Hello, manny13-ga!
The gray-black clay tiles that are common in China are produced in an
oxygen-starved, or reduction atmosphere, which results in
"carbon-soaking" of the fired material. If you hope to achieve the
blackened effect, you will have to create a very sooty atmosphere with
very little oxygen to fan the fire.
I have found descriptions of several simple firing methods that can
be tweaked and adapted to suit your "Backyard project". All of them
allow you to create the same atmosphere....a hot fire that is then
allowed to smolder for a long period to create the black look you
desire.
The primary factor to remember in achieving blackened pottery is that
some type of lid or material must be place over the "kiln", pit, or
whatever you choose to use in order to starve the atmosphere of oxygen
and allow for carbon coating.
Of course, I cannot guarantee the end result! You will have to
experiment until you have discovered the optimum amount of time for
smoldering. I also imagine the type of clay you are using will be a
determining factor in how blackened your tiles will become. The use of
coconut waste as fuel is also an unusual variable that will affect the
process.
Your question is extremely unique and interesting. While I cannot
tell you exactly how to reconstruct your "kiln" since you have to work
with the materials at hand, the following information should help you
understand the process of creating a "reduction atmosphere" and allow
you to gather some ideas to create a firing process that might work to
produce blackened tiles.
There is quite a bit of information on reduction, but most of it
centers on firing techniques which also use glazes to help achieve
certain results. I have sifted through numerous sources and found the
information that is most applicable to your "primitive" situation.
Again, I want to reiterate that you will have to experiment with the
technique and the materials you have at hand. Hopefully, you will
achieve the desired results!!!
====
"There are two types of fires - reducing and oxidizing. The former
removes oxygen while the latter, a smokeless fire, adds it. Reduction
and oxidation change the color of the fired clay and gave early
potters their palette of red, buff, and black."
From "Pottery." The Columbia Encyclopedia, Sixth Edition. 2001
http://www.bartleby.com/65/po/pottery.html
====
The reduction technique is also used to achieve black colors in Raku
pottery:
"The hot clay work is placed in combustible material and the fire
covered with a lid, to produce an oxygen deficient atmosphere (
reduction). This reduction step produces the wonderful metallic
lustres and inky blacks.."
From "Malvin Malkin." Online Exhibitions.
http://www.vandopgallery.com/malkin.html
====
From a description of Pottery from Tenth Century Britain:
"Some pots would have been almost black due to a process known as
reduction. This happens when oxygen is excluded from the kiln by
clamping off any airways, and leaving it for a period of time."
From "Pottery." http://www.regia.org/pottery.htm
========================
OXIDATION VS. REDUCTION
========================
Oxidation:
"This is a reference to the quality of kiln atmosphere during firing.
It means there is plenty of oxygen in the kiln and that the fuel is
burning completely. It's important because things react to oxygen and
carbon in the kiln. Such as oxides. Copper is the best example. Copper
when oxidized like a copper roof or copper bells on a church is green.
Copper when polished and all the copper oxide is removed is metallic
golden/ reddish color. So copper oxide in and oxidation atmosphere
stays as copper oxide, therefore, staying green. The same copper in a
reduction atmosphere looses it's oxygen because the atmosphere is
starving for it, and turns back to it's original copper color,
reddish. Oxidation atmospheres also affect the color of the fired clay
and it's strength, it is stronger. This strength is due to the
formation of mullite crystals. These crystals form a thin spidery
latus that helps resist thermal and impact shock. Also, problems like
bloating are eliminated because no unburnt carbon can become trapped
in the clay because there is not unburnt carbon."
Reduction:
"The opposite of Oxidation. This is a kiln atmosphere where the fuel
is not burning completely from lack of oxygen. It is induced by
decreasing the air to the fuel or increasing the fuel to the air
ratio, and/or reducing the amount of air circulation by restricting
the chimney flue. Reduction can be judged by ignorant back-pressure
inside the kiln, (this is air pressure build up from the lack of
airflow), as seen by flames shooting out any holes into the kiln,
(chimney, spy-holes, leaks in the seams and bricks). This flame is
yellowish or greenish and can be smoky, however it doesn't have to be
to achieve a good reduction atmosphere. Reduction starves the oxieds
of oxygen creating different color responses. It also can lead to
carbon trapping in the case of Shinos and clay color. It can effect
cone readings and inhibit Mullite crystal formation."
From "Vocabulary." http://www.geocities.com/Hut4L0rn/pottvocab.htm
====================================================
CHINESE "REDUCTION FIRING" PROCESS AND KILN DIAGRAM
====================================================
A good description of the process and a diagram of the kiln used in
the reduction process can be found under "Shang Developments" which I
have excerpted from the following article:
"Chronology of Chinese Kiln Development."
http://mcel.pacificu.edu/as/students/ceramics/kiln.html
Shang Developments
"Shang Dynasty potters made the first step toward what would later
become the down draft kiln. They developed a kiln with flues which
exited from the bottom of the ware chamber and up through the walls to
the domed top. The first stage of the firing began with a low fire and
the central and side flues all open. This slow heating was continued
long enough for the kiln and ware to dry, and for a sufficient heat
rise to begin the next stage of the firing. Next the dampers were
completely closed over the floor vents at the sides of the kiln near
the side flues. This caused all the heat to blast up through the
center of the kiln and out the central flue. Immediately after this
something was placed, possibly a clay tile, over the central flue to
force the heat to circulate back down through the ware and up through
the side flues. This was an ingenious development which allowed a more
even heat flow at higher temperatures. This allowed potters of the
shang period to approach high fire temperatures and to produce the
first glazes, which were either ash or feldspathic in nature.
*****
"Also of great significance was the advance toward reduction
techniques. The fast heat rise and the closing of the dampers together
produced an oxygen starved atmosphere in the kiln, to such a degree,
in fact, that the nature of the firing was one of carbon-soaking. In
this type of atmosphere the clay bodies are turned gray to black from
the excessive amounts of carbon that is not completely combusted."
*****
The Reduction Process:
-----------------------------
"Reduction is a term describing the inner atmosphere of a kiln when
the ratio of fuel to oxygen is such that there is insufficient oxygen
to accommodate the amount of fuel available for combustion. This is
accomplished either by limiting the draft, or by increasing fuel. When
done correctly the heat in the kiln causes the release of carbon from
the chemical compounds in the fuel and clay body while, due to the
lack of oxygen, much of the carbon experiences incomplete combustion.
There are various possible results to this firing technique.
*** Carbon-soaking is an extreme reductions process in which the
atmosphere is so carbon-saturated that the pottery, if unglazed, will
turn permanently gray to black in color.
===================================================
SOME OTHER FIRING TECHNIQUES FOR CREATING REDUCTION
===================================================
Building a "Reduction Pit"
==========================
http://www.stlcc.cc.mo.us/fv/users/lmosley/Firing/creatingreductionpit.html
Pit Firing:
*First, make a shallow pit or low enclosure of common brick, and build
a fire in order to produce a bed of hot coals.
*Preheat the pots on the edge of the pit, and gradually moved them
closer to the coals.
*When they have reached about 650° F, it is safe to put them into the
coals and begin to build a fire over them.
*Once the pots reach about 850-1100° F, or a dull red glow, smother
them with combustible material such as sawdust, or dried cow/horse
dung to create a reduction atmosphere which will turn them black.
*After about 15-20 minutes, they may be taken out and allowed to cool.
==
From "Pit Firing." Ceramics Today
http://www.ceramicstoday.com/articles/090197.htm
"Depending on the amount of work to be fired, a pit of the appropriate
size is dug. A bed of dry leaves and twigs and possibly coal, which
will burn slowly, is placed at the bottom of the pit and the pottery
placed on top of this. The work is then covered with more leaves and
twigs and dung, if available, building up a mound over the pieces.
Once the stacking process is finished, the pile can be lit around the
edges and left to smolder for several hours, if not until the next
day. Towards the end of the burning process, it is possible to bury
the pit in earth or sand, which will cut off the oxygen supply and
create a strong reducing atmosphere inside the mound.
Sawdust Firing
==============
http://www.stlcc.cc.mo.us/fv/users/lmosley/Firing/creatingreductionsawdust.html
(This technique might also work for you, especially if you leave the
tiles in the loosely-lidded can to let them smolder until black)
* Make sure it is not accessible to children or pets who might touch
the hot can if it will be left unattended.
* Drill 1 inch holes all over a metal garbage can, about 6 inches
apart.
* Put in a bottom layer of sawdust, and then begin layering pots and
sawdust. Metal hardware cloth can be placed in between to keep the
pots from settling onto one another as the sawdust burns away.
* Other combustibles may be added (see saggar firing below). The finer
the sawdust the slower the burn (lack of oxygen).
* Light the top of the sawdust with a bit of newspaper and kindling.
Let it flame until it's going well and then put the lid on loosely
(prop with bricks). It should smolder but not flame to fire at a safe
speed. . Let it burn down on its own, about 24 hours.
The "Soenga" Hole
==================
(This is a type of pit firing, using firewood and pine needles to
create a smoky atmosphere that blackens the pottery. Unfortunately,
there are no specific details of the process)
"Molelos is a village of the municipality of Tondela, in the district
of Viseu, that is located in the region of Beira Alta, in an area
flanked by two ridges of mountains at the central region of Portugal
(the mountain of Caramulo and the mountain of Estrela).
* "This village is one of the last places where ceramics is still
manufactured by the reduction cooking process, which allows the
obtaining of pieces of black clay."
"This process consists in the complete obstruction of the earthenware
at the end of the cooking creating an atmosphere loaded with carbon.
It deposits all over the pieces and a physical-chemical transformation
of the metallic oxides of the clay takes place due to the carbon
monoxide. This is done in a little deep hole named "Soenga"."
* "Nowadays there are only few potters who use this traditional
cooking process in the soil, with firewood and pine tree needles,
using a firewood oven instead, more practical and functional."
During the process of "Soenga", the potter's experienced eyes observe
the colour of the pieces, to determine when the process is finished.
** "At the same time, he puts in green branches quickly to create the
smoke, which prevented to escape, impregnates the pieces that in this
way acquire the characteristic black-leaded colour."
From "António, The Potter of Molelos Black Earthenware."
http://www.verytypical.com/artisan_details.asp?Cod_Artesao=9
Korean Wood Firing
===================
"One last category of wood-fired kilns in Korea is a small tube kiln
that produces earthenware as black as the wares of Oaxaca in southern
Mexico. Chung Jin-Kook, a studio potter near Seoul, has two such smoke
kilns. Instead of traditional rice steamers and roof tiles, Chung
makes Silla-inspired wares and patio furniture. His kilns are fired in
a day, and then, as in the Silla kilns, cooled to a dull red heat,
stuffed with green pine needles, and sealed."
"Korean Wood Firing: A Ceramics Monthly Portfolio," by Everette
Busbee.
http://www.jeonju.ac.kr/~busbee/ceramics/ceramics_monthly/korean_article/korean_cover.html
Black Pottery from New Mexico:
==============================
Anita Suazo, Santa Clara:
"In firing, to make a black pottery piece, the fire must be smothered
to create an oxygen free atmosphere, a technique called reduction
firing. The Santa Clara potters accomplish this by covering the
outside "cage" of the kiln with pieces of tin or metal. this protects
the unfired ceramic pieces. Then this is all completely covered with
pulverized horse manure. It is the manure that keeps out all of the
oxygen, and produces the beautiful black finish."
http://www.canyonart.com/sanpots.htm
=====
You might want to read the following article about pit firing and a
"blackening method" with smaller objects placed in cans filled with
horse manure. I know you would not be using small cans for tiles, but
I wanted to further demonstrate the low-oxygen principle.
"13th Annual Sand Canyon Primitive Pottery Workshop."
http://www.goffpottery.com/kelly/kelly.html
"Smudging or Blackening - Maria Montoya Martinez, "The Potter of San
Ildefonso", and her husband, Julian, are credited with the revival of
interest in the pottery of the Pueblo Indians. Black pottery is
identified with Maria and the techniques she used have been documented
in both books and films, however, success in reproducing her black
pottery has been difficult. At Kelly's Place, the items we selected to
blacken were placed in containers (large coffee cans) filled with
horse manure. These containers were placed upside down in the center
of the pit where they would be subjected to the most intense heat. The
horse manure was prevented from flaming due to being deprived of
oxygen. The high carbon environment permits the pottery to be smudged
or blackened by the carbon being absorbed deep into the clay."
=====
Read the following message called "Tile Weirdness" where a tile maker
was discouraged by the black centers in her tile....the result of
unburned carbon, which is exactly what YOU want!
http://www.potters.org/subject67696.htm
"Tile manufacturers use this property to make tiles go from red edges
to black. The black is unburnt carbon. In red clays it is probably
also reduced iron bearing clay. Only the edges of your tiles got
enough air to burn off the everpresent organic material."
=====
For a good overview of firing outcomes depending on type of clay and
the kiln atmosphere, read:
"PPP: Influences on Clay Color," by Jeff Zamek. Ceramic Industry
(2002)
http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,88297,00.html
======================
AN INTERESTING WEBSITE
======================
A type of Celtic pottery known as "Black Burnished Ware" is shown on
Bill Crumbleholme's Pottery website at
http://dspace.dial.pipex.com/crumbleholme/bbw.htm
See the photo of pottery using the reduction method:
"These are a few of the pots fired in an Iron Age type pit during a
weekend in February 2003 at Viables in Basingstoke with Clair
Lamerton. A high level of reduction and carbonisation was achieved so
the pots are well blackened. Several pots were also made with locally
dug clays, with crushed flint temper.
An excavation of the Bestwall kilns are shown on the following page.
The potter's attempt did not come out as black as anticipated.
http://dspace.dial.pipex.com/crumbleholme/bbware/bestwallbc.htm
See "One of Bill's BB reproductions - not so black as it should be."
====
You may be able to use the reduction technique with your existing
kiln if you can sufficiently close it down to create the necessary
oxygen-starved atmosphere. I imagine you will be successful if you
continue to experiment with the amount of heat and length of
smoldering time.
The poverty alleviation project sounds like an extremely interesting
venture. I wish you the best of luck!
Sincerely,
umiat
Google Search Strategy
chinese kiln reduction
Heavy reduction black pottery
how to build a "reduction atmosphere" kiln
chinese black clay tiles AND reduction
"atmosphere reduction" kiln
reduction kiln AND black tiles
firing blackened tiles |
Clarification of Answer by
umiat-ga
on
03 Nov 2003 16:20 PST
Mannie,
I have looked under every resource I can think of for primitive kiln
designs. The primary problem is that there just aren't any detailed
drawings or descriptions available that are applicable to your
situation. There are designs for electric kilns, portable kilns out of
ceramic fibers, gas kilns, etc. but these do not help!
The best I can do is refer you to the more primitive methods (which I
have already done) which are primarily the firing pits, the rural
kilns previously referenced and the Shang period design.
Unfortunately, these are primarily verbal descriptions. A few other
links to non-detailed drawings follow, but they will do no more than
give you a crude picture of the outside of the kiln.
* The first kiln design link I provided from the Shang period which
shows the various stages in draft control is the most detailed drawing
I can find online. It describes (and shows in the diagram) the process
of gradual damping down to create a reduction atmosphere.
http://mcel.pacificu.edu/as/students/ceramics/kiln.html
Perhaps your original domed kiln exploded because it was TOO damped
down. As you can see from the Shang diagram, there is still some
escape flow on the side flues even when the top is capped. If the oven
is sealed too tightly, an explosion is likely. Gases have to escape
even if it is at a minimal level. In lieu of side flues, you can have
some small cracks or holes in the kiln that you can damp and then open
when the top is sealed so the kiln is not so airtight that it will
risk explosion.
As described under "Reduction Process", one way to judge the air
pressure buildup inside the kiln (or the back pressure) is by the
appearance of the flame color from any escape holes. A yellow or
greenish flame with some accumulated smoke is indicative of a
reduction atmosphere inside the kiln.
** More sophisticated kilns have ways to gage temperature that you do
not have. The monitoring of the escaping flame color is the only way I
have read about to judge the atmosphere inside such a primitive kiln.
You said that the pit method DID work for you but was very limited in
production. Is there a possibility you could build a deeper, longer
pit to make production more efficient?
In lieu of that, the Korean "tube kilns" referenced in my answer
might be a design you could adapt. Unfortunately, again there are no
diagrams or detailed pictures.
http://www.jeonju.ac.kr/~busbee/ceramics/ceramics_monthly/korean_article/korean_cover.html
Some very crude kiln diagrams are available on the following webpage:
http://www.stlcc.cc.mo.us/fv/users/lmosley/firing/kiln.html
Some pictures of sawdust and pit kilns are available here:
http://micaweb.com/purple/emine/outdoor.html
Some more links to pit firing methods:
Open pit firing:
http://cator.hsc.edu/~mollusk/ChemArt/pottery/openpit.html
Closed pit firing:
http://cator.hsc.edu/~mollusk/ChemArt/pottery/coveredpit.html
Sawdust firing:
http://cator.hsc.edu/~mollusk/ChemArt/pottery/sawdust.html
==
Some primitive kiln drawings and descriptions are avialable from the
following web tutorial:
http://www.victor.bryant.hemscott.net/histx103.html
Since Victor Bryant seems to be an authority on primitive kilns, it
might not hurt to email him if you cannot figure out a viable
primitive firing method:
victorbryant@ceramicstudies.com
===
You might want to read the section under "Sawdust Firing" on the
following site, followed by a description of the kiln.
http://grafik.sdsu.edu/ceramicsweb/articles/primitive.html
"Sawdust kilns are usually built with common house bricks. The size
depends on the number and size of the works you wish to fire. A common
size is a box about 80 cm cubed., with four brick walls, a brick floor
and sheet metal for a lid. It is probably as well to mortar the bricks
together in construction because too much air entering the kiln tends
not only to burn away(oxidize) the carbon effects (smoking) but can
also cause the pieces to dunt (crack). Metal drums function quite well
as sawdust kilns- at least for a few firings
===
From "Roman Kiln." http://www.potters.org/subject34227.htm
"The roman kiln is described in the book Pottery in Roman Britain by
Guy de
la Bedoyere, published by Shire, ISBN 0 7478 04469 9.
"The basic plan is that an oval shaped area is dug out. Half is formed
into the kiln and the other half, the windward half, is the area for
the stokers to stand and get roasted. The wood is put in the stoking
hole, slowly and only up to half its area, so that sufficient air can
get in and be pulled through the firebox and upwards through the ware.
The ware sits on a perforated clay table above the lower area where
the hot gases circulate. The upper area of ware is then covered with
old crock, turf, etc. It can be closed when 700 or if your lucky 800
degrees have been reached for reduction or left open for oxidation.
Last year we closed it at about 700 because the stokers, slaves, were
suffering from heat stroke and the supply of slaves was limited :-)
The next week it was opened and the ware taken out."
** "What was particularly interesting was that most was black or dark
grey and very hard and durable, but some identical pots, next to the
grey pots stayed red with no reduction
at all."
===
PotteryVideos.com
http://www.potteryvideos.com/
"Building Your Own Potter's Kiln" with Graham Sheehan.
Complete with a materials list and working drawings
"This video guides you through the construction of a 25 cubic foot
kiln that is a perfect project for the novice builder, yet large
enough for a serious hobby potter or a beginning production potter. It
begins with layout and design for the foundation, and includes the
placement and cutting of bricks for walls, and the building of the
firing chamber, the door, and the sprung arch kiln roof. The detailed
materials list and working drawings that come with this video make
this the most comprehensive kiln-building resource available today.
Nothing is taken for granted, from the initial layout to the final
brick in the chimney."
The review of the video follows:
http://www.studiopotter.org/reviews/?review=video006
"The premise of this comprehensive film is that with its aid and the
accompanying working drawings "anyone can build this kiln." It opens
with Sheehan going over his completed, 25-cubic-foot, down-draft,
reduction kiln, while he explains the connection between the kiln's
proportions and the principles of combustion.
Keeping these principles well in mind, he then walks the viewer step
by step through the construction process, from establishing the
"footprint" of the kiln to insulating the arch. Sheehan's considerable
experience with kiln building shows in his attention to detail and
demonstration of ways to prevent potential trouble spots. His relaxed,
conversational manner creates a feeling of intimate dialogue with the
viewer.
For someone thinking about building his own kiln, this is a
staightforward, informative video. Unfortunately, it is washed out and
hard to follow in places. Also, I would have liked more emphasis on
the limitations and saftey hazards of working with Fiberfrax. A final
kiln firing and viewing of the fired pots would have made a satisfying
conclusion to an otherwise useful film.
Plans for the Bourry Firebox (a design that is not very applicable)
=============================
http://sidestoke.com/bourrybox/bourrybox.html
A single bourry box kiln:
http://sidestoke.com/bourrykiln/bourry.html
*************
Aside from what I have provided, I am at a loss for further diagrams.
You may be able to get some more precice building designs by
contacting the three parties listed on the Shang kiln webpage:
http://mcel.pacificu.edu/as/students/ceramics/kiln.html
(The email addresses are shown for three individuals.)
One other person you might contact is noted on the following website:
http://www.studiopottery.co.uk/html/pgal-amcf.html
In her resume, she says "In May 2000, I was selected to travel to
Tanzania, East Africa to set up a roof tile-making workshop with a
group of villagers from the village of Magima. The aim of the project
was to start up a sustainable business, which would provide an income
to support the Magima villagers throughout the dry season when they
are unable to farm. On this first trip, we were working at the clay
source in the bush using very simple equipment. We built a primitive
kiln and made our iirst roof tiles."
(I don't know if she required blackened tiles, but she may be someone
to network with. Her email is noted at the top of the page)
===
Since your original question asked for an explanation of the
reduction technique used in Chinese tile-making, I provided a thorough
explanation for your understanding. You mentioned that you already had
a square brick oven kiln in which you had manufactured "reddish"
tiles, and you asked how to create a "reduction atmosphere" to produce
a blacker look.
Your original question did not ask for detailed drawings of other
kiln designs and I have not come upon any such sketches in my research
other than the Shang design. Aside from the pictures and descriptions
of other reduction kilns and methods I have provided, there is little
else out there.
I still believe you can use your original square kiln, or the domed
kiln you mentioned in your clarification, if you provide adequate, but
small, vents during the reduction. If that is the case, it should be
no problem for your engineers to design a simple sketch that can be
used in the field. Otherwise, the Shang kiln design is a very good
sketch and worth experimenting with!
Sincerely,
umiat
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