Thursday, April 12, 2012

A Brief Overview on Glaze Effects of An Anagama Kiln

So I am currently in the midst of writing a monograph on Anagama Firings. That is why I have been away, because the reading and research is just overwhelming. I am giving a talk about this stuff in the Student Research Conference here about this stuff. However, just for your reading pleasure, 1/6th of my monograph. It is kind of technical and really detailed, so unless you are a potter, or interested in pottery, you might find this a bit dry.

Even though the glaze in an Anagama is formed by the action of fire on ash that fell naturally on the clay body, there are many ways in which a potter can control the glaze effects. Before I go into the ways Glaze effects can be controlled, I would like to go in some depth into the effects themselves. I would also like to inform you that this is the first time, at least to my knowledge, that someone has compiled a list of Anagama kiln glaze effects with the technical names given to them in Japanese, which can serve as a useful tool to explain, and catalogue wood fired pots. I shall detail the formation of Shizenyu effects in the order of their achievement as the firing progresses.
Shizenyu:  This is the term for a natural ash glaze, which, looking at it objectively, is a kiln effect. The main component of ash is Calcium Carbonate, (CaCO3), which when pure, melts at 851*c. However, due to various impurities present in the ash, the rule of thumb says at least the temperature of 1100*c needs to be achieved. It is important to note that the very impurities that drive up the melting point of ash also contribute to the delicate colouration of the glaze itself. In addition, the higher firing temperature contributes to partial vitrification of the clay body, lending it more tensile strength and heat shock resistance.
Shizenyu initially appears in form of clumps resembling sesame seeds. This is called Gomabai. As the firing progresses, the areas between the spots is filled up with Shizenyu. Soon, the ash forms a web like structure around the clay body called Amibai. Soon, due to the excessive melting, the ash begins to run, and leads to Tamadare which is ash runs on vertical faces, and Yu-Damari which are ash pools on the horizontal surfaces. If the firing is continued at the same, or higher temperature, the clay body begins eroding, called Shinshoku which is followed by bloating, deformation and collapse, called Buku  and Tsubare. In areas that do not get ash deposition, Hi-Iro occurs, which can be called flame marking, which is due to the direct contact of the flame on the clay body. A lot can be told about a pot from these glaze effects, because they depend not only on the pot and the clay body, The various factors are:
The type of wood: each type of wood has a different chemical composition and thus behaves differently. Also hardwood and softwood burn differently and lead to different temperature changes per hour.
Temperature in the general area of the kiln:  The kiln temperature, contrary to expectation, is not always uniform, because the kiln is not an empty chamber. It has stacks upon stacks of pots. On top of that, there is considerable ash deposition. (In the firing I took part in, we went through 4 cords of wood in 3 days. That is 512 cubic feet of wood.) This leads to areas of unequal heating.
Position Of the pots: The way a pot is positioned will affect not only the pot, but also the entire kiln. This is so because the kiln is a dynamic system and each pot effectively serves to block draft. If the pot is too big and in the front, It can even stop the draft in conjunction with the other pots, which will result in a failed firing from under firing of the entire kiln. Indeed, the pots placed in a tunnel kiln show the direction of flame in their glaze formation, and someone with an experienced eye can tell where the pot was placed from the glaze formation.
Kind of firing: Using dampers in the chimney, a potter can control the draft and the airflow in the kiln. The airflow is important because if there is a lack of oxygen, the firing goes into the reduction state, which can lead to a large temperature drop. However, reduction gives the glaze a very interesting colour not normally expected from it. Correct usage of dampers is a art.

Considering that a kiln is so dynamic, I shall not go further into the variables that control the firing, but shall concentrate on the effects and how they are created more.  In doing so, I shall also give some pointers on how to identify the pot’s various effects and what went into achieving the said effects.
Gomabai: commonly referred as the sesame seed pattern, it occurs in areas of the kiln where there is low fly ash deposition, and where the flame rises slowly. It can be further divided into various subtypes depending on the colour.
Ki-Goma- has a yellow to reddish brown colour. It is indicative of an oxidising fire in the kiln.
Aoo-Goma- has a blue-green colour, and is indicative of a reduction firing.
Furthermore the difference in the gloss shows us the difference in the cooling. A glossy colour indicates a slow cooling after the kiln has been sealed.  This is called Kase-Goma. On the other hand, a glossy colour shows a fast cooling to 800*c and then a slow cooling. The 800*c limit is there to prohibit dunting of the pots#.
Furthermore, the matte texture of the glaze itself has categories. Enoki-Hada has a texture resembling tree bark, and indicates a low firing temperature of 1,100*c (cone 9). Melon-Hada on the other hand has a texture that resembles a melon skin, and also indicates a low firing temperature of 1,100*c.
The to and fro between the reduction and oxidation also leads to some dynamic kiln effects that are called Shimi. These occur in some sections of the kiln which are away from the stoke holes and the fire box. Here, the temperature in reduction can drop down to under 1,000*c in reduction firing and rise to 1,150*c in oxidation. Due to the reduction fire, the clay body acts as a carbon sink. However as the temperature rises in the oxidation stage, the clay violently releases the carbon, which oxidises, affecting the glaze by leaving pockmarks on the glaze surface, and changing the colour in the localised areas. The resultant area of Shimi  is inset. This is contrasted by Hanten, which can be classified as spots on the surface of the glaze itself. These are caused by external action like splattering, or getting hit by an external object. It is also different from Gomabai, which is ash spots on a clay surface.
Amibai is a delicate ash netting that forms on the clay body. It is only observed in pots that have high ash buildup, and high temperatures during the entire firing. Thus, it could be observed in pieces near the firebox and the stoke holes. The clay body also affects the Amibai formation. A clay with more grog or coarse sand will lend itself to Amibai rather then a finer clay like porcelain. Tamadare is the next stage, where running lines of ash glaze with a ball at the end form over the clay body. This happens in areas of very heavy ash buildup, and very high temperatures. This means it is most probable to find them near the firebox and the stoke holes. The temperature differences lead to different types of Tamadare, where the slower drops, formed in cooler (comparatively) environs are matte, and faster drops, formed in the warmer conditions are glossy. Also to be noted is the fact that the higher the temperature, the thinner the width and the smaller the ball at the base. Again, as with Amibai, a coarser, more grogged clay body is more suited to Tamadare. Biidoro is a special form of Tamadare. It can be translated as a “Kiln Teardrop”. It is characterised by glossy running drips with a glossy green/ blue drop at the end. It is a result of a long firing with fluctuating temperatures, and reductions, followed by a high temperature finish and a fast cooling. Tamadare  only occurs on vertical surfaces. If it meets a horizontal surface, it starts pooling up, it is called Yu-Damari. It again exhibits the usual characteristics based on the firing, i.e. a glossy surface shows high temperature and faster cooling, and a matte surface means a slow cooling.
As we can see, temperature is a major factor in the firing. Sometimes in a firing though, a pot gets covered, in part, or full by embers. They insulate the pot from the temperature, and can also contribute to a localised reducing reaction. This leads to a situation where an entire section of the kiln near the stoke holes will reduce when the rest of the kiln oxidises. This localised reduction is called Sangiri. Sometimes, the embers will cover the pot while Tamadare is occurring. In that situation, embers begin to stick to the running glaze. If there was a moderate ash deposition, the result will resemble a delicate moss. This structure does not have much strength and can break off very easily in processing, and thus is prized. It is referred to as Koke-Koge. If the ash buildup was higher, than the pots take on a rough stone like texture. This especially occurs when the embers also contain large amounts of ash. This effect is called Ishi-Koge. Sangiri and Koge effects are so prized that many a time, a potter will deliberately take a pot off its base, and roll it around in embers while the kiln is still at its peak temperature. This technique is called Korogashi.
As previously mentioned, a heavily grogged clay body will lead to more chances of achieving Tamadare, however, it has its disadvantage. Most potters do not have access to clay with coarse silica, quartz, or feldspar naturally in it. As such, most potters will add external grog or coarse sand or feldspar to the clay body. These materials behave differently under high temperature and cooling then normal clay, because their coefficients of expansion and contraction are different. Sometime, clay will contract around a feldspar or silica pebble so much so that it will crack. This fault is called Ishihaze. On other times, feldspar pebbles in the clay body will actually melt and raise to the surface. This leads to a shiny white extruded spot on the clay body, called Tombo-No-Me, which means dragon fly’s eye. In other cases, the feldspar will rise out, but will melt back flush with the clay body, leaving a black spot. This is called a Kani-No-Me, which means a crab’s eye. As is evident, Kani-No-Me occures at a higher temperature then Tombo-No-Me.
In some cases, the absence of Shizenyu can lead to kiln effects. At the rear of the kiln, where the least ash deposition occurs, a peculiar effect called Hi-Iro occurs. Somewhat analogous to flashing that is observed in western glazes, Hi-Iro only occurs on the clay itself. It can be explained as the result of the flame’s interaction with the clay, and its constituent compounds. Hishoku is a special type of Hi-Iro that has a carmine colouration. It is rarely seen, and occurs seemingly at random. The only thing known to cause it is high temperature, and even that alone can not cause Hishoku. According to my source, it is so rare, that it is regarded as a gift of the kiln gods. Hi-Iro can also be called flame marking. As such, the clay body actually needs to be in contact with the fire to show this effect. As such, areas that are not exposed to flame do not show Hi-Iro. This effect is called Nuke. To more precisely define Nuke, we need to take into consideration the fact that flame in the kiln has a direction. The flame moves from the firebox towards the chimney outwards, and always follows the path of least resistance. As such, if there is a pot that is obscured by either another pot, or wadding, or kiln walls, then it will not have contact with the flame. Similarly, if the pot is partially obscured, it will partially come in contact with the flame. The partial obscuring of a pot leads to Nuke. It is important to note the even the pot obscures itself, and the face opposite the firebox also receives Nuke. Nuke can be controlled by the wetness of the wadding, with wet wadding leading to sharp outlines and little to no colour variation. Dry wadding on the other hand leads to soft outlines and major colour variation. A special type of Nuke can be seen in ware with natural ash glaze, called Bota-Machi, which occurs when the wadding prevents the ash from depositing on a certain area.

With this brief look at kiln effects, I shall move on to firing schedules.
Phase one is the stage where greenware turns to bisque ware. Thus it first loses water, both free and chemical.  The stages in the first Phase are as follows:
1) Kemuri Toshi: removal of free water. The free water is water added to the clay while working and in processing to make it plastic and workable. When all the free water is removed, the ware is bone dry. This stage ranges from 0*c-150*c
2) Aburi: Removal of residual free water. This stage is a midway stage which serves to stop the rise in the temperature.
3) Seme Aburi: Removal of chemical water, quartz inversion and bisquing. In this stage, the water in the molecules of the clay constituent is removed, leading to the chemical change. Also the process of quartz inversion takes place. Quartz at room temperature is in its α state. However at roughly 600*c it inverts to its β state in its crystal structure. As typical in cases of isomerism, there is a change in size. Finally it turns to bisque ware.
  Phase two involves all the high temperature firing activities. Ash glaze is achieved in this phase. Side Stoking and sustained front stoking is conducted in this phase. There are only two stages in this state:
1)Seme: This state is when the highest temperature is reached in the firing. Constant stoking is required. This is the stage where the ash deposits on the ware in large quantities and starts melting. The peak temperature achieved depends entirely on the kiln and the intent of the potter, but it is not advisable to exceed 1350*c, because of the risk of deformed or melted pots.
2)Nerashi: This stage involved sustained high temperatures. This stage is important because this is where the bulk of the glaze formation happens. Side stoking and front stoking takes place, and, depending on a kiln, up to a day can be spent on this stage.
The final Phase is cooling. The kiln can either be sealed up and left to slow cool, or it can be left open to cool until 800*c and then sealed up to slowly cool. This is to prevent dunting when quarts reverts to its α state and shrinks.

1 comment:

Trevor Youngberg said...

Hey there
Any insight on crystal hanten?
Been finding them in shino's lately and am looking to research a bit...can't fing much on the web

Best, Trevor Youngberg (.com)