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Raku Firing Facts

Raku Pottery Firing

Raku is a technique that carries with it many myths, confusion and questions.
Here is a short explanation of what raku is and some of the most common questions asked during workshops and demonstrations.

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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 pottery workers under Japanese rule but the exact way that it was developed and how it was discovered is a mystery.
The raku technique, like other pottery techniques such as salt glazing and pit firing is about the firing process, but association with raku often goes much deeper into its philosophy, roots and cultural significance.
Raku from across the pond and the western version are similar in many ways but there are also some significant differences.

Just about 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 to 24 hours or even longer.
When the kiln has reached the right temperature, which is generally determined through the use of pyrometric cones, it is shut off and allowed to cool enough to remove the pottery or ceramics using bare, or lightly gloved hands.
The cooling cycle may last from 12 to 24 hours or longer.
The pottery is considered finished when it is taken from the kiln.

In raku, the pottery 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 pottery reaching the desired temperature in as short a cycle as 15 to 20 minutes.
Some 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 pottery is immediately removed from the kiln.
Since at this point the glaze has melted, 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 pottery is treated to an after firing reduction phase.
The pottery is put into a container with combustible material such as sawdust, or leaves and allowed to smoke for a certain length of time.
The carbon rich atmosphere reacts and affects the glazes and clay and produces unique effects on the surfaces of the pottery.
Some of these effects are metallic and crackled glaze surfaces and black unglazed clay.
When the pottery has cooled, they are washed with an abrasive cleaner to remove all residue of soot and ash and then dried.

Who Invented Raku?

Paul Soldner introduced the new method of firing and was one of a few responsible for making raku popular in this country beginning in the 1950's.

What Is The Difference Between Pit, Sawdust, Smoke firing and Raku?

These types of firings are often confused with each other because they do share some similar characteristics.
Raku pottery is fired in a more or less conventional type kiln where glaze technology is understood and used.
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 or a pit.

Do I Have To Bisque Fire First?

Raku firing greenware is a sure way to line the bottom of your kiln with broken pottery pieces!
Always bisque fire your pottery to at least Cone 08 before glazing and raku firing.

Can Raku Be Used For Functional Use?

The handed down use of raku pottery in the Japanese tea ceremony has contributed to the confusion about the functional use of raku.
With very few exceptions, all raku fired pottery is fragile, porous and generally unsuitable for functional use.
Unless such fragile pottery is treated after firing with a non-glaze material, such as a polyurethane or acrylic sealer or an oil of some kind, the pots will soak up water and eventually breakdown.
You should 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 swallowed.
Food use and storage is not recommended for Raku Fired Pottery.
The fragility of the pottery also causes the reliability of handles and other attachments to be extremely questionable.

Is There A Special Raku Clay?

Raku clay is any clay that can be successfully raku fired.
That includes just about any type of clay out there!
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 the right clay body.
The other characteristics that you are looking for are your personal ones such as plasticity, color, texture and so forth.
Nothing needs to be sacrificed in order to have a good, reliable raku clay.

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 will all mature to your satisfaction at around the same temperature, you will be faced with varying degrees of maturity.
Now, 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.

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 pottery workers fire their pottery using the actual glaze melt as the visual indicator of maturity.
Some pottery workers, do use pyrometers or cones to warn of coming glaze maturity and then check the pottery visually through the peepholes.
Some temperatures of raku depends on the glazes that you are using.
Most raku is fired in the range of cone 010 to 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 and doesn't exhibit any other characteristic that you find desirable, you need to adjust the firing.

What Can I Do If I Don't Get Bright Metallic Effects Or Crackle?

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 after firing reduction technique.
You must quickly get your pottery from the kiln to the reduction container and covered before the pot has a chance to cool too much, otherwise the after firing reduction will not be effective.
Pronounced crackle effects are also often dependent on a thick application of glaze.

Bright Metallic Effects And No Colorful Glaze Effects.

This is the exact opposite of the question and problem above.
Brighter colors need a degree of after firing oxidation in order to develop.
There are several ways to achieve this.
When you remove the pottery from the kiln, spray the areas with water where you would like more color to develop before placing the pottery in the reduction container.
This will oxidize and cool the glaze.
Another method is to hold the pottery in the air for 10 to 30 seconds before reducing the pottery.
Allow the combustible material to ignite and cover the container only after flames have clearly developed.
Other pottery workers will uncover the reduction container after a short time allowing the material to ignite again while fanning the pottery.
Using these and other similar techniques you will learn how to control the amount of metallic and colorful effects on your pottery as well as gray to black unglazed areas.

Can I Preheat My Pottery In The Kiln As It Is Warming Up?
Using A Fiber Garbage Can Kiln With A Lift Off Lid, Will My Pottery Get Preheated Enough By Setting Them On The Lid.
Do They Get Hot Enough On A Fiber And Expanded Metal Lid?

This question shows a common misunderstanding about the raku process.
The only time you need to be concerned about preheating your pottery is when you are doing multiple loads in the same kiln.
The first load should always be in a cold kiln taking the temperature up slowly.
Now, if you are going to fire additional loads, you should be preheating your pottery on or along side the kiln.
If the lid of the kiln is a wire mesh and fiber lid then you might get enough radiant heat.
You can also place the pottery 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 pottery around the base of the kiln and 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 relight the burner.

Can I Fire Raku In An Electric Kiln?

Almost 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 pottery.
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, but there are some special considerations that require careful attention.
You must remember that you are exposing yourself to a live electrical current when you open an electric kiln, so you must protect yourself from any possible contact with the current.

How Does The Terms Flue, Damper, Stack And Chimney 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 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 chimney.
The damper is a device, usually a kiln shelf, that slides in and out of the lower part of the chimney in order to control the size of the opening and 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 broken kiln shelf piece.
The kiln chamber is essentially the chimney with the opening for the flue and the kiln shelf the damper.

Can't Reach Temperature No Matter How Much Gas Is Use?

This is the most common question regarding the raku firing.
The 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 and 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 opening at the top of the kiln.
Most pottery workers use a meter controlled 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!

Do The Tongs Used To Lift The Pottery Leave Marks?

Sometimes they do.
More often though the glaze is still melted 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.

What Does The Term Reduction Mean?

Reduction is a firing term that refers to a lack of oxygen in the combustion process.
This lack of oxygen in the atmosphere causes the fuel to search for oxygen elsewhere to allow for more combustion to take place.
This can take place during a firing as in reduction stoneware.
In this ordinary situation, the additional oxygen is found chemically bound in the the clay body and glazes.
The result is characteristic reduction effects.
Reduction as it is referred to in raku usually takes place out of the kiln separate from the actual firing as described earlier.
Now, just because you are doing raku doesn't mean that you can't experiment with actual reduction firing in the more ordinary sense.

Does Pottery Have To Be Remove From The Kiln To Apply After Firing Reduction Or Can Reduction Material Be Insert Into The Kiln Chamber?

As described earlier, after firing reduction is normally done by removing the pottery from the kiln and placing it in a container with your combustible material.
Certain situations may make it difficult to actually remove the pottery 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 after firing success by shutting 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 an 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.

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 pottery.
In other words, how much smoke your pottery is quickly subjected to.
Some materials have the potential to release more carbon than others.
The condition of your material such as wet, damp or dry, as well as the particle size as in the case of wood materials like sawdust, shavings or chips can be important. The type of wood can also affect your results.

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 after 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 after firing reduction and simply cooling your pottery when it comes from the kiln, then there is no smoke created.

What Is Smokeless Raku?

Smokeless raku is not really smokeless at all.
It is an after 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 turning your container upside down 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 take your pottery out of the kiln and 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.

What Is Meant By Naked Raku And Slip Resist In Raku?

Naked raku or slip resist refers to a variety of techniques whereby a clay slip is applied to the surface of the pottery.
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 pottery to varying degrees of after firing reduction.
When the piece is cooled, whatever slips remains on the pottery is removed by scraping and cleaning.
As a further decorative process, you can scratch and draw through the slip prior to firing to create other designs.

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