Compiled 5/31/96 by Steven Branfman
Author, Raku: A Practical Approach and The Potters Professional Handbook
The Potters Shop 31 Thorpe Rd. Needham MA 02494, 781/449 7687
Revised 2/99, Last Posted 1/13/99
COPYRIGHT (C) 1996. All rights reserved.
Contact the author for permission to issue a copy in any form.
R.0 What is Raku?
R.1. Didn't Paul Soldner invent raku?
R.2. How do pit, sawdust, smoke firing, and raku differ?
R.3. Isn't raku a once-fire process where you don't have to bisque first?
R.4. Is raku suitable for functional use?
R.5. What constitutes a raku clay?
R.6. What is a raku glaze?
R.7. Which cones do I use in raku?
R.8. How do you get those bright metallic effects? Sometimes my glazes don't crackle as much as I like. What can I do?
R.9. All I ever get are bright metallic effects. How do I get more colorful glaze effects?
R.10. Can I preheat my pots in the kiln as it is warming up (starting with a cold kiln)?
R.11. Must I use a certain type of kiln or can I fire raku in an electric kiln?
R.12. When it comes to kilns I’ve heard the terms flue, damper, stack, and chimney. How do they apply to raku kilns?
R.13. Why can't I reach temperature no matter how much gas I use?
R.14. Don't the tongs used to lift the pots leave marks?
R.15. I’m confused about the term reduction. Can you explain what this means?
R.16. Must I remove my ware from the kiln to apply post firing reduction or can I insert reduction material into the kiln chamber?
R.17. Do different types of reduction material give different effects?
R.18. Must their be clouds of smoke when doing raku?
R.19. What is smokeless raku?
R.20. What is meant by slip resist in raku?
R.0 What is raku?
Raku is a pottery technique that has it’s origins in 16th century Japan. We are
pretty sure that it was developed by Korean potters under Japanese rule but the
exact circumstances of it’s development and in what context it was discovered
is a mystery. The raku technique, like other pottery techniques such as salt
glazing and pit firing, primarily revolves around it’s firing process although
involvement with raku often goes much deeper into its philosophy, roots, and
cultural significance. Traditional raku and our western version of raku are
similar in many ways though there are some significant differences.
To briefly describe the raku process we must understand that most all other
types of pottery are loaded into a cold kiln where the firing proceeds slowly
until the desired temperature is reached. This firing cycle may take anywhere
from 8-24 hours or even longer. When the kiln has reached temperature (which is
generally determined through the use of pyrometric cones), it is shut off and
allowed to cool enough to be able to remove the ware using bare, or lightly
gloved hands. The cooling cycle may last from 12-24 hours or longer. The ware
is considered finished when it is taken from the kiln. In raku, the pieces may
be loaded into a cold kiln but are often preheated and loaded into a hot kiln.
The firing proceeds at a rapid pace with the wares reaching temperature in as
short a cycle as 15-20 minutes (though raku firings can last up to several
hours depending on the individual pieces and their firing requirements). Glaze
maturity is judged by the trained eye without the use of cones or measuring
devices. When the firing is determined to be completed the wares are
immediately removed from the kiln. Since at this point the glaze is molten,
tongs or other lifting devices are used.
This is the stage in the process where traditional and contemporary raku differ
in technique and treatment. In our western version the wares are now treated to
a post firing reduction phase. The wares are put into a container with
combustible material such as sawdust, or leaves and allowed to smoke for a
predetermined length of time. The carbonaceous atmosphere reacts and affects
the glazes and clay and imparts unique effects and surfaces to the wares. Some
of these effects are metallic and crackled glazes surfaces and black unglazed
clay. When the wares have cooled, they are washed with an abrasive cleaner to
remove all residue of soot and ash.
R.1. Didn't Paul Soldner invent raku?
Not exactly. Soldner is an innovator and one of a few responsible for
popularizing raku in this country beginning in the 1950's. Raku was first
developed by Korean potters under Japanese rule in the 17th century. The
circumstances that led to its launch and spread are somewhat of a mystery
though.
R.2. How do pit, sawdust, smoke firing, and raku differ?
These types of firings are often confused with each other because they can
share some similar characteristics. Briefly, raku ware is fired in a more or
less conventional type kiln where glaze technology is understood and utilized.
The others are forms of primitive firing where temperatures reached are
generally lower and glazes are not commonly used. In primitive firings, the
"kiln" may consist of a simple hole in the ground (ie, a pit).
R.3. Isn't raku a once-fire process where you don't have to bisque
first?
Raku firing greenware is a sure way to line the bottom of your kiln with
shards. Always bisque-fire to at least Cone 08 before glazing and raku firing.
R.4. Is raku suitable for functional use?
The traditional use of raku ware in the Japanese tea ceremony has contributed
to confusion about the functional use of raku. With very few exceptions, all
raku fired ware is fragile, porous, and generally unsuitable for functional
use. Unless such fragile ware is treated post-firing with a non-glaze material,
such as a polyurethane or acrylic sealer or an oil of some kind, the pots will
sweat water and eventually breakdown. Treat raku as decorative. The occasional
use of raku in a functional setting is OK but keep in mind that the glaze is
soft and can be easily chipped and end up being ingested. If you must use your
pots for food try to limit the use to dry food. The fragility of the ware also
renders the reliability of handles and other appendages extremely questionable.
R.5. What constitutes a raku clay?
A raku clay is any clay that can be successfully raku fired. That includes most
any type of clay out there! As I say at my workshops; "I've never met a clay
that I couldn't raku." Generally though, a clay suitable for raku needs to
contain a lot of fireclay and similar refractory materials so that it can
withstand the sudden heat shock of the raku process. This includes most stock
stoneware clays. The clay does not have to be loaded with coarse grog but it
does have to be open enough to expand and contract without cracking. Most clay
suppliers can help you choose an appropriate claybody. The other
characteristics that you are looking for are your personal ones such as
plasticity, color, texture, etc. Nothing needs to be sacrificed in order to
have a good, reliable raku clay.
R.6. What is a raku glaze?
Similar to the question of raku clays is the answer to this question. Any glaze
that you can successfully raku fire is a raku glaze. The most important factors
in identifying raku glazes is the temperature at which they mature, how you
plan on using them, and what kind of effects you are looking for. You must also
keep in mind that if you are using a variety of glazes on the same pot or in
the same kiln load, unless you know that they all mature to your satisfaction
at around the same temperature, you will be faced with varying degrees of
maturity. However, just because a glaze is formulated to fire at a temperature
higher than your usual range doesn't mean that you should eliminate that glaze
from your pallet. Experiment with your glazes to achieve a variety of surface
effects from dry textures to surfaces with a high gloss. Don’t limit yourself
to homemade or personal glazes either. Try low fire commercial glazes for some
unusual results.
R.7. Which cones do I use in raku?
The only cones that should be used near a raku kiln are ice cream cones.
Because of the fast firing, varying atmosphere, multiple loads and other
factors, pyrometric cones are generally poor indicators of heat and temperature
so they are not used. Most raku potters fire their wares using the actual glaze
melt as the visual indicator of maturity. Many potters, however, do use
pyrometers or cones to warn of coming glaze maturity and then check the ware
visually through the peephole(s). It takes some temperature of raku depends on
the glazes that you are using. Most raku is fired in the range of cone 010-06.
You must remember though that you as the maker of the pots are the final expert
on whether a glaze is mature, underfired, or overfired. If a glaze is not
glossy enough, doesn't have the expected crackles or metallic effects, or
doesn't exhibit any other characteristic that you find desirable, you need to
adjust the firing.
R.8. How do you get those bright metallic effects? Sometimes my glazes don't
crackle as much as I like. What can I do?
Assuming you are using the correct glazes, both metallic effects and dark
crackle lines are a result of firing the glazes to their maximum maturity
followed by a fast post-firing reduction technique. You must quickly get your
pot from the kiln to the reduction container and covered before the pot has a
chance to cool too much, otherwise the post-firing reduction will not be
effective. Pronounced crackle effects are also often dependent on a thick
application of glaze.
R.9. All I ever get are bright metallic effects. How do I get more colorful
glaze effects?
This is the exact opposite of the previous question and problem. Brighter
colors need a degree of post firing oxidation in order to develop. There are
several ways to achieve this. When you remove the ware from the kiln, spray the
areas with water where you would like more color to develop before placing the
ware in the reduction container. This will oxidize and cool the glaze. Another
method is to hold the ware in the air for 10 - 30 seconds before reducing the
ware. Allow the combustable material to ignite and cover the container only
after flames have clearly developed. Other potters will uncover the reduction
container after a short time allowing the material to ignite again while
fanning the ware. Using these and other similar techniques you will learn how
to control the amount of metallic and colorful effects on your wares as well as
grey to black unglazed areas.
R.10. Can I preheat my pots in the kiln as it is warming up (starting
w/a cold kiln). I will be using a fiber garbage can kiln with a
lift-off lid, not one where the entire kiln lifts off.
I don't see how my pots will get preheated enough by just setting them on the lid.
Do they get hot enough on a fiber/expanded metal lid?
This question shows a common misunderstanding about the raku process. The only
time you need to be concerned about preheating your ware is when you are doing
multiple firing loads in the same kiln. The first load should always be in a
cold kiln taking the temperature up slowly. Only if you are going to fire
additional loads should you be preheating your pots on or along-side the kiln.
If the lid of the kiln is a wire mesh/fiber lid then you might get enough
radiant heat. Place the pots near the flue opening so they get the heat coming
from the draft. Be careful not to place them too close, too soon, or they may
break. After preheating on top of the kiln you can place the pieces around the
base of the kiln after you remove the fiber chamber, let them heat up there for
a minute or two, then put them on the kiln shelf letting them sit for another
minute, then put the chamber back in place for a minute, then re-light the
burner.
R.11. Must I use a certain type of kiln or can I fire raku in an electric
kiln?
Most any type of kiln can be used for raku as long as it meets certain
requirements of the raku process. It must be located outdoors or in close
proximity to the outdoors. You must be able to easily reach into the kiln to
remove your ware. If you are going to be firing multiple loads the temperature
of the kiln must be easily controlled. An electric kiln is perfectly suitable
for raku although there are some special considerations that require careful
attention. You must remember that you are exposing yourself to live electrical
current when you open an electric kiln so you must protect yourself from any
possible contact with the current. There'll be more on this in a subsequent
revision of this FAQ.
R. 12. When it comes to kilns I’ve heard the terms flue, damper, stack, and
chimney. How do they apply to raku kilns?
All fuel fired kilns need some way for the gases to exit, the air flow to be
controlled, and air to enter the chamber. Conventional kilns have a flue,
damper, and stack (or chimney). The flue is a path in the kiln for the gases to
move through after they have circulated through the kiln on their way to the
stack. The damper is a device, usually a kiln shelf, that slides in and out of
the lower part of the stack in order to control the size of the opening, thus
controlling the exit of the gases and their mixture with air. Raku kilns are
generally small and overpowered. Most have a simple opening in the top of the
chamber covered by a kiln shelf shard. The kiln chamber chamber is essentially
the stack with the opening the flue and the kiln shelf the damper.
R. 13. Why can't I reach temperature no matter how much gas I use?
This is the most common question regarding the raku firing. Successful rise of
temperature in any fuel-fired kiln depends on the correct mixture of air and
fuel. A hotter kiln is not necessarily dependent on more fuel! If there is too
much fuel, the atmosphere will be smoky, full of flames and the temperature
will not rise. Also, if there is too much air, the
temperature will stagnate or fall. Usually, the solution to a situation in
which the kiln doesn't reach temperature is to increase the air in the air/fuel
mixture. In a gas-fired kiln this can be done by increasing the available air
in the burner or around the burner. You can also simply open the flue (the
opening at the top of the kiln). Most potters use a venturi
type burner that has a disk that screws open or closed. Open it for more air.
If you are using a power burner in which a fan is attached, increase the air
flow. Experiment to arrive at the correct mixture but be patient!
R.14. Don't the tongs used to lift the pots leave marks?
Sometimes they do. More often though the glaze is still molten enough that once
the pot is released from the tongs, the marks tend to smooth out. In either
case the tong marks should not be treated as defects but rather as
characteristics of raku.
R.15. I’m confused about the term reduction. Can you explain what this
means?
Reduction is a firing term that refers to a lack of oxygen in the combustion
process. This lack of oxygen causes the fuel to search for free oxygen to allow
for more complete combustion to take place. This can take place during a firing
as in reduction stoneware. In this conventional situation, the additional
oxygen is gotten from the clay body and glazes, resulting in their
characteristic effects. Reduction as it is referred to in raku usually takes
place out of the kiln separate from the actual firing as described earlier.
However, just because you are doing raku doesn’t mean that you can’t experiment
with actual reduction firing in the more conventional sense.
R.16. Must I remove my ware from the kiln to apply post firing reduction or
can I insert reduction material into the kiln chamber?
As described earlier, post firing reduction is normally done by removing the
ware from the kiln and placing it in a container with your combustable
material. Certain situations may make it difficult to actually remove the wares
from the kiln while they are hot. For instance your piece may be too large or
awkward to handle. In this case you can achieve some post firing success by
shuting off the kiln, adding your reduction material to the chamber, and
closing off all open ports including the flue, peep hole, and burner ports. At
best, the chamber will be only marginally sealed and since effective reduction
depends on on air tight chamber, your reduction will be only partially
successful. If your kiln is a lift off fiber type then you might try removing
it and replacing it with a metal drum or can for the reduction phase.
R.17. Do different types of reduction material give different effects?
The short answer is yes. The long answer is much more complicated! Here is a
medium answer: Your reduction effects are certainly influenced by how much
carbon is in the atmosphere that surrounds your pot. In other words, how much
smoke your pot is quickly subjected to. Some materials have the potential to
release more carbon than others. The condition of your material (wet, damp,
dry) as well as the particle size as in the case of wood materials (sawdust,
shavings, chips) can be important. The type of wood can also affect your
results.
R.18. Must there be clouds of smoke when doing raku?
Raku doesn’t require smoke at all. There are two aspects of the process that
have the potential for creating smoke. If you are using a fuel fired kiln (as
opposed to an electric kiln) then it is likely that there will be at least some
smoke generated during the firing. If you are doing post firing reduction,
there will be smoke created then as well. The amount of smoke is determined by
the efficiency of your reduction technique, the material you are using, and the
amount of material you are using. If you are reducing in a container, the
tighter the lid fits, the less smoke will exit the container. There are other
techniques of reduction that create less smoke than others. Of course if you
are not doing post firing reduction and simply cooling your ware when it comes
from the kiln, then there is no smoke created.
R.19. What is smokeless raku?
Smokeless raku is not really smokeless at all. The term was coined by Jerry
Caplan who has been developing his techniques for many years. It is a post
firing reduction technique that is designed to produce minimal smoke by keeping
the reduction chamber as air tight as possible. This can be done using a
combination of tight fitting lids with gaskets of wet paper or cloth. The
tightest chamber is one made by inverting your container onto a bed of sand or
dirt. Arrange a ring of sand or dirt on the ground for the lid of your
container to fit onto when placed upside down. Place your reduction material
within the ring. Quickly place your ware onto the material and cover it with
the container pushing the rim of the container into the sand. Bury the rim with
additional sand to keep the smoke in.
R.20. What is meant by slip resist in raku?
Slip resist refers to a variety of techniques whereby a clay slip is applied
to the surface of the ware. This slip is formulated to peel away during the
firing, and not permanently adhere to the surface. As the slip peels, shrinks,
and separates from the surface it exposes the ware to varying degrees of post
firing reduction. When the piece is cooled, whatever slips remains on the pot
is removed by scraping and cleaning. As a further decorative process, you can
scratch and draw through the slip creating designs.
Compiled and first posted June 1996. Revised 2/99. Last posted 2/17/99 Steven Branfman.