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Basic Kiln Designs

The history of kilns spans thousands of years and ranges from the firing pit and ancient Japanese noborigama kilns to 18th century English bottle kilns or the modern computer controlled electric kilns of today.



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Depending on your practice as a ceramic artist or potter, your kiln needs will vary greatly.
A raku artist will need a raku kiln for quick low firing and post-firing reduction, a porcelain artist might fire in an electric kiln and a salt-glazer or soda-firer will need a salt-kiln.

Some issues though remain similar to most kiln types, such as stacking economically, knowing about safety devices, temperature measurement or burners.
Other questions such as the lifetime of kiln furniture and bricks also have to be considered.

Whatever ceramic technique involved, be it wood-firing, raku, paperclay, porcelain, vessel, sculpture or something else, the material must be transformed in the fire to achieve the durability expected of the ceramic object.
If you are considering buying a kiln, a search on any one of the major search engines for your region will yield a couple of companies making kilns near you or why not build one yourself?

PIT KILNS

The earliest kilns were no more than the hearths used by primitive peoples for cooking, warmth, light, and protection.
In fact, very simple pit kilns are still in use today.
Clay has been used since prehistory to create vessels for storing foods and grains in, bowls to eat out of, figurines, representations of animals and people, but the date of the actual discovery of the firing process is unknown.
When agriculture began, it is generally thought of as the origin of fired clay objects, approximately 10,000 years ago.
These early farmers needed containers for seeds, harvested foods to be stored in and for water transportation and storage.
Fired clay served these needs well, and was locally available and easy to form.
The earliest kilns were nothing more than a shallow pit dug in the ground.
Pottery was loosely stacked on top of each other.
Combustible materials were placed around and above the pottery and the fire was allowed to burn down.
After cooling, the pots were cleaned of the ash and residue and were then used.
Pots fired in this way were very fragile and porous due to the low temperatures possible in such a firing, 1000°-1200° Fahrenheit.
At this low temperature glazing is not possible and was not discovered until much later.
Advantages of this type of firing are its relative ease of construction and low cost.
Disadvantages are the low temperature limitations and the fragility of the ware.
Also, many pieces broke during the firing process due to the erratic nature of the firing and poor insulation.

The minimum requirements for a fuel burning kiln are:

INSULATION
LOADING AREA
FUEL
OXYGEN

Although the pit kiln does not appear to be much of a kiln, it is considered one.
Look at the picture below and notice that this kiln does have insulation, which is the earth itself.
Earth is a decent insulator, is not flammable, and is certainly abundant.
The loading area is the pit itself and the fuel is any flammable organic material such as wood, straw, or manure; and the oxygen is available in the air surrounding the kiln.
So, simple as it is, this kiln meets the requirements.
Pit Kiln
The design defects of this kiln are quite obvious, because the kiln is upside down!
The insulation should be on top, and the fuel should be on the bottom.
The pit kiln looses most of its heat out the top.
Early potters tried putting the fuel at the bottom of the firing, but found that as the fire burned down, the logs would fall, and so would the pottery, breaking everything and that is why they had to put the fuel on top.
They had no architectural technology enabling them to construct an arch.
With the rise of settled agricultural communities construction techniques improved and better kilns were built.

BEEHIVE KILNS

Beehive KilnBarrel Kiln

The BEEHIVE kiln was the first kiln constructed that looks like what we consider a kiln today.
See the kiln cross sectional diagram at left, and notice that now the fuel and fire are below the ware, the insulation, in the form of an arch is on top, retaining the heat better.
The pots are stacked within this chamber allowing greater retention of heat.
The enclosing of the kiln presented a problem, the oxygen access was restricted and without ventilation this kiln will not burn properly and this is why a hole at the top of the kiln, known as a FLUE was included in the beehive design.
The DAMPER is the device that regulates the size of the opening of the flue.
Oxygen does not enter the flue, instead it exits the flue by nature of the tendency of heat to rise.
As the fire burns and the kiln gets hotter the hot air rises and leaves the kiln through the flue.
And the cool air enters the bottom at the FIREBOX.

CLIMBING KILNS

Climbing Kiln
An interesting variation on the beehive design was first built in China around 500 A. D.
This is called the CLIMBING KILN, stepped kiln or hillside kiln.
This kiln utilized the basic format of the beehive, but multiplied the chambers so that total kiln capacity could be increased.
This modification worked well in villages where pottery making was the primary activity, and where a large volume of pottery needed to be fired at once.
In the diagram above each chamber has the arch construction typical of the beehive, but the chambers are joined so that the draft passes through from one chamber to the next.
After the kiln is loaded, the fire is lit in the bottom chamber's firebox.
The heat rises through the first chamber, and rather than passing out a flue at the top of this chamber, the heat is cycled down and into the opening at the base of chamber number two.
After the first chamber has been fired to its ultimate temperature, the potters begin stoking firewood into the firebox at the base of chamber two.
The heat follows the same round about path as before, rising, then falling, and entering the base of chamber three.
This was designed to utilize the tendency of the heat of a fire made at the entrance of the lower end of the kiln to go upward.

This process is continued until all chambers have reached temperature.
Notice that the draft of the kiln is eventually up even though along the way it has taken several downwards turns.
Such a kiln is referred to as a DOWNDRAFT kiln even though the ultimate draft is UP.
It is the observation that the draft is DOWN during part of the cycle that causes this kiln to be called a DOWNDRAFT.

The biggest disadvantage of this kiln design is that large quantities of pottery are required to fill these huge kilns, making it an impracticable design for the individual potter.
Its big advantage is that large quantities of work can be processed at once making it ideal for pottery communities.
This kiln was first built in China, probably to increase the volume of pottery available for trade.
A significant difference in these kilns is that they were able to reach higher temperatures than any kilns before.
The recycling of waste heat, the increased thickness of the walls necessary to reinforce these huge chambers and the multiple fireboxes all combined to cause higher temperatures to result.

It was in such kilns that the earliest stonewares and porcelains were developed.
Certainly not on purpose initially but over time the art of porcelain manufacture was perfected by the Chinese potters and held secret for over 700 years.
These kilns were huge, often 10-12 chambers, and therefore difficult to conceal.
Eventually neighboring villages began to copy the design, and the concept spread out of China to Korea, Japan and ultimately the West.
However, by the time this idea traveled to the United States pottery villages were all but extinct and their role was taken over by machine made pottery.
One additional interesting feature of this kiln is the use of saggar boxes, which were used to protect the pottery from flying wood ash.
These saggar boxes, which were made of clay, are indicated in the diagram as the square boxes stacked in each of the chambers.
Without these protective boxes the pottery would have been subject to attack by the wood ash, which at these higher temperatures would form glaze and stick pieces together.
Ancient kilns found, fires the imagination.

DRAGON KILN

YulingTing Song Dynasty Dragon Kiln Just west of Wuyi, two long narrow buildings snake up facing hillsides like dragons which in fact they were built to resemble.
Dragon kilns originated in south China about 2,000 years ago, and this site is all that remains of two of the Song Dynasty’s longest dragon kilns (2m by 113m).
Shards of Song Dynasty porcelain and pottery molds are so numerous that locals use them to build field walls.

The extinct dragons lie with heads at the base of the hill and tails at the top.
Each kiln had a fire bore, kiln house, and smoke outlet, and could produce about 80,000 porcelain pieces at a shot.
The wares were fired at 1100? to 1200? for forty hours or so, and cooled for another 24 before they could be taken out to produce the exotic gold-accented black-glazed porcelain so prized by the Japanese and Koreans

But at which kilns were the works for royals kneaded and fired?
The answer remained elusive as ancient imperial annals recorded that the one and only kiln complex with the five centuries' patronage from two dynasties was destroyed in a peasants' rebellion in the late 19th century.

The restored version later disappeared without even an oral or paper trail, following the downfall of the Qing Dynasty in 1911.

Since the founding of New China in 1949, the porcelain town sprawled.

Houses and porcelain factories were built and roads were opened in Jingdezhen.

It was not until the early 1980s that researchers stumbled upon the possible ruins of the royal kiln complex in the central area of Jingdezhen.

Liu Xinyuan, professor of ceramics history with a local college, spent years searching every possible building site.
In early 1982, he was walking by a construction site when he watched piles of broken porcelain pieces fall from a bulldozer.

The construction workers had seen too many broken pieces in the area to pay attention to them.
But Liu was different.
With his knowledge and a pair of keen eyes, he believed the broken pieces he saw could only have come from the royal kiln complex.
It was said that Liu practically had to stand in front of a bulldozer to stop the construction.

But extensive excavations at the ruins started some 20 years later in 2001.

A group of archaeologists from Peking University, Jingdezhen and provincial archaeological institutions have kept very busy over the past four years on the northeastern and southern part of the kiln complex site.

Kiln shapes

Judging from their position and age, the archaeologists concluded that the size of the royal kiln complex was much larger in the early stage of the Ming Dynasty than in the Qing Dynasty.

Two shapes of kilns were unearthed, one resembling a gourd and the other steamed bread.

Excavated in the northeastern part of the site, the Gourd Kiln Hulu Yao is surrounded by a brick wall. The kiln is huge in size, and the kiln bed slightly rises from the front to the back.

Drawing on the best functions of the common Dragon Kiln (Long Yao) and the Steamed Bread Kiln (Mantou Yao), the Gourd Kiln enabled the craftsmen to better control the fluctuation of temperature.

"The Gourd Kiln was already common during the Yuan Dynasty (1279-1368), but this is the first time we have unearthed this type of kiln in the royal kiln site," Quan said.

Another 14 so-called smaller Steamed Bread Kilns were also excavated in the southern part of the site.
These were built with small bricks.

The excavation showed that the Gourd Kiln was mainly used before the reign of Emperor Xuanzong (1426-35) of the Ming Dynasty.
Afterwards, the Steamed Bread Kiln was more utilized.

Though both kilns were already commonly used before the Ming Dynasty, they were not simple copies, but went through further reconstruction at the time.

What greatly aroused the archaeologists' interests was the "kiln sweats."

The glass-like sinter produced in the high-temperature chemical reaction, between the inside walls and the substances volatilizing from the burning wood and pottery glaze, hung inside the walls of some Steamed Bread Kilns, accumulating in thick layers and indicating a high firing temperature of over 1,200 C.
But such a phenomenon is not widespread among all Steamed Bread Kilns.

"It is absolutely not accidental.
We think work divisions among the kilns already existed at the time," said Quan.

Some kilns were used for firing the small blue utensils of qingci celadon requiring a high temperature.
Comparatively, in some low-temperature kilns, the colorful ceramic glaze could be fired.

"The wide varieties of exquisite pottery of the Ming Dynasty should not be separated from its advanced kiln structure and the reasonable work division of the kilns," said Quan.

During the Ming Dynasty, in order to avoid any copies of the royal tributes from the civilian kilns, the defect products and the works failing to meet imperial standards were all shattered and buried with discarded kiln tools and slag.

A Dragon Kiln

Dragon Kiln at Green Glaze Factory

At the Green Glaze Factory, behind the modern buildings in a secluded green and leafy plot, we find the ancient climbing kiln which used to fire the factory's output. This is the "Dragon Kiln" we were promised, and standing at its base, watching it curve snake-like up the steep hill, I understand the name.
It is easy to imagine it firing, with workers stoking the many ports, emitting smoke and flame up the hillside as if it were in truth a living dragon.

It was built as one continuous chamber with internal "steps" for stacking pots. There were three entrances for the potters to get inside to stack, and numerous stoke holes.
It's about two-thirds dug into the hillside, one-third arched roof above ground level, and it extends for some 100 feet.

No one's fired it for at least 75 years; near the top one whole section has caved in completely.
When we inquire about firing, our factory hosts dismiss the idea. Today they have modern tunnel kilns, where cars laden with pots roll on tracks into the kiln at one end and emerge at the other end with fired pots ready for shipping.

The Dragon Kiln took something like two weeks just to fire, never mind the amount of wood it needed.

The old dragon kiln at the Green Glaze Factory sprawls up the hill out of sight.
Part way up the second stacking entrance is visible as a brick arch.
Dragon kiln, which is 50 meters long and more than four feet high.
Made of bricks, its roof is designed with dragon’s scales. The jars are cooked in 1200°C heat, vitrifying the sand and clay to form a metal glass-like surface that resonates like a bell when struck by stone.
After three days in the kiln, it takes another 20-22 hours for the jars to sufficiently harden
Inside the Dragon Kiln
In the long history of porcelain-making, Jingdezhen's porcelain-firing kiln was a good combination of both Dragon shaped kilns popular in the south and U-shaped kilns popular in the north and was then developed into Jingdezhen Kiln which was later called the egg-shaped kiln.
This marked the highest level of that time in the building of traditional kilns & the techniques of firing.

The famous Jingdezhen kiln was set up and improved by drawing from the best of the Dragon Kiln, the Stairs Kiln and the Gourd Kiln and the Mantou Kiln in the North.

It has no type of shape bricks, complicated fume extractors, or any auxiliary equipment in its structure.
The gaseous atmosphere and temperature in the kiln is appropriately controlled only by judging the time and amount of wood to throw on the fire.
It has overcome the shortcomings of the Dragon Kiln and the Stairs Kiln both of which have difficulties in heating up the tail sections, and the high temperature difference of the Gourd Kiln.
Therefore, the Jingdezhen Kiln has held an important position in the history of porcelain kilns in China

Nowadays, few of the porcelain workshops in Jingdezhen are set up completely adhering to these design criteria.
The majority of traditional kilns and workshops have been dismantled in the process of industrialization and urbanization, while old builders have passed away.
Statistics show that only 23 traditional workshops exist in Jingdezhen.
The traditional techniques and architecture are in imminent need of rescue.

NATURAL GAS KILNS

The most common kiln design utilized by contemporary potters is the natural gas UPDRAFT kiln.
Notice in the diagram at right how very similar this kiln design is to the BEEHIVE kiln.
Basically, it is the same in all respects. Rather than using firewood, natural gas is the fuel.
We now have better quality insulating brick, but otherwise nothing has really changed.
Note that the damper and flue are in the same places and have the same function.
The updraft design is not the only one used with natural gas however.
Many natural gas kilns are based on a variation of the downdraft design described above.
Advantages of natural gas as a fuel are that it is environmentally desirable in that it produces very low levels of pollution, and that the fuel is relatively inexpensive compared to electricity.
In some parts of the country propane is more commonly used as a fuel, however propane is heavier than air.
This means that if the flame should get blown out, then care must be taken to dissipate the gas which will stay at the bottom of the kiln before relighting, otherwise an explosion can result.
Natural gas will disperse on its own since it is lighter than air.

Updraft Kiln

ELECTRIC KILNS

The electric kiln is the only really new kiln technology of the 20th century.
Instead of a burning fuel, these operate by radiant heat generated from an electrical current passing through coiled wires.
A toaster operates on the same principle.
Since these kilns have no fireboxes and no burning inside, they have no need of a damper or flue, since no draft in necessary.
Thus electric kilns have no hole at top.
They are neither updraft or downdraft, more like NO DRAFT kilns.
What they share with the fuel-burning kilns is insulation and a loading area, but not a fuel or a need for an oxygen draft.
Thus OXIDATION firing is the firing of choice in the electric kiln.
Most modern electric kilns are equipped with electronic shut off devices, called kiln sitters, to monitor the firing process.

FIRING DIFFERENCES: ELECTRIC vs. FUEL BURNING

These two types of kilns give quite different results in firing.
Remember that fuel-burning kilns require oxygen; electric kilns do not.
A fuel-burning kiln(fired with its damper open, providing the kiln with adequate draft) will fire with results identical to an electric kiln.
However, partially closing the damper during the firing process will have a dramatic effect on glaze colors.
Here's how: A FUEL is a material that can combine with OXYGEN to create a fire in a process called COMBUSTION.
Generally, the fuel takes the oxygen from the atmosphere during the firing.
If the damper is partially closed, the draft is reduced, providing the fuel with not enough oxygen to combust completely.
The fuel will then try (chemically) to 'find' the oxygen it 'needs' from any other source in the kiln.
What other sources are there?
The clay and glaze materials contain oxygen in the form of the metallic oxides such as silicon dioxide, cobalt oxide, iron oxide, copper oxide, etc.
A chemical reaction takes place such as:

Note that the original form of iron oxide (which is rust red in color) contains two atoms of iron to every 3 of oxygen.
During the firing process, the fuel has reduced two oxygen atoms from the iron, leaving us with a new form of iron oxide (which is jade green in color), in which the ratio is 1:1.
The only reason we care about any of this as potters, is that these two forms of iron are different colors.
This process results in forms of the metallic oxides that are REDUCED in oxygen.
A similar reaction takes place with the other coloring oxides as well, explaining why glaze colors behave so differently in gas and electric kilns.
We have come to call this chemical process REDUCTION, and this firing process, REDUCTION FIRING.
In an electric kiln, in contrast, there is no draft, no oxygen demand, and no damper.
Thus closing it is impossible; it does not exist.
So, therefore, reduction firing is impossible in an electric kiln unless the kiln itself is on fire or a combustible material is introduced into the electric kiln.
Firings in which the oxygen levels in the oxides are not reduced are termed OXIDATION firings, referring to the observation that the oxygen in not changed.
Colors are thus more predictable in an electric kiln (this is good and bad).
To summarize, a fuel-burning kiln is capable of REDUCTION or OXIDATION depending of the damper position.
An electric kiln is capable of only OXIDATION.

Wisdom is the reward you get from a lifetime of listening, when you would have preferred to talk.



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