Monday, April 16, 2012

Why wood firing?


This is part 2 of my three pare series on Woodfiring. As it is supposed to be a part of a paper, it is still a first draft, so editing and adding stuff is gonna take up most of my week. But yeah, Why fire pots with wood?





Pottery , until a few decades ago, was considered a “craft”. There was a distinction between an “artist”, who made art, and a craftsmen, who made furniture and consumable goods. This distinction has been destroyed in most circles now, with pioneering work of Robert Arneson, Peter Voulkos, Shoji Hamada, Bernard Leach, Grayson Perry, Peter Voulkos, and many other pioneering ceramic artists and potters. However, to answer the question why?, we need to look at the roots of pottery, what it implies, its visual, and aesthetic underpinnings and its ethos. Rather than being factual as in the previous sections, I shall attempt to find a personal reason and explanation for the question. We need to keep in mind, that art is an intensely personal process, and there can be infinite reasons. I can only claim my own reasons.
The first question we can ask is, Why Pottery? In most cases, people take up pottery for the sole reason that it is a very enjoyable process. Most of these “hobby pots#” can be put in the category of art brut for reasons of lack of pedagogical underpinnings and unintentional and unsubstantiated choices made in the process itself. Simply put, if it is “art”, it is unexamined art. On the other hand, there are pots made by professional potters in large batches and for large scale consumption. These pots are mostly well made, and the potters are technically proficient. However, the aesthetics of the pot, the form, shape, style, glazing, are mostly conventional and conservative. This is so for the reasons of mass sale and ease of production. There is a small section in the middle of these two groups, populated by studio potters, who use techniques from both sides of the spectrum to make creative, technically proficient work.
The question arises, that what place does art have in such a utilitarian trade such as Pottery, which makes tools to make our life easier. An argument could be made that all a potter has to do is to make competent pots, that fulfill their function. However, the same argument could be made for a house painter, and an “art” painter. The major point of difference, in my opinion, that differentiates “art” pottery, from “consumption” pottery, is the intent. Does the potter make choices that lead to the creation of a “art” pot, or does he make no choices, and ends up with an “unexamined” pot. The thing to keep in mind is that pottery has certain processes that need to be completed before the potter can even start making the pot. The choice of clay is the first step. Each clay, stoneware, earthenware, other misc. clays, have characteristic properties. They behave differently in each stage of wetness, they behave differently when dry, and they behave differently in a firing. For example, stoneware clay fire properly at a temperature that would reduce earthenware to a puddle of weak glass.
I recently went out on a limb and decided that I wanted to work with something other then stoneware and earthenware, and so I decided that I will make my own clay. I was however constrained by the lack of material at my disposal. On further looking, I found a bag containing 5 kilos of pine ash that I had saved up from a camping trip. I decided that the ash would be the base for my new clay. That put some problems ahead of me. Firstly, the question of plasticity. Ash mixed with water would give me a paste that would be corrosive and unworkable. Thus I needed something that would neutralise the corrosive nature of the ash, and make it more pliable. The first thing that came to mind was Bentonite. Bentonite is a clay of volcanic origin, that expands on contact with water, which makes it an excellent sealant. Another advantage is the fact that it is commercially available as cat litter. A small amount of crushed bentonite was thus mixed with the ash. Now come the question of its heat stability. Ash has a relatively low melting point, which can easily be achieved in a cone 10 firing. I obviously did not want a glaze on the kiln instead of the pot, so I had to correct the lower melting point. I used quartz powder that led it to have a higher melting point. I also added coarse silica sand to the mixture to open it up. I made a few pots with the clay, with the intention of firing them in a stoneware kiln, because I predicted that the ash and bentonite would give the body a soft texture. I recently had a chance to fire a test tile with the material, and found all my predictions were correct, except the temperature sensitivity. The test tile had stuck to the trough I kept it in. This was possibly because the clay body was its own glaze. This tangent serves to show that even the process of choosing the clay body is one that leads to many choices.
The choices that follow are equally complicated. To glaze, or not to glaze? Underglaze? Enamel? The reader needs to keep in mind that glazing is not a simple chromatic process, but instead is a physical and chemical process, where compounds show colours in high temperatures, and undergo physical and chemical changes due to these temperatures. As such, they also react with the compounds in the clay. For example, Shino, a japanese family of glazes yields white to light orange colouration. However, in certain conditions, it becomes black, due to carbon trapping, which occurs in reduction stages of a firing. Also, the Shino can react to certain clays to give reds, greens and browns. ( In the anagama firing I took part in, the Shino pots were in a noburigama chamber, which reached cone 14. At this temperature, the glaze on the vertical surfaces ran to the inside of the pot, leaving a very thin residue on the walls, and the bottom had all the glaze. However, this glaze filled up half the pot, because it was mostly bubbles. Apparently, Shino takes on soap-like elastic tendencies under extreme high temperatures. The glaze on the walls was green, which is an uncharacteristic colour for a Shino.It could be argued that the batch of clay the pot was made from was adultrated by lead-oxide, but that seems implausible because the clay was sourced from a clay mine in Washington state.)
I realise that this seems like a continuation of the kiln firing variables mentioned previously, however, it is important to keep in mind that these questions all lead us the the movements that have made our art world as it is. I especially refer to Abstract Expressionism, and Process Art.
Process art can be called a thought process in which the objet d’art is not the principal focus. Instead of that, the process of making art itself has higher meaning, and value. Pottery, I argue, can be classified as an intrinsically process based based art. This is so, because of the various steps going into its creation. Again, I shall rehash a list of things that need to be done to make an ideal pot, with approximate times needed to make a pot from a kilo of clay.
1) Collection of clay- involves finding correct vein of clay, and collection. 4-5 hours
2) Drying and processing of clay- clay needs to be dried, ground, and sieved. 1-2 days
3) Mixing, pugging of clay- any additives like ball clay, kaolin, need to be added and mixed in. A mechanical pugger makes this easier. 6 hours.
4) Seasoning the clay for a year- Seasoned clay is always better. Hamada used clay that was 4-5 years old. Fresh clay can be used, but yields less than optimum results- 1 year
5) Wedging the clay- Clay needs to be wedged before use, to ensure uniformity in the clay body and to ensure there are air bubbles. I wedge a half kilo ball of clay for 15 minutes. Hamada got his balls wedged for 3 hours.
6) Adding any additives to the clay itself, like sand- This could be called a part of wedging, however it is a separate step, that also necessitates further wedging. 30-45 minutes.
7) Centering/ slab/ ball/ coil formating- Either the wedged clay can be centered, or it can be worked into slabs, coils or balls. Centering can take from 1-2 minutes, to hours depending on the size of the ball. For a ball of clay a kilo heavy, it takes me 40 minutes to perfectly center the clay.
8)Making the pot- Making the pot itself takes maximum 5-15 minutes.
9)Drying- Proper Drying can take a week.
10) Bisque firing- A bisque firing makes a pot workable and stable. It takes a day or two.
11)Glazing- Glazing can take up to an hour per pot. Most of my glaze work is done in 5 minutes per pot.
12) Drying- The glazes need to dry, otherwise they develop glaze defects like crawling etc. 1-2 days
13) Kiln loading- loading a kiln is an art, with an improperly loaded kiln giving defective results.1 hour - 5 days
14)Firing- The firing itself can stretch from a day to a month.

In the 14 states illustrated, the potter actually directly handles the pot’s “look” in two stages, making the pot, and glazing. The rest of the states influence the pot, but indirectly. If they are incorrectly done, they will yield a bad pot, and properly done would yield a good pot. Thus it could be said that making a pot is a journey, and not just a end product. Similarly, it can be argued that amongst the greatest catalysts for development in ceramic forms, the Japanese Tea Ceremony is itself a process oriented performance art.
Abstract Expressionism was an American artistic movement that mixed the radical subjectivity and powerful lines and colour of expressionism, exemplified by artists like Munch, Kandinsky, Chagall, and Otto Dix, and combined it with Abstractions pioneered by cubists (Gris, Braque, Picasso), Die Stijl (Mondrian, Doesburg), and Futurists (Boccioni, Goncharova). In making a mixture of these different styles, Abstract Expressionists came up with a style that was expressive, powerful and which symbolised the rise of America’s ascendancy in the art world. Artists like Motherwell, De Kooning, Pollock, Rothko and Hoffman took the art form and constantly explored their consciousness, and making strong marks on the canvas or the workbench.
It can be argued that each pot is a work of abstract expressionism. While throwing a pot on a wheel, the pot is extremely response to every movement of the potter. This starts off before the artist begins to throw the pot. The first step taken is to center the pot. For right handed potters, the wheel spins in the anticlockwise direction. The left and is used to press in the clay horizontally, and the right hand presses in the clay downward. The centering is a very important part of throwing, because a poorly centered pot will be off balance and, indeed, off centre. Also important is the fact that during this stage, the width of the pot’s base is determined. All these things are done by the simple actions of pushing inside or downwards. Next, using only tactile feedback, a hole is made in the centered clay’s top. This hole is then widened using upwards and outward motions of the hand. All these times, the traces of the potters hand remains as throwing marks. These dry into the pot unless expressly taken out.
It has to be kept into consideration though, that wheel throwing is not the only method to make pots. Entire styles are based on hand building of pots. Case in point being pinch pots. Traditionally Raku pots are pinch pots. A pinch pot is made by pinching into a ball of clay to make a bowl. This leads to a bowl showing the pinch marks and finger marks of the potter.
In both of these cases, the potter’s hand marks are intrinsic parts of the pots, and a major part
of using and appreciating the potis the visual and tactile exploration of the pot. This not only leads to a more well rounded experience of using the pot, but also leads to a better understanding of the pot and how it behaves. This step is an important step in the tea ceremony. The creation of the marks on the pot itself is strongly reminiscent of the action paintings of Pollock , and the visual exploration of the pot is analogous to the exploration of a Pollock. The piece is the actual account of the creators movements and actions, and can not be appreciated in proper gravity without the observation and analysis of each mark as a part of the greater scheme.
The rise of studio pottery in America was engineered by a handful of artists, including Arneson, Voulkos, and Soldner. Arneson used ceramics as a form of sculpture, whereas Voulkos and Soldner used Japanese techniques to make ceramic forms, that stood on the fine line between pots and sculptures. Raku and wood firing were brought into the consciousness of the American masses, and began to flourish. On the other side of the pond, Leach and Hamada had made made considerable inroads into the traditional pottery market with their Mingei influenced pots. The confluence of the european and the american pottery lead to the ceramic world today.
We have established that
1) “Art pottery” is comprised of informed and well examined technical and aesthetic judgements that may, or may not, be based on conventional pottery aesthetics.
2) Pottery, specially “Art Pottery”, is a process based art, and can be seen to have abstract expressionist tendencies.
We still haven’t touched on the question of why do wood firing in an Anagama (or any tunnel/ chambered wood-fired kiln for that matter). To answer that question, we will need to keep in mind the two assertions that I proposed in the previous paragraph.
As a part of my work with George, the wood firing potter in Payette, Id., I kept a diary to record my daily activities at his pottery. Presented here are pertinent extracts from the first four days. The next 4 days were taken up by the firing itself, which shall be discussed in some detail later.

Day 2: Wadding: 50% fireclay, 50% grain. Make lots of wadding.
Loaded kiln back. Rougher wadding on the supports. 3 levels.
Chopped wood into smaller pieces for firing.
Cleaned kiln surroundings to make circling it easier.
Chipped off old wadding off slabs.
Applied lots of wadding.
Day 3: Wadding tends to fall off. Removed and stuck on new batch of wadding on most pots of kiln load.
Loading Kiln. Anagama chamber almost full.
Make wind shelter around shed.
Carried wood for preheating.
Day 4: Increased height of chimney.
After the first 4 days, the remainder of my trip, 4 days, were taken over by 12-15 hour shifts at the kiln, stoking, and making sure that everything was as it was supposed to be. A log that I kept of the kiln temperature and the time is affixed in the appendix. After we stopped stoking the kiln, and sealed it up for cooling, we waited a week, and opened it up. The unloading took two days. All in all, two weeks of firing that took a full month of planning, 6 month’s worth of pots, 4 cords of wood, 12-15 hour shifts by 6 people over 4 days. Keep in mind that the kiln we fired was a small Anagama compared to most used by potters.
Diametrically opposite to this style of firing is the gas firings done at the college’s ceramic studio. The loading takes all of 6 hours, and after the gas is switched on, all that needs to be done are checks once in a while to see if everything is going as planned. After the firing is complete, the gas is stopped, and the kiln cools in a day and the pots are out. Making concessions for the size of the kiln, in my opinion, it is still a more easier, economic, and in all seriousness, sane method of doing a firing.
However, an artist is not often accused of sanity. In this case, let us refer back to my twin assertions. There have to be informed and well examined choices, and pottery is process art, with elements of abstract expressionism. Now we need to look at the disadvantages of wood firing.
It is Highly Inconvenient: The amount of wood that needs to be stoked is prodigious, and on top of that, the kiln has to be stoked at regular intervals. Thus there needs to be considerable manpower at the disposal of the potter.
It is Highly Expensive: The wood alone puts a steep cost on the firing. On top of that, the slabs used inside the kiln are made from a special compound, and are very expensive. They also progressively disintegrate with each firing. The kiln also gets damaged with every firing, due to excessive deposition of ash glaze on the walls.
It is Very Unpredictable: There are many ways to predict how the glaze will act on a pot in an Anagama kiln, however, it is never possible to predict with absolute confidence the final “look” of the pot. The kiln is a very dynamic system, and a simple error like putting too much wood in too fast could make it a reducing kiln, drastically changing the pots. In addition, a misplaced stoke could cause a pot to tumble off the wadding, or move it and make it stick to another pot, both of which are irreversible.
On the other hand, the advantages are:
The Ash Glaze: The ash glaze, and its characteristic effects are impossible to achieve on any other kind of kiln.
The Unpredictability: The very thing that is a disadvantage can also be an advantage, with the fire and the ash taking control of the pot’s “look”. This gives the potter a measure of uncertainty in his work.
The Process: Wood firing is the ultimate expression of process in the firing. The firing is dependent on every detail of the process itself, and each little thing leads to massive differences in the outcome.
The brief overview of the alternatives available to a potter are shown above. Each potter can do a cost benefit analysis of sorts, to see if wood firing is for him/ her. From my personal experience, and from my interactions with practicing potters, I have gathered that not everyone can work with a wood fired kiln. It is not an issue of technical acumen, but of personality and preference. In a manner, Anagama  firing strikes at the root of pottery. It is less of a profession as much as it is a way of life. A potter can not go back home and proclaim day off, pottery is too involved a process. It is a very hands on method of production, and the potter stays in the mode of production most of the time. A wood fired kiln is, in a manner, the potters style of pottery, for it is the most involved style of firing. It is hard, takes skill, patience and a personality that is rarely found amongst people who are not “in” on pottery.
Moving back to my personal experience with Anagama, after working at the studio for a week, and coming back, The first question I asked myself was, “Why?” Indeed, it was, and is a perfectly valid question. In the 4 days of the firing itself, I slept for 7-8 hours, and spent all my waking hours in the kiln, save for lunch and dinner breaks. In process, I lifted 4 cords of wood to and fro, personally chopped a cord, endured extreme heat and cold, built a 7 foot high chimney in a day, wadded and loaded an entire kiln. Was it worth it? I would say, yes. But then I am also an art student so it was akin to a class. And in addition, I got to take two weeks off from college. I pondered why would any sane person ever go through such a long convoluted process like a wood fired kiln, when there is a perfectly decent, cheaper, safer, and more environmentally friendly alternative in a gas or electric kiln. I talked to George, the potter I was working with, and I talked to my Japanese contacts. All were surprisingly cryptic on this matter, especially considering the wealth of information they provided to me on other matters. And one day, out of nowhere, which I was working in our college’s ceramic studio, I came across the answer, at least an answer that works for me.
An Anagama firing atleast in pottery is the ultimate expression of renunciation. It encourages you to do your duty and not worry about the results. The results of an Anagama are unexpected. They may not be perfect every time, but then, if everything was perfect, what challenge would there be in it? It forces the potter to do everything necessary to get the results. Most processes force you to get the results too, but in this case, the results are given to you, as a reward for your work. Somebody at George’s kiln said that each kiln unloading is like christmas. I would tend to agree.

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.