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CONTENTS
P1.
Guidelines for rec.crafts.pottery
P1.1.
What is rec.crafts.pottery?
P1.2. What topics are appropriate to this news group?
P1.3. Is it OK to post for sale messages?
P1.4. May I post binaries to this group?
P1.5. How can I help with the FAQ?
P2.
Pottery and Ceramics for newbies
P2.1.
What is clay?
P2.2. What is the difference between earthenware, terra
cotta, stoneware, porcelain, and raku clay?
P2.3. What are slips and engobes?
P2.4. What is terra sigilatta?
P2.5. What are greenware and bisque ware? What does
leather-hard mean?
P2.6. What are cones?
P2.7. What temperature should I fire to?
P2.8. What is the difference between oxidation and reduction
firing?
P2.9. What are pit-firing, saggar-firing, and sawdust-firing?
P2.10. What is raku?
P3.
Potter culture
P3.1
Some of the things that make potters special
P3.2 How do you pronounce "kiln"?
P3.3 Why do potters immediately turn over pots to look
at the bottom?
P3.4 Why do many potters grimace at anything blue and
white?
P3.5 How can I recognize other potters?
RELATED
FAQs POSTED SEPARATELY
FAQ
Pottery Glazes - Email Tom Buck Tom.Buck@HWCN.ORG
FAQ Pottery Books and Journals
FAQ Pottery Internet Resources
FAQ Raku Pottery - Email Steve
Branfman sbranfpots@aol.com
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P1.
What is rec.crafts.pottery?
P1.1.
What is rec.crafts.pottery?
rec.crafts.pottery
is a news group for discussing the construction,
decoration, and firing of pottery and ceramic art. We try to take a
broad
view of the meaning of pottery. Functionality and craftsmanship are
not
prerequisites for inclusion in this group. Discussions do not need to
be
strictly recreational either -- Issues affecting professional potters
are
welcome. We hope that the amateurs (in the sense that an amateur is
a
devotee of a subject) have something to offer the professionals. I know
the
recreational potters will learn a lot from the professionals and that
there
is a broad range of semi-professionals in between. If you are new to
pottery, you will soon discover that potters are by and large nice people
and
we place a high premium on being nice in this group.
P1.2.
What topics are appropriate to this news group?
Appropriate
topics include discussion of construction and decorating
techniques, glaze recipes, firing methods, and other aspects of making
ceramic objects. Questions, answers, and opinions about pottery making
equipment are suitable for this group. Discussions of history of ceramics,
collecting, studying, and where to see pots and potters are also welcome.
It is fine to discuss the business aspects of being a potter, but this
is not
a forum for marketing your work.
Inappropriate
topics include spamming, blatantly not-ceramic posts, personal attacks,
and commercial activities. Help from manufacturers and suppliers is
appreciated in the spirit of the free-flow of information. However,
people are bound to complain if it sounds like you are fishing for customers.
Some people in the "support" side of the pottery business have generated
a lot of good will by offering assistance and opinions without marketing.
It is a delicate balance and those who might complain about "commercial
posting" should consider that and also realize that there is nothing
wrong
with suppliers being proud of their work.
Some
questions posted to the group are likely to be met with at best a stony
silence. The poster may be left wondering at this slight. Although some
posts are difficult or weird enough that no one has a good answer, more
commonly this is because the question is overly broad, uninteresting,
or can
be answered easily by checking in a text book or two. Requests such
as,
"Please send me some nice cone 6 glaze recipes" fall in this category.
The best response to that request is, "Check out the glaze files on
the SDU
ceramic gopher, the ClayArt archives, any of a number of books, and
be
prepared to test your glazes. The success of a glaze depends not only
on
the formula, but on the clay body, the firing cycle, and the skill of
the
potter."
Questions
on techniques are fine, but some are either too basic or not
amenable to answering on the Internet. I doubt anyone can really describe
how to pull a handle using only ASCII characters. Get a book or better
yet,
personal instruction.
P1.3
Is it OK to post for sale messages?
By
straw poll, short "for sale" messages by individuals are OK.
Commercial for sale messages by ceramic suppliers, galleries, and others
"in the business" are not. Posting of workshop and other educational
opportunities are welcome in moderation. By moderation, I mean special
events of general interest such as a recognized potter in for a 3 day
workshop. Your complete schedule of beginner pottery classes would not
be appreciated by the majority of readers. If you post a for sale notice
please include "FS" in the subject line and try also to identify the
geographic location. The Internet covers the globe but people are not
likely to be interested in something that weighs a ton and is located
half-way around the world. An example of a useful subject line would
be:
"FS:
20 cu ft gas kiln, Bay Area, California, USA"
P1.4
May I post binaries to this group?
Some
sites do not have storage for binaries such as pictures and programs,
so do not appreciate binaries in rec groups. Please post binaries elsewhere
and describe where to find them, here.
P1.5
How can I volunteer to help with the FAQ?
Write
up the answers to some questions. Better yet pick a topic to help
maintain. Send or post questions of general interest. Send constructive
comments. We are new at this so appreciate your interest. The author
list includes:
Evan
Dresel pedresel@aol.com
Michelle Lowe mishlowe@desertdragonpottery.com
Tom Buck Tom.Buck@HWCN.ORG
Steve Branfman sbranfpots@aol.com
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P2.
Pottery and Ceramics for newbies
P2.1
What is clay?
Clay
is the stuff we shape into pots. The definition gets a bit more
complicated when you look at what makes this earthy material so unique
and
wonderful. The word "clay" has several meanings and all aspects contribute
to the craft of pottery.
To
a mineralogist a clay mineral is one of a group of silicate minerals
with
a flat sheet-like structure. Clay minerals are mostly made up of atoms
of
silicon, oxygen, and aluminum. Other substances such as magnesium, iron,
potassium, and calcium are usually present in smaller amounts. Clay
minerals are "hydrated" -- that is they contain hydroxyl ions (OH) chemically
bound within the structure. (You can think of clay as being made of
silica (SiO2), alumina (Al2O3), water molecules (H2O), and some other
stuff). The hydrous nature becomes very important during firing of the
clay and distinguishes clay from other sheet structure aluminosilicates
like micas. The composition and plate-like structure of clay minerals
is largely responsible for the properties like plasticity which make
for good pot-making materials.
Clay
minerals typically exist as extremely small particles, which leads
to the second meaning of the word "clay" which is a mineral particle
sized
between 0.001 and 0.15 millimeters in diameter. By this definition not
all
clay-sized particles are clay minerals. However, clay minerals not only
are
found as tiny particles, but the flat structure of the minerals tends
to align
like a deck of playing cards giving the cohesion and workability of
the
material. If you chew on a bit of clay between your front teeth you
won't be
able to feel any grittyness because the particles are too small. Silt
sized
particles will feel gritty.
Finally
"clay" is often used to mean a "claybody" which is the stuff we use.
Claybodies usually contain a variety of clays an other minerals in order
to
promote the right properties for working. Often claybodies contain
materials such as sand or grog (ground up pre-fired clay) in order to
add
strength and resistance to thermal shock. Other minerals often used
in clay
bodies include flint (microcrystalline quartz) which helps resist warping
and
strengthens the fired material, feldspar and nephelene syenite which
act as
fluxes to lower the maturation temperature, and talc which acts as a
flux and
helps resist thermal shock.
P2.2
What is the difference between earthenware, terra cotta, stoneware,
porcelain, and raku clay?
Earthenware,
terra cotta, stoneware, porcelain, and raku clay are different
types of claybodies. Earthenware is a clay body that is fired to a low
temperature (approximately 1000 C or 1800 F). The resulting material
is still
very porous so it will absorb water or even leak if not glazed. Terra
cotta
is generally used to mean a red earthenware. Stoneware is a generally
buff,
tan, or brown clay which is matures at high enough temperature (approximately
1200 C or 2200 F) to form a dense ceramic having a low porosity and
which will not leak, even if unglazed. Porcelain is a very white clay
body which is fired to the point of becoming almost totally vitreous
(glassy) so it is translucent when thin. You will often see almost any
white clay body referred to as porcelain but many are better termed
white stoneware. True porcelain is fired to cone 9 to 12 (see below).
Raku clay is a clay body designed to take the stress of raku firing
process. Raku clay typically is white and contains a lot of sand, but
see the FAQ Raku for a more complete explanation.
In
the past, potters had to dig clay and prepare their own claybodies.
Now,
in industrialized countries, potters are fortunate to have suppliers
which
provide an almost overwhelming choice of claybodies. Even potters who
mix
their own claybody can purchase raw materials, avoiding much of the
labor and quality problems associated with digging their own clay.
The
variety of claybodies available can be confusing to the potter just
starting to work on his or her own. You may be best off exploring a
couple of
bodies in the firing range of interest at a time, rather than trying
too many
kinds at once. It sometimes takes a while to get to know a body and
how to
work with it. Try a variety of forms and forming methods. Glazes will
look
and act differently on different claybodies, so test-tiles for testing
your
glazes on a new body are very useful. Conversely your glazes may need
some modification so that they fit the body.
P2.3
What are slips and engobes?
Slips
and engobes are colored clay slurries used to decorate pots. A
simple slip may be made up of a clay body mixed in water. Engobes may
mean colored slips or may be used to describe a slip modified to melt
slightly
producing a surface somewhere between a slip and a glaze. Slips and
engobes may be covered with glaze to produce an "underglaze" decoration.
Slips and engobes are generally applied to greenware (see below) but
may be applied to bisque-ware (also below) if formulated to shrink less
during firing.
P2.4
What is terra sigilatta?
Terra
sigilatta (or terra sig. to cool potters) is a slip made of extremely
fine clay particles. The terra sig is applied to the "leather-hard"
greenware, or even bisqued pot and when mostly dry can be burnished
(or
polished) with a stick, smooth pebble, back of a spoon, or whatever
works.
The resulting surface is rich and shiny. Burnishing the surface was
important
in many cultures because the resulting surface made the pot more waterproof
when fired to low temperatures. This is because the burnishing lines
up the flat clay mineral particles along the surface of the pot. The
alignment minimizes the porosity of the surface. Burnished pots must
be fired to only very low temperatures to maintain the shiny surface
because the clay minerals begin to recrystalize at higher temperatures
and the structure is lost leading to a dull surface.
P2.5
What are greenware and bisque ware? What does leather-hard mean?
Pots
that have not been fired are called greenware, particularly when dry
enough to load into a kiln. Pottery is at its most fragile at this stage,
It has lost its plasticity but has not attained the strength of a ceramic.
Generally pots are "bisque" fired to a low temperature (approximately
950-1050 C, 1750-1950 F or cone 08 to cone 04) prior to glazing. These
bisque-ware pots (or biscuits) are porous but fairly strong. They have
already undergone much of the shrinkage expected during firing. Bisque
pots
are soft so be careful not to chip or scratch them. The pots are then
re-fired after glazing.
Leather-hard
is the stage in drying a pot where the pot will hold its shape
but has not lost all its water. This is the stage for applying handles,
using terra SIG, or doing a variety of other tasks in making pots. When
pots
are as dry as they get, prior to bisqueing they are called "bone dry".
P2.6
What are cones?
When
clay and glazes are fired the result depends on the rate of temperature
rise and the time it is left at high temperatures as well as the temperature
reached. If you just measure the temperature with a thermocouple, that
does
not tell you enough to get consistent results. Cones are specially formulated
ceramics formed into triangular, pointy shapes. When placed in a kiln
they
will soften and bend (or "mature") at a specific temperature ** when
the
temperature is increased at a specific rate **. If fired faster or slower,
the cones will mature at a different temperature. Cones thus account
for the
firing history as well as the temperature.
Cones
with different numbers are made to mature at different temperatures.
From lowest to highest temperatures the cones count from approximately
022, 021, 020 ... 03, 02, 01 then to 1, 2, 3, and on upward. The "0"
at the
beginning is like a negative sign although there is no 0 cone.
When
firing by cones, typically three cones are used. The guide cone is one
or two numbers lower than the desired end point. When it bends over,
you know you are close to being done. The firing cone bends over at
the desired end point. The guard cone is a cone higher than you want
and lets you know if you have over fired. The cones are placed in front
of a peep-hole in the kiln and monitored periodically when the end of
the firing is near. It is a good idea
to put cone packs in different parts of the kiln so you can see if the
firing
was even (after the kiln cools).
On
the net people will either spell out cone or use the ^ symbol to represent
a cone (e.g. ^06).
Cones
manufactured by the Orton Foundation are the most commonly used in
North America. Large cones are the standard cones used by potters. Orton
provides a chart of maturation temperature for the large cones when
the rate
of temperature rise is 150 C (270 degrees F) per hour. The chart for
small
cones is for a temperature rise of 300 C (540 degrees F) per hour and
the
maturation temperatures are higher than the large cone of the same number.
I would include a cone temperature equivalent chart here, but I'm too
lazy
to type it in. They are available in many books and pottery supply catalogs,
and often come with a box of cones.
Generally
the small cones are used in kiln sitters which are devices which
automatically shut of the kiln at the proper cone. The small cone sits
horizontally on top of two metal bars and holds a trigger bar up. When
the cone melts it bends, the trigger drops, releasing a weight on the
outside
of the kiln. The weight hits switch which shuts off the electricity.
Rube
Goldberg would be proud. Kiln sitters are reliable but not infallible.
Back
up timers are a highly recommended option on your kiln sitter. Once
you know how long a firing is expected to take (from experience and
careful records of your firing schedule) you can set the timer for a
little longer time. If the sitter fails, the timer then will shut off
the kiln before a major melt-down occurs. The best backup, however,
is a set of large cones visible through a peephole and yourself baby-sitting
the kiln, particularly near the end of the firing, (I use a sitter/timer
and hang around at the end of the firing). Most people find that they
need to use a small cone in the sitter which is one number hotter than
the large cone target temperature.
P2.7
What temperature should I fire to?
Obviously
this is a matter of personal choice but the firing range will
affect your pots greatly. Remember different clay bodies are designed
for
different ranges of temperature and if you get them mixed up you risk
a real
mess in the kiln! Earthenware clay will slump and "melt" into a disgusting
heap at stoneware temperatures.
Earthenware
is generally fired to cone 06 to 02. One common technique is to
bisque fire to cone 04 and then glaze at cone 06. Firing to cone 03
or 02
will promote the formation of mullite in the body which can help the
glaze fit the pot properly. Firing to earthenware temperatures (or lower)
is
called "lowfire".
Stoneware
is traditionally fired to cone 9 or 10 which is called "high fire".
However, in the USA, cone 6 claybody is usually referred to as stoneware
and is frequently fired in an electric kiln. People in other parts of
the world may
refer to cone 6 as "mid-fire".
Porcelain
is usually fired to cone 10 or 11. Although there are white
lower-maturing clay bodies, you may find that the body is not truly
dense at
cone 6 and may not even hold water.
P2.8
What is the difference between oxidation and reduction firing?
Pottery
may be fired in oxidation or reduction. Oxidation firing takes place
in air with its 21 % oxygen content while during reduction firing excess
fuel
is introduced at times to consume the oxygen. The reducing agents present
during reduction firing react with the claybody and glaze. The type
of firing
chosen has a huge impact on the final appearance of the pottery. Many
glazes such as copper reds and celadons only work in reduction. Oxidation
firing often promotes brighter colored, glossy, and some would say harsh
surfaces. Reduction firing promotes warmer clay body color and often
softer, more visually textured glazes. Oxidation can be achieved in
any type of kiln where the atmosphere is not depleted in oxygen through
the combustion process. Electric kilns, for all practical purposes fire
in oxidation. Fuel burning
kilns may be fired in reduction by making the flame richer at certain
times
during the firing by adding more fuel, cutting the oxygen to the burners,
and mainly by partially closing the damper. The oxygen starved atmosphere
in the kiln reacts with iron and other elements in the clay and glaze
causing
chemical changes that affect the appearance. Reduction firing involves
the
potter in the process more, is more involved, and may be less predictable.
These traits appeal to many potters who value the variety and "life"
produced
in pots which are fired in reduction.
P2.9
What are pit-firing, saggar-firing, and sawdust-firing?
Pit-firing,
saggar-firing, and sawdust-firing, are types of low-temperature
firing where the pots are stacked in combustible material such as sawdust,
dung, or wood shavings which produce strong reduction during the firing.
This is one of the earliest firing techniques and often lends a "primitive"
quality to the pots. In pit or sawdust firing, the material around the
pots
is ignited to produce the heat source. In saggar firing, a large covered
pot
called a saggar is loaded with the organic mixture and one or more pots
then
placed in a conventional kiln for firing. (Saggars were originally used
to
fire pots prior to the development of kiln shelves. This allowed glazed
pots
to be stacked and provided protection from wood ash in the kiln.) Modern
potters may add a variety of material such as sea weed, orange peels,
copper
wire, or oxides in order to produce different colors and effects. Pit
firing
is a good technique to explore if you have some space where you can
fire but
only have minimal equipment.
Pit,
sawdust, and saggar firing are usually to very low temperatures and
the
soft, porous pots that result are mainly decorative rather than functional,
although these techniques have served the functional needs of some cultures
for thousands of years.
P2.10
What is raku?
Raku
is the name given to a firing technique, where the pots are removed
from the still-hot kiln when the proper temperature is reached. Commonly
raku involves post-fire reduction where the pots are immersed in a can
containing organic material (e.g. sawdust) and covered so that reducing
conditions are attained. The clay body turns black, the glazes craze
(or crackle), copper bearing glazes are often reduced to produce metallic
lusters. This is an exciting process often leading to surprise results.
The
origin of raku was in Japan where it has been practiced by the Raku
family
of potters since the 16th century. The technique is somewhat more subdued
producing black raku and (less commonly) red raku ware. Some of the
most
famous tea-bowls are raku.
Sometime
in the 1960's Paul Soldner, Robert Pippenberg, Hal Riegger, and other
American potters got into the act and produced innovations in post-fire
reduction. The term raku has stuck for their techniques in spite of
vast
differences from the Japanese origins.
The
handling of hot pottery and rapid cooling required by raku firing means
that the clay body must be able to withstand a lot of thermal shock.
Typically raku involves a white body that contains a lot of sand for
strength.
Also the body is general fired only to an open porous ceramic that helps
it withstand thermal shock. For example the raku body used at Penn State,
when I was a student, made a fine cone 10 white stoneware body even
though raku is typically fired to about cone 011 to cone 06.
Because
of the porous, fragile, nature of the less-than-mature raku pot, raku
ware should generally be considered decorative rather than functional.
Also
some raku surface decorations will fade with time unless preserved with
non-ceramic coatings such as acrylic spray, or wax.
See
the FAQ on raku pottery and the pottery balkiest FAQ for further
information on raku.
P3.
Potter culture
P3.1
Some of the things that make potters special
Potters
and ceramic artists, like any other group, develop their own way of
communicating, their own approach to things, and other markings of a
subculture. This isn't to say that potters are not a diverse lot, but
certain
things can seem strange or bewildering to beginners. Clay isn't the
basis for
a secret society, but is is a way of life for many of us so here are
a few
things to help put you "in the know".
P3.2
How do you pronounce "kiln"?
Kiln
is either pronounced the way it is spelled "kiln" or the final "n" may
be
silent so it is said "kill". Is this loss of the final consonant a holdover
from our rural potter roots, a habit from long time, or a sign that
you are a
cool potter in the know? Probably all of the above in various instances.
I'm
open to suggestion.
P3.3
Why do potters immediately turn over pots to look at the bottom?
The
foot of a pot tells much about the potter. It is this humble area that
often shows the individual style of the maker. The line of the cutoff
wire, the angle of the trimmed foot, and even the transition from glaze
to
raw clay are the intimate details which make a pot a unique thing. Besides
the base is where you can usually find the potter's mark and the price
tag.
P3.4
Why do some potters grimace at anything blue and white?
Cobalt
blue decoration on a white background is thousands of years old and
there is no sign of it going out of style yet. Cobalt is a reliable
and
intense colorant for a wide range of temperatures. It's an easy seller,
and
very common. This isn't to say that there is anything wrong with blue
or
white, and there may well be much wonderful work to be made in the style.
Some of us do wish the public would be more receptive to some of the
pieces
which show some of the other wonders of pottery. Some of these other
pieces
are "potters pots". They attract other potters but don't sell well.
By the
way, green glazes have a reputation for not selling but apparently is
becoming
popular, in the USA at least.
P3.5
How can I recognize other potters?
By
the glazed look in their eyes. 8-)
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