spot on the rim of the plate is porcelain slip with 2% of Copper Oxide, so it is a good indication of the heavy reduction. In oxidation it would be green. The green slip trailing is also porcelain, but with 2% Chrome.






There was also more orange flashing towards the fire face than usual, which will balance with the heavy carbon trapping from the previous firing when I display them together at my next exhibition.










There were more losses than I've had recently, but that is mainly because of the large number of plates with the black glaze. Just one grain of sand or flake of kiln wash will ruin the whole piece.










Two hundred of the pieces from this firing will be going into my next exhibition and will be available for purchase at New York Takashimaya Home, 10F Takashimaya Department Store, Shinjuku, Tokyo, from next Wednesday, July 15th till July 28th.
I will be at the exhibition on the 15th, 18th, 19th, 25th, 26th, 27th and 28th, if you would like to visit while I am there.

The Principles of Shino

Over the past few weeks I have been asked by several potter friends about the materials for Japanese shino glazes and their equivalents in the west. I haven't actually used a shino glaze for about fifteen years, but with some of the restaurant collaborations and requests from chefs, I may soon be revisiting the shino arena.

It is probably useful to firstly put shino into a frame of reference.

Once upon a time there was a tea master named Shino Soushin (1444-1523) who directed that the potters of Mino produce a white glaze. Their solution was to coat a low iron clay, either mogusa clay or gotomaki clay, with a local feldspar, occasionally mixed with some clay to make it more user friendly. The resulting glaze, wood fired of course, tended to be slightly grey in oxidation (Nezumi Shino meaning Mouse Shino) or pearly white with red flashing in reduction.
Generally Shino glaze is accredited to the Momoyama period (1568-1600), and, yes, some of the finest shino glazes come from that period, but it had its roots in the Muromachi period (1336-1573). Interesting to note the overlap.

So...what was the feldspar?!

The simple answer is Hiratsu Shino Choseki. Choseki is Feldspar, the Kanji "長石" means "long rock" referring to the rhombic crystalline structure of feldspar. This feldspar has a high Alumina to Silica ratio, combined with fairly high Sodium/Potassium content. It is in fact the Sodium and Alumina under reduction that give the distinctive soft pink and orange flashing of Shino. Which is why Nepheline Syenite lends itself so well to shino style glazes. Compare the analyses;

Hiratsu Shino Choseki ;

SiO2 65.4
Al2O3 20.5
Fe2O3 0.09
TiO2 0.01
CaO 0.14
MgO 0.03
K2O 6.89
Na2O 5.13
Ignition loss 1.59

Nepheline Syenite;

SiO2 60.1
Al2O3 23
Fe2O3 0.09
TiO2 0.02
CaO 0.37
MgO 0.02
K2O 4.75
Na2O 10.6
Ignition loss 0.47

Many recipes have been based on this material, especially with its high sodium content. With the addition of up to 40% Clay (Ball clay, Kaolin, Porcelain, Terracotta or Fire clay) a huge range of variations can be achieved.

The representative Ball clays added to the feldspar in Japan were either Gairome;

SiO2 49.72
Al2O3 34.55
Fe2O3 1.23
CaO 0.16
MgO 0.24
K2O 0.74
Ignition loss 12.8

Or Kibushi Nendo;

SiO2 48.56
Al2O3 33.48
Fe2O3 0.87
CaO 0.30
MgO 0.15
K2O 0.36
Na2O 0.63
TiO2 0.12
Ignition loss 15.76

Other recipes have taken note of the high alumina to silica and high Alkaline flux and have included Soda ash into the mix. Others depend on fluxes other than Soda, the most significant being Lithium. In the west many Shino glazes use Spodumene as the base, which is basically;

Li2O. Al2O3. 4SiO2

This is as rare as hens teeth in Japan, and the best replacement is Petalite;

Li2O. Al2O3. 8SiO2

Of course the Silica present in the rest of the glaze needs to be reduced to compensate. This can sometimes be done by replacing Kaolin for Ball Clay, but it is important to test with the materials one has at hand, as there are huge regional variations. Alternatively an addition of Lithium Carbonate and Alumina will compensate, but you will need to do the math!

It is this high alumina, high alkaline flux that gives my pottery its distinctive peach blush, as I am using a porcelaineous, high alumina, low iron, clay in a wood firing, from which the natural salts vaporise from the wood, with the addition of a small amount of soda ash at the end of the firing. Not Shino, but based on the same principles.

There are books that purport to tell the secrets of Japanese wood firing, with lists of "American" shino recipes and no examples of any actual representative Japanese ceramics whatsoever. OK, I can live with that. As recipes they stay within the parameters of the original Hiratsu Shino style, and if you are satisfied with that, fine.

But that is NOT all, oh no that is not ALL!

Japanese shino glazes are not all made with Hiratsu Shino Choseki, and not all flashing is Soda/Alumina. One must not forget our old friend Iron, who so happily serves the potter with a huge gamut of glaze effects.

"And so", said the Cat in the Hat, "So, so, so, I will show you another good trick that I know!"

As my sempai Ken Matsuzaki demonstrates so well, any feldspar can make a Shino, given the presence of Iron either underneath the glaze or within range of the external glaze surface. It is the nature of Iron oxide that in a heavy reduction atmosphere, between the temperatures of 1090 and 1130, over an extended period of time, it will volatise and migrate to the surface of the glaze. This must be done before the glaze cinters, sealing it and preventing the Iron from seeping through the porous glaze matrix. By painting a decoration in Iron under the glaze, or by utilizing a small amount of Iron in or on the body beneath the glaze an Iron blush can be created on the opalescent white feldspathic glaze. A wash of Iron inside a saggar surrounding the work can give a similar effect.

Maintaining a heavy reduction during cooling is essential, but in the end the micro fine layer of Iron will re oxidize to give the distinctive Shino blush. Merely seeing Iron through a semi transparent glaze is not Shino. There is a lustre and warmth that springs from the natural process which makes shino special. Colours that resonate with sunrises and sunsets, that remind us of the soft warmth of human skin.

The Japanese traditional teaching system does not put the onus on the teacher to teach; It is the responsibility of the student to actively learn. Shimaoka sensei told me that there are no secrets. You just need to ask the right questions.

Vulcan

It is interesting to note that the Lava flowing from the volcano in Hawaii has a temperature of about 1200 degrees centigrade, whereas the temperature inside my wood fired kiln is 1300 c. Effectively I have a man made volcano in my back yard. (Photo; Sakurai Mitsuyo/桜井光代)


It isn't really such a strange thought. The glazes which I apply to my pots are mixtures of powdered minerals, rocks if you like, which I am trying to melt into a glass. The last time they were molten was when they were either lava or magma, so it stands to reason that the temperature which I need to achieve should be the same. Hotter in fact because I am trying to achieve geological effects in human time scales.



In this last firing I included some rock glaze tests. The traditional Mashiko Kaki glaze is made from the powder of one rock, a local stone formed from volcanic ash called "Ashinuma Ishi". Ashinuma, where it comes from is a five minute walk up the road, and many of the storehouses and building foundations in this area were built from this stone, including our house and the shed which burnt down. A mashiko potter many years ago used some of this rock to close his kilns firemouth after a firing, and when he opened the kiln the rock had fluxed into a beautiful iron rust glaze. It wasn't long before it became the signature mashiko glaze, and because of its similarity in colour and surface to the seeds of the persimmon, it was dubbed "Kaki" glaze by Hamada. It is no longer being mined, and the Mashiko cooperative will only sell the remaining reserves to members. HOWEVER....as I just mentioned, the foundations of the shed which burnt down were built from this stone. One wonders how much of this glaze would be used in one potters lifetime. I put a sample into the kiln as a control, which you can see here. It is usually applied over a "Nami jiro" standard white glaze, but I have applied it over my porcelain clay.



The question which I ask myself is, Does tradition lie in the materials and tools which are used or in the spirit in which they are used? Every tradition has at some point started as a cutting edge development, the repetition of which over generations becomes "Tradition" with a capital "T". The risk is that subsequent generations will make copies of old works which are no longer appropriate in a contemporary context, anachronisms which lack the spirit and relevance of the original works. If the tradition lies in the spirit of the process, the use of natural local materials which are inexpensive and readily available, to create work which is relevent to the society in which it is made, then change itself becomes tradition. What we consider local now, places we can flit to within the hour, may have been days of travel away a century ago. And so I have begun to experiment with some different "local" materials.



Just on the other side of Utsunomiya, the prefectural capital, about 40km from here, is a place called Oya. Oya stone, a vesicular stone once again formed from volcanic ash, has been quarried for centuries for building purposes throughout Japan, particularly the Kanto area. It is hugely abundant, far more so than Ashinuma stone, and is easily worked with hand tools, being light and porous. It is sawn into blocks, and the dust from the sawing process is freely available. When you mix the dust with water, apply it to a pot and fire it to 1300 it looks like this.



To the south of Mashiko in the neighbouring prefecture there are large deposits of Granodiorite in Iwase and Makabe. This is a much harder rock, very much like granite, but it contains a plagioclase (sodium) feldspar rather than an orthoclase (potassium) feldspar. It also contains quartz and muscovite and has a large crystalline structure. It cooled slowly underground which allowed the different minerals to separate and crystalize independently, giving it a distinctive appearance. The dust from these quarries is also cheap or free and there are mountains of the stuff. It looks like this. (You may notice that nature has left the message "CRAP" in black muscovite flakes in the left corner of the picture, this has not been tampered with!) The glaze is a smooth transparent toffee type glaze, similar to traditional "Ame".



I don't know whether I will ever use these glazes, but the point is that they are there. Simple, inexpensive and functional. Similar materials are available virtually anywhere in the world, probably around the corner from you, just waiting for a potter with a personal volcano to bring out the best in them.