Successful Tips and Techniques for Raku Firing: How to Select Raku Clays, Glazes, Kilns and Combustibles
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Raku firing is expressive, exciting and fun. Whether you’re raku firing in your own studio or taking part in a group raku firing at a school, workshop or community center, raku offers many rewards. Raku firing is one of the most exciting processes in ceramics. After you place your pottery into a raku kiln, the anticipation builds as you wait for that final moment when the intense heat begins to melt the raku glazes. When you remove the pieces when the glazes begin to melt, you can feel the heat and hear the pings your red hot work rapidly cooling, then it’s into the raku combustibles for a round of flame and smoke. Many surprises await you as you clean the surface and reveal the wonders of raku pottery.
Check out this excerpt:
How to Raku
by John Ramer Sherrill
Raku pottery has become tremendously popular in the United States. The wide range of glazing and firing methods, and the surprises that lurk in every firing hold the interest of potters year after year.
Many achieve consistent results, but I’ve talked to dozens of established potters as well as students who have been unhappy with their raku attempts. Most complained that they had been unable to find specific information to properly guide them in their efforts.
For my own early efforts, I obtained several books on the subject, but found them frustratingly long on philosophy and short on technique. I still couldn’t properly fire a raku pot, but I could use my new-found knowledge of Zen to cope with the situation. I don’t believe there are many raku enthusiasts who are interested in my philosophy, wondrous as it may be, but I know for certain that some want to know how to do raku.
Select a Raku Clay
A wide variety of clays can be used to make raku pottery. Be aware, though, that the clay used determines much of the character of the finished piece.
Most suppliers sell a body designated as “raku” clay, which is usually a grogged clay that includes Kyanite. It is the clay of choice for really massive raku pieces. Suppliers also usually have bodies that are designated as “ovenware” clay. These clays, which contain less grog (easier on the hands), often make an ideal raku body. I use ovenware clay almost exclusively.
Form and Dry Raku Pieces
Raku pots are usually wheelthrown or handbuilt. I’ve heard from several sources that cast pieces cannot be raku fired, but I’ve never had a problem with them although you’ll need to test them.
Some consideration needs to be given to proper drying. As a general rule, drying pots of 3 pounds or less does not require special handling. I often force dry and bisque fire the same day. Flat pieces and large pots must be dried slowly and evenly, though. Large ovenware pots will often survive fast drying, but the stresses that are thereby induced will cause them to crack at a later stage.
Decorate with Slips
If you want a colorful pot, you may use oxides or stains in the glaze, but they may mask the dark crackles to some degree; some almost entirely obliterate them. For that reason, I use colored slips under a clear glaze. My slip base is simple—1 part ball clay to 1 part EPK kaolin. Just mix it with water to a cream consistency and add stain. I use commercial stains in percentages ranging from 2% (dark blue and green stains) to as much as 30% (pink stains).
In order to choose the stains that will work well, it is good to know their chemical components. Most commercial stains will block carbon to some extent, with the worst offenders being those that contain iron or vanadium. Vanadium is present in most warm-tone commercial stains, so you should use titanium yellow, praseodymium yellow or zirconium yellow in combination with other vanadium-free stains to formulate your own palette.
Ideally, the slip should be brushed or dipped on at the leather-hard stage, but it works on bisqueware as well. When brushing, you should apply three coats for dark colors (when you don’t want the body showing through) and two coats for lighter colors (a warm-tone body showing through some, such as pale green, can be very attractive). The pot should then be bisque fired in the cone 08 to cone 04 range.
Glaze Raku Ware
After bisquing, the pot is ready for glazing and the final firing. Glaze should be applied fairly thickly. If you dip, the consistency should be about that of thick cream, and one dipping should suffice. If you brush, the glaze should be somewhat thicker, and two or three coats should be applied. Evenness of application is not particularly important.
It is somewhat traditional to leave the area near the base unglazed. This will turn quite black in a good postfiring reduction, when carbon penetrates the still-hot pot. Other areas may be left unglazed as well. These areas may be random or symmetrical, and can greatly enhance the beauty of the finished piece.
Select a Raku Kiln
Because red-hot pots are removed from the kiln, it is apparent that many models simply are not appropriate for raku firing. Large kilns of any type, when opened at temperature, radiate heat so fiercely that it would be foolhardy to attempt rakuing. Top-loading kilns are not ideal since you must position yourself above the kiln in order to reach inside, and the rising heat can be overpowering.
Small (2 cubic feet or less) front-loading electric kilns may be used, but most raku firing is done in gas kilns especially constructed for that purpose. See “Buying a Raku Kiln” page 8.
Fire a Raku Kiln
Raku kilns, unlike conventional kilns, are usually loaded on a single level, and spaces between pots are left a bit wider in order to facilitate their removal. It is certainly possible to use multiple levels, but it isn’t worth the hassle, as far as I am concerned.
Traditionally, the maturity of the glaze is determined visually. The kiln is opened, and the pot surfaces are examined for complete glaze melt. If mature, the glazes will appear wet and reflective. I strongly recommend using a pyrometer in conjunction with this technique, noting the temperature at which maturation occurs. After a few firings, you will need to look only at the pyrometer to determine unloading time. There is some medical evidence that prolonged or repeated staring into a red-hot kiln can damage vision.
When examining the pot for complete glaze melt, look for bubbles in the glaze, as these can mar an otherwise perfect pot. Even if the bubbles burst when the kiln is opened, unsightly craters will remain. Bubbles are almost always present on my pots because I fire rapidly, so I simply assume their presence, and take steps to remove them.
To accomplish this, partially open the kiln just long enough to drop the temperature by 200°F or so (I give it about a 10-second count). Close the kiln and bring the temperature back up to near maturity. Give it a couple of minutes for the craters to heal. If you have clusters of bubbles, you may have to repeat the procedure.
Post-firing Reduction in Raku Firing
The final phase of raku firing requires the still-hot pot to be placed in combustible materials inside a fireproof receptacle that can be covered, the tighter the better. The combustible material can be sawdust, straw, leaves, newspapers or anything else that readily catches fire. I prefer a bed of sawdust covered with crumpled newspapers, but I suggest trying different materials to discover what best suits you. See “Finding the Right Combustibles for Raku Firing” on page 16. In any case, the bed of combustibles should be prepared in advance of the firing.
Post-firing reduction is where the novice usually runs into problems. It is potentially a dangerous process, so always take precautions and exercise extreme care. You will be working closely with temperatures up to 1800°F, so you must train yourself to touch nothing without first considering whether or not it may be hot. After a long rakuing session, I actually catch myself hesitating before entering my home, considering whether or not the doorknob is hot. It is a useful habit to cultivate.
Cover as much of your body as possible (always wear long sleeves), but don’t wear polyester. Taste in clothing is not the problem; the problem is polyester will melt and conform to your body like hot glue. Heat-resistant gloves are a must. A hat and face mask are not absolutely necessary, but are a good idea. I prefer to remove large pots by hand, but for this, special heat-resistant insulated gloves must be used. For smaller pots, long metal tongs are suitable.
Reduction techniques vary quite a lot, so I will simply describe my own; modify as you wish. I remove the pot and place it on a fire proof surface, then wait for cracks to appear in the glaze on the rapidly cooling surface. In bright sunshine, these cracks generally appear as shiny lines. They will announce their appearance with audible pings or pops.
Only then do I place the pot into the reduction receptacle. This action takes place for a small pot, such as a bud vase, in as little as 10 seconds. A very large vessel (5 pounds and up) may require 90 seconds or even longer. This timing from kiln to post-firing reduction is very important, as it will, in large part, determine what kind of crackle effect will be achieved. I find that the sequence I have described gives deep, dark, widely-spaced crackles usually interspersed with networks of finer lines. Varying the timing should soon show you how to get the effect you prefer.
How to Quench a Raku Pot
A lot of pots are lost in the raku step called “quenching.” After reducing for a minute or more, the pot is removed with tongs and submerged immediately in a container of water. The water hisses and bubbles, and the hot pot rolls about as if in pain. Those with narrow mouths will gyrate wildly, and will sometimes rocket themselves clear out of the container. All this commotion by a suddenly animated pot is undeniably a lot of fun, but I no longer enjoy it because I no longer do it.
These days, I just leave small pots in the tightly covered reduction chamber for about 5 minutes, remove them and place them on the ground to cool. I leave large pots in reduction for up to 30 minutes, long enough for them to drop below the quartz-inversion temperature (1063°F), because that is when a large exposed pot is in serious danger of cracking. As far as I’ve been able to determine, the only thing I lose by not quenching pots are pots and, yes, a bit of fun. After the pot cools enough to handle, all that remains is to scrub it vigorously with a metal pad or wire brush.
by Bill Jones
The raku ﬁring process requires a porous non-vitriﬁed clay that can withstand rapid heating and cooling without cracking or breaking from the thermal shock. By this deﬁnition, any clay that can withstand such stresses can be considered a raku clay; however, some clays will provide a greater degree of success. When deciding on a suitable raku pottery clay, your chances for success increase with bodies speciﬁcally formulated or adjusted for the raku pottery process.
by Steven Branfman
A raku glaze is any glaze you use in the raku pottery method. It doesn’t have to be a glaze speciﬁcally designed for raku, formulated to ﬁre at the temperature you ﬁre your raku ceramics to, nor homemade or commercial. Read how your raku glaze can be most anything. The key to success is understanding the raku ﬁring process and the ability to predict how a particular glaze reacts to that process.
Buying a Raku Kiln
There are many conﬁgurations for raku kilns—top loaders, front loaders, top hats, car kilns, and clam shells. Here’s a brief overview of what you need to know to buy the raku kiln you need.
Pop Goes the Slip! Naked Raku
by Kate and Will Jacobson
Naked Raku gets its name because an outer covering of slip falls off during firing leaving the naked clay body behind. Discover what’s underneath by trying out Kate and Will Jacobson’s naked raku techniques.
Nature Inspired Firings
by Sinead Glynn
Want to up your raku game? Try the ferric-chloride firing technique–a unique alternative firing process that involves dipping and pouring ferric chloride onto fired clay. This technique is often used alongside other bare clay techniques and produces a wide variety of surfaces.
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