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Sunday, August 23, 2020
Saturday, August 22, 2020
Tips and Directions for firing updraft Alpine Kilns
Updraft Kilns vs. Downdrafts: The Alpine is a classic updraft. Flames enter the kiln from the bottom, and exhaust fumes exit from the top. Such a kiln is vented so that the exit gasses may be observed exiting the kiln before being gathered up by the kiln vent hood, and departing the kiln room via a fireproof flue.
In an updraft kiln the flames naturally assume the key features of a candle flame, yellow reduction at the top of the flame, blue oxidation near the bottom. So Alpines naturally reduce at the top, with some oxidation at the bottom and around the burner ports, with some turbulent areas around the middle and bottom offering the potential of being either oxidizing or reducing. Therefor when speaking of 'putting the kiln into reduction', what we mean is that the kiln will reduce a greater portion of it's loaded volume, and/or the areas at the bottom that naturally oxidize will be less heavily oxidized. Placing the kiln into heavy oxidation, may not totally eliminate some areas near the top which naturally tend to reduce, nor will putting the kiln into heavy reduction totally eliminate all oxidation.
By contrast, the physics of a downdraft kiln more naturally mixes the kiln atmosphere, due to natural turbulence caused by the tendency of hot gasses to rise up against the exit direction which is down. The beauty of the Alpine design is that with experience, once can load a variety of wares, requiring a variety of firing atmospheres. This means that temoku, and copper green glazes will do well on the bottom, while glazes requiring reduction or heavy reduction such as Shinos, Shaner and Copper Reds, etc. can be placed at the top.
Therefor, in loading the kiln, one wants to find ways to introduce turbulent airflow as the hot gasses move upwards if one wants an even firing on all shelves in the load. My HF-24 has a stacking area composed of (2) 11" x 28" shelves. I stack them spaced apart as much as 1" so there is some flame flue space through the load itself. The art of firing an updraft, may really thus be said to more significantly the art of loading. In no other kiln type, is loading so crucial in determining subtleties of oxidation and reduction.
Use of Digital Controls: I use the kiln's digital controls to set a basic rate of rise particularly at the beginning of every bisque. Running large burners even at 1/4" water column, will start the firing too fast for pots that may still have damp insides. I therefor begin all firings, particularly bisque firings, at the lowest possible gas setting which in my case is about 1/4” of water column, with a healthy supply of blower air, or about 45 cfm. Potters with smaller Alpine updrafts, or even the same size with smaller burners, have more leeway to manually control firing speeds at low temperatures.
A more expensive control valve for the Alpine may be purchased at a quality plumbing supply that allows gas pressures to be feathered to 1/8" water column or even lower. Installing this is a bit of a hassle, and I've managed thus far without this improvement.
The digital controls merely switch the burner gas supply on and off, and attempt to makes the kiln's thermocouples match a user entered program that specifies various ramps, and particular rates of rise. It is up to the potter on a digitally controlled Alpine to set burner blower, damper, and gas pressure settings, even when running with automatic digital controls. It is assumed that the reader of this document has some experience in programming digital controls, so the information given here will be on the basics of flame color, inducing reduction, and using the dampers, blowers, and gas pressure to control the kiln.
Using Pilots to Candle Soak the Kiln load: If your Alpine gets too hot from candling with just the pilots, then it is likely you’ll have to adjust the pilot pressure feeds to a lower setting. You want pilots at a setting, where, after 8-12 hours of initial soak, the kiln as a whole has reached a temperature that is above boiling, say 400 degrees on the top and 250 degrees on the bottom but not higher.
Biscuit firing presents the ideal opportunity to learn Alpine’s automatic controls. One can set a rate of rise schedule and then walk away and forget it. This is particularly useful when:
a) bisque firing . .
b) down-firing in relative oxidation after a reduction firing cycle has been completed,
c) holding the kiln at a particular temperature for a protracted period,
d) glaze firing to Cone 6 or Cone 10 where the entire firing cycle calls for oxidation.
I’ve included links to firing schedules from my own studio, from Vince Pitelka, University of Michigan, and from Creative Arts Workshop (CAW) of New Haven, CT.
History and Renovations of this Kiln: Most owners of Alpines have inherited or made modifications in some way. No two kilns look or fire the same. Here I'll supply details of what modifications I made during the time I owned it, and how that has worked for me as a high-fire potter.
MODIFICATIONS TO MY ALPINE KILN
Door: The 1" of Portland/vermiculite insulation was also chipped out of the door, after cutting off the outer layers of steel. In addition I welded an additional 1 1/2" of steel framework so that the door would benefit from an additional 1 1/2" of fiberboard insulation. The entire void of 2 1/2" was lined with new material, then the original steel sheeting and diagonal braces welded back in place.
Spyhole: As part of making the door on average about 2 3/4" thicker, new spyhole linings needed to be thrown from porcelain, and spyhole plugs crafted. My plugs are a full 9" in length, each 
carved very carefully from a single brick. The additional thickness has forced me to use new techniques for placing cone-packs in the exact location where they can be viewed through the slightly narrower field of view. (See photos)
Sides of the Kiln: The original Portland/vermiculite insulation was chipped out of the upper portion of the side walls of the kiln, approximately one-quarter way down the side walls, and replaced at the same time as the same castable used to redo the top. Another 1 1/2" of supplemental board and fiber insulation to the SIDES of the kiln are planned, but haven't been completed. (See photos).
Top of the Kiln: The kiln top benefitted from a new poured castable replacement of all the original Portland/vermiculite filler. I then added an additional 2 1/2" layer of soft brick. This required a re-engineer of the damper controls, so that they could operate at a slightly higher elevation. The dampers slide on bearing surfaces made from kiln shelves. (See photos).
Back of the Kiln: Steel sheathing was cut off the back of the kiln, and the vermiculite/Portland mixture chipped out entirely. This was replaced by 4 1/2" of soft brick insulation, top to bottom. My intention is to add yet another 1" of fiber followed by 1" of fiberboard. The reason for all this extra insulation? To slow the cooling of the kiln to promote crystal growth and complexity in complex glazes.
New Generation Alpine Digital Controls: New generation Alpine controls rescued from a kiln in a Manhattan basement, were installed by my good friend Joe Catenzaro who actually bought the Alpine Company, with all its plans and parts. Over the past few years he has been building brand new quality Alpine kilns with ultra-safe digital controls, and selling to schools. The burners on this particular HF-24 are from a larger Alpine, most likely a HF-30.
Bag Wall: I've seen many iterations of how the side bag walls adjacent to the burner ports are arranged. Originally, Alpine kilns shipped with two carbide kiln shelves slipped into slots built integrally into the kiln body. My kiln preserves this original configuration. There also were ceramic curved grates that sat on top of the shelf edge, designed by Alpine to produce turbulence in the flame body as it exited the burner port area. Most Alpine high fire potters I know remove these baffles since they impede firing efficiency somewhat.
Recently I saw an HF-20 where the carbide shelf baffle was less tall than typical, and spaced above the floor of the kiln so that the flames could exit above the shelf, or below. This seemed like a good idea and one I'd like to try. The very bottom of my kiln oxidizes, no matter what. I'm wondering if perforating this bag wall design by making the shelf narrower, spacing it up off the floor, or replacing it completely with a brick bag wall with lots of spaces in it, might help in the mixing of oxidation and reduction flames.
Gas Supply: The kiln is connected to a 2’ natural gas line, and runs 150 feet through my building from the hookup supplied by the local gas utility. It delivers 5-7 inches of water column at the point where the kiln is connected. Pressure does vary.
Notes on Numbers, Pressures, Rates of Rise, CFM: Please note that potters using propane or Venturi burners will not be able to directly translate the numbers represented here. Instead try to adapt your firing technique with the principles I am presenting.
After all, the Alpine kiln is an insulated box, with flames entering from the bottom and exiting at the top. The same issues of a) confining the flame, b) giving it air, c) letting it rise in temperature, or letting it cool, apply regardless the type of fuel that is supplied.
Temperatures in Degrees F: All temperatures are quoted in degrees F (Fahrenheit)
BISCUIT FIRING THE ALPINE
Cone Packs: I always use cone packs. If my target cone is Cone 07, I make two packs, each with a 08, 07, and an 06 cone. As a backup, the Alpine has two thermocouples, top and bottom, one can easily use the thermocouple temperature to determine when to shut the kiln off. Bear in mind the degree of biscuit is determined by the rate of rise as well as by the thermocouple temperature at the time of shut off. (Cone = Time x Temp).
The key to a successful bisque firing in any kiln is SLOW!
The Alpine has more than enough horsepower to break pots. Gas and power settings need to be kept very low, particularly at the beginning of the firing, with good air supply so there is NO REDUCTION AT ALL IN THE KILN.
A bisque firing may take as long as a glaze firing. This is normal. Don’t rush it!
Please bear in mind that pots break most frequently when liquid water boils off from the bodies of the pots at temps around 212 degrees F, and later when chemical water and other volatiles depart the clay between 600-1400 degrees F. Please refer to John Britt’s work High Fire Glazes, for ample description of what happens when clay fires.
Please refer to the handwritten Bisque Firing Schedule by production potters at CAW (found at the end of this document. This is a Cone 07 firing by pottery professionals working at Creative Arts Workshop in New Haven.
I’ve divided directions for bisque firing into four stages. They are:
I - CANDLING - ROOM TEMP to 300 degrees (some point in excess of 212 degrees F)
I live and work on the coast, humidity is high, and pots are never completely bone dry, except in the winter. Reducing moisture content of a kiln load can be accomplished by candling the kiln load. This is essentially a pilots only soak. usually overnight, or for a long afternoon and evening, adequate to take the kiln and its contents above 212 without breaking pots.
I have spoken about the importance of pilots not being adjusted to create too fierce of a flame. The primary reason is that though the pilots may at a very high setting advance a kiln in temperature, too aggressive a pilot flame will cause carbon to enter the pores of the unfired pots. Remember there's no air, so the pilot flame is creating some soot. Candle to above boiling, but not much above.
Once the kiln is safely hotter than 212 F top and bottom for a protracted period of time, it is then safe to start the main burners and begin the firing process.
Sometimes running just the pilots is not enough to take the bottom of the kiln above 212. If this is the case, and you are absolutely sure your pots are bone dry, you may light the burners and run them at the lowest possible setting to get the kiln above 300 degrees. Remember that on the inside of a pot, the water doesn't ‘boil’ at 212 degrees, or even quite a bit higher, due to the higher pressures of gas phase water inside the pot. The pot itself can contain some pressure. Bangs heard from within the kiln usually arrive at 250-400 degrees and mean that the slow start process has been rushed and, a pot (or cone pack) has exploded. The higher the temperature when the bang is heard, the faster you were running the kiln.
Now, mid-winter firings are different for us. If it is a cold dry January, and I know my pots are dry I let her rip. On to the next step.
II - INITIAL RISE - FIRING from roughly 300 degrees to 800 degrees (50 degrees per hour ideally)
Again the strategy is SLOW!!
Burners should be supplied with a maximum of 1/4” to 1/2” of water column of gas, or as low as they can be set without a chance of blowing out. The burner blowers should be set to medium. This ensures a vital oxidation flame with plenty of air.
During this period the gas should burn with a healthy roar, but not a rough cadence, and the entire flame should be BLUE.
When biscuit firing I set my dampers at 2” and let them stay there pretty much for the entirety of the firing. In our example firing the potter has commenced the firing with dampers set at 3” and raised the latter part of the firing to 4”.
The key takeaway here is that none of this is RIGHT or WRONG. You have to do what works for your kiln. In general opening dampers lets heat move from the bottom of the kiln on up to the top, and constricting the dampers has the effect of slowing the firing, and even putting the kiln into reduction.
Contrary to what one would expect, OPENING THE DAMPERS usually has the result of unifying temperatures between top and bottom of the kiln. IT IS ABSOLUTELY NORMAL to have the top of the kiln race ahead in temperature, especially at the beginning of the firing. However this discrepancy can become much larger if the dampers are kept too far shut.
The rate of rise during this period should be 50-100 degrees per hour. (My experience says that 100 degree rate of rise at this stage is too fast especially with thicker pots). Since the gas is at a very low setting, why rush it? Once all the ware is past 750 degrees most of the chemical water that is contained within the pots will be boiled off.
III - MID FIRING RISE - From 800 Degrees to 1600 Degrees (75-100 degrees per hour)
Not all of the chemical water will have burned off by the time the kiln is at 800 degrees. However it is not as critical to keep the rate of rise on the very slow side, but can be increased to 100 degrees per hour.
[Note: There is one key moment in clay firing development and that is called “Quartz Inversion” that occurs around 1063 degrees F. At this moment all of the quartz (silica) molecules in all the pots, the ware, kiln furniture, and bricks undergo a transformation which results in a sudden expansion in size of around 1-2%. If quartz inversion occurs at different times and places on a pot that has not achieved a uniform temperature, then the pot will most likely crack.]
Note on the CAW Bisque Firing: In this instance the potter increased gas pressure to 1/2” water column and left the kiln unattended overnight until the following AM. I don’t recommend abandoning a bisque or glaze firing for such a long period until a successful schedule has been established that can ensure that the kiln will not have shot past the point for optimal body reduction. Heather and Margie however have fired this kiln hundreds of times. That is key. They have gotten to know their kiln.
At around 1200-1400 degrees F a burner flame will become evident emerging from the top of the kiln. The flame color should be BLUE, and it should exit the kiln with some force. Until the beginning of body reduction this exit flame should not be yellow!
Check exit flame at the top spy hole: Remove the spyhole plug. At no point during a bisquit firing should the flame exit with a yellow or smokey exhaust. If so you need to increase the burner blower speed, open the vent, or lower the gas pressure to the burners.
AUTOMATIC vs. MANUAL CONTROL - Because my set of burners are very powerful, I have sometimes opted to let the kiln manage a slow rise in temperature via computer control. Other firings I operate the kiln manually. The key thing is RATE OF RISE. If you cannot manually a low enough rate of rise using manual techniques (changing gas and blower settings) then use the Alpine computer to control the firing (which it does by turning burners on and off).
IV - END PERIOD - 1600 Degrees (roughly) untill end of firing. (108 degrees per hour)
Returning to our sample firing, in the morning the potters Heather and Margie increased the gas pressure to 1” of water-column and increased the burner speeds from 35 to 80. I finish my own bisque firings with the gas set no higher than 2 1/2” This will vary between kilns and burners.
Notice that the main dampers were opened from 3” to 4” during this period. While this firing appeared to be successful, I believe that having the dampers open as much as 4” wasted a lot of heat, but they did manage to contain the difference between top and bottom kiln temperatures.
Note on the Dampers: An updraft kiln will run most quietly when there is steady even combustion within the chamber. Find the magic spot to match damper settings with gas and air to maximixe combustion within the chamber. Avoid over-managing a natural differential in temperature between top and bottom. Aim for temperatures to converge by the end of the firing.
UPDRAFT GLAZE FIRING
Glaze Firing is very different from Bisque Firing.
The first part of a glaze firing proceeds MUCH FASTER because the pots have already been biscuit-fired and there is less danger of them breaking from expanding moisture. On the other hand the latter part of a glaze firing is much slower, primarily due to the fact that reduction slows the rate of rise, and the target shut off Cone to be met is much higher.
Straining the speed of the kiln will waste energy. Aim for a steady sustainable rate of rise that brings all the ware along with it in pace. A slow(er) rate of rise is generally preferable to fast. However naturally the first period of a glaze firing will see temperatures climb more quickly, for no other reasosn than the glaze firing contains less ware, and all the pots are at least biscuit fired.
Packs: Two cone packs in front of each spyhole for a Cone 10 Reduction Firing.
Reduction Pack: If I’m reducing at Cone 010 I use a pack with a 09, 010, and 011 cones facing to the RIGHT in the same pack and place that pack directly in front of each of the two spyholes.
Firing End Cone Packs: Behind the reduction pack I have my Cone 10 firing packs, each comprising a Cone 9, 10, and 11 cone set facing in the opposite direction to the reduction packs.
Bear in mind that it may be cooler in the kiln directly in front of the spyholes, particularly at the lower spy where a slight draft may enter from the spyhole plugs. Therefor it is advantageous to set the two cone packs consistently in terms of distance from the kiln door face in order to ensure repeatable results.
Automatic vs. Manual Control: Whereas I bisque fire automatically, I glaze fire entirely with the kiln under manual control. In actual practice with the Alpine this means entering a one-step control program that is completely impossible, such as ramp to 2400 degrees in 1 minute. This ensures burners remain on for the entire firing thus are only subject to manual control.
I use the automatic controls heavily when downfiring..
A Cone 10 Glaze firing may be divided into five stages:
I - CANDLING the KILN - FIRING from ROOM TEMP to 300 degrees (some point in excess of 212 degrees F)
A good soak prior to a glaze firing ensures that glazes are adequately dry on the pots and that kiln packs are well dried out before going into a substantial rate of rise. A hastily made cone pack can blow up early on in what ordinarily would be a perfect firing. Give them time to dry!
II - INITIAL PERIOD - FIRING from 300 degrees - (212+) to 800 degrees (Rate of Rise: 100 degrees per hour)
While I might advance the temperatures of a biscuit firing through these temperatures with the gas pressure at just 1/4” here we can safely begin at 1/2” and advance to 1” by the end of this firing stage.
So far this stage of the firing is like the biscuit firing previously mentioned, except that it is somewhat faster. With experience this period can be speeded up significantly. Some glazes do not like being elevated in temperature too quickly.
Heavily applied Shinos that have not dried completely may crawl or crack and fall off the pot in scabs.
Again . . . you can rush . . . . but why?
Burner blowers should be set to medium or medium-high. This ensures a vital oxidation flame with plenty of air.
During this period the gas should burn with a healthy roar, but not a rough cadence, and the entire flame should be BLUE.
III - MID PERIOD - From 800 Degrees until Body Reduction or approx.1600 degrees (Rate of Rise: 100 degrees per hour) During this period it is vital that the kiln be run in full oxidation, with no unburnt hydrocarbons exiting the kiln. In addition there is one dangerous passage, a period responsible for the most dunting (cracking) in fired pots, and during this transition I purposefully slow the kiln down.
During good oxidation the upper spyhole will not exit any flames at all when the plug is removed.
Use your sense of smell to verify Good Oxidation before Body Reduction: " In the very early stages of the firing, before any red heat is present, the only indication of excessive reduction is the unpleasantly sweet, acrid smell of hot, unburnt gaseous hydrocarbons." Vince Pitelka, Tennessee Tech University Appalachian Center for Crafts Clay Studio, 2009.
Caution, a word on Safety: It is very important when opening spyholes of any kiln, to employ great caution before bringing one's face close to the opening, either to look at cone packs, or scent the exhaust. Do this by first opening the spy, then pass a hand or finger in front of the spyhole rapidly, to check the direction of flame. Pitelka continues . . .
"To check for reduction smell, remove the top spyhole plug, stand off to one side, wave a gloved hand in front of the spyhole towards your face, and sniff the gases. If the kiln is oxidizing, you will smell nothing, because no gases will be exiting the spy holes. If the atmosphere is neutral, there will be slight back pressure and the fumes at the spyhole will smell like a clean burning gas stove. . . . If you smell reduction, open the damper by 1/8” increments until you get the clean neutral smell, and recheck to make sure that the gas pressure and primary air shutters are properly adjusted." Vince Pitelka, Ibid.
Quartz Inversion: This clay transition occurs around 1063 degrees F and is a moment when all firings, bisque and glaze should slow down. At this precise temperature all of the quartz (silica) molecules within the pots, ware, kiln furniture, and bricks all undergo a transformation which results in a sudden expansion in size of around 1-2%. If the kiln has heated quickly, different parts of the wares have different temperatures, so expand at different times, causing cracks. This is called dunting. Also know that If a kiln is not well enough insulated, and if a fired load cools too quickly, (which may be caused by under-loading), dunting may occur at exactly this temperature during the cooling cycle. Listen for the sudden expansion or contractions - you can actually hear them!
The potters at CAW slow their firings at this juncture, by letting the kiln soak fully for a half hour at 1060 degrees F.
After Quartz Inversion run the kiln as before, in full oxidation until you approach Body Reduction.
IV - BODY REDUCTION PERIOD - At around 1600 degrees. (1 hour with no particular degrees rate of rise)
I normally body reduce at Cone 010, around 1580 degrees. Body reduction may begin as early as Cone 012 if the firing has pots slated for carbon trapping, but ordinary stoneware may be reduced at Cone 010 or even as high as 08. It is very difficult with the Alpine kiln to catch reduction reliably at the last possible moment, particularly if the kiln is running fast.
Therefor Cone 010 is almost always the target cone for initiating reduction, unless one has a lot of material one wants to carbon trap. The heaviest reduction almost always occurs at the top. The importance of beginning body reduction at exactly the same moment is critical if one wants repeatable results. It is important to use a set of separate Cone Packs for body reduction. Creep slowly up in temperature on the dropping of the desired cone!
Body reduction is a time-based thing. The reducing atmosphere should be held for a minimum of 1/2 hour but I reduce for 1 full hour. It is important that the kiln not advance much in temperature during this period, certainly not past Cone 08 (Val Cushing). There is not much danger of a rapid temperature advance if heavy reduction has been commenced.
I start heavy reduction much earlier when carbon-trapping Shino glazes. For this I initiate the reduction at Cone 012, and getting the timing of this part of reduction exactly right is not an easy feat. I'm still learning how to refine this process. It helps to really slow the kiln down as one approaches the Cone 012 temperature zone, and hit it at a rate of rise of 108 degrees or optimally, less. Remember that thermocouple temperature is not the same thing as what is happening to the insides of the wares. For this reason a chart of Cones vs. Rates of Rise is very useful for determining the optimal moment to start shutting dampers and increasing gas pressure.
The important idea here is that unless you are making rate of rise calculations off the Alpine's digital controls, use cones to decide when to start body reduction. Using only thermocouple temperatures risks at best missing the target cone completely, or worst, carbon trapping a lot of pots. Cones have a vital role to play even in this world of digital controls.
Body Reduction reduces metallic oxides in stoneware bodies to their metallic form, giving the clay of the pot an attractive color. It develops a speckled look in higher iron bodies, and helps the glazes themselves stay reduced via contact with their inner surface. One can fire and reduce glazes without body reduction, but to attempt to just reduce the glazes later on, does not yield reliable results, nor is the reduction even around the pots, because the glaze layer is so thin.
Body Reduction is accomplished by:
a) Increasing Gas Pressure. If the kiln was running at 2 1/2” of water column or less, increase the pressure to 3” or even as high as 3 1/2”. I have done firings where I needed to advance gas pressure to 4", and others firings where just 3" water column was necessary to maintain reduction.
b) Reducing Blower RPMs by about 50%. Whereas the kiln normally oxidizes with the blower motors set to 45-50, lowering to between 25-35 will be sufficient to maintain a good reduction.
c) Choking Intake air on the burner ports. I do this with pieces of duct tape - the potters at CAW use heavy artist's card (mat board) and let the suction keep the card attached to the blower surface. I use the tape to block as much a 60-80% of the area of the burner port intakes. [Alpine burners have a flap to do this but the blower vent controls bend easily and don't always fit flush to the blower surface, that’s why we use tape or paper.]
d) Closing Side Vent Plugs (at base of kiln on both sides).
e) Make minimal adjustments to the damper openings. Usually this setting is revised down about 25% from 2” to 1 1/2” during body reduction, but is later opened up again in order to facilitate an even rate of rise between top and bottom. On some firings. I’ve found changing damper settings is not necessary.
KEY INDICATORS OF GOOD REDUCTION: As soon as body reduction starts, the flames exiting the main flue will turn to a smokey yellow orange, or a yellowish-green. Ideally the flame will be something in between, not entirely orange, but absent any blue color. Also pull out the top spyhole plug. A deep yellow sooty flame should exit the port and be at least 1 1/2”-2” in length. (Val Cushing).
The aforementioned steps will vary between kilns, and even between firings, because loading affects kiln behavior so much. You will be judging reduction based on the color and length of the flames exiting the top of the kiln and the upper spy hole when the spy plug is removed. Those flames are your best indication of oxidation vs. reduction. The picture below shows HEAVY REDUCTION during a carbon-trap firing.
Shinos tolerate a huge range of firing atmospheres.
After a good hour of body reduction you may lighten up on reduction and get the temperature rising again. Do this by:
Throughout the body reduction period there are no temperature ‘targets’. You simply want to reduce the ware for a period of at least 1 hour. The kiln may even drop slightly in temperature during heavy reduction.
Oximeters may be used to check reduction levels from the moment body reduction starts.
From Cone 5 through Cone 9, when the surfaces of Cone 10 the glazes begin to vitrify and ‘trap in’ the reduction from the atmosphere against the previously reduced clay body.
Without some body reduction, obtaining glaze reduction becomes extremely difficult, since the outer body layers of clay are what take or give oxygen molecules to the glaze components. As the clay, so goes the glaze.
END OF FIRING - (Rate of Rise: will vary between 150 and 60 degrees per hour)
a) Decreasing gas pressure somewhat. It may have been as high 3 1/2” of water column. In which case you might reduce the pressure to 3”.
b) Increasing blower motor rpms. I increase the rpms from 20-30 to about 45-50.
c) Opening the damper if you had closed it, to about 2” to 2 1/2”
d) Removing the paper or tape choking the air intakes of the burner ports. I sometimes do this in stages.
e) Removing the side port plugs and inserting what I call ‘donut spys’ into the side vent ports of the kiln. These are essentially 4” round kiln posts with a hole through the center. This allows some air to enter through the side ports, but not so much as to over oxidize the kiln load.
From this moment on you are essentially managing a lesser degree of reduction until the end of the firing. With my firings, once body reduction is finished the kiln never leaves a state of partial reduction. The flame is never brilliant blue again, is always instead at minimum, a greenish yellow.
For some potters managing this stage of the glaze firing is the most difficult. They complain, ‘my kiln has stalled’. In reality small changes to kiln settings take a few minutes to notice. Be patient. Don’t over control the kiln. If the dampers are at a non critical setting such as 2” try to manage the kiln entirely through a balance of burner blower speeds and gas pressure.
As the kiln advances higher in temperature it becomes more important to use light fingers on the controls. Controlling a kiln at high temperatures is akin to controlling an airplane at high altitudes. The wrong move can cause a rapid decrease in altitude.
It is critical to manage the combination of gas pressure, blower rpms, and damper grate opening in such a way as temperature increases steadily, but without losing some degree of reduction (as evidenced by the exit flame).
I fire this way all the way to my target Cone for shutting the kiln down, which for me has usually been Cone 10.
Downfiring: There is an advanced technique called down-firing following a typical glaze firing cycle during which time the potter may lower the temperature of the kiln slowly in a series of downward saw scale steps in order to promote crystalline growth in glazes and deepen the results of glaze reduction. This is different than strike firing (see John Britt's book High-Fire Glazes on this topic). The key difference is that a strike is performed to glaze the ware on the way down, and is done when the previous body reduction step has been missed. Down-firing is a technique for essentially performing a fuel assisted cooling cycle, and is a process ideally suited to control by the Alpine computer. Once the main firing has completed lower the kiln temperature slowly in a series of downward steps starting at around 2000 degrees F and ending at 1850 degrees F using the computer, over a period of 4-5 hours.
Many Japanese and wood fire potters downfire. The objective is to duplicate the effects of a much larger kiln that naturally cools more slowly.
One way to achieve some of the benefits of the slow down-fire cycle is to leave the pilots on during the cooling cycle of the kiln. The damper grates are shut almost entirely, and the main burners turned off at the gas source, but the pilots are left on to slow the cooling process.
SHUTDOWN - Shutting off and sealing up the kiln once the firing or down-fire cycle is completed is an important step. Damper grates should be closed entirely, some manner of blocking the burner ports ideal (I use pieces of ceramic plates and fiber), also the side ventilation plugs inserted. The main gas supply needs to be shut down, but I leave power on to the main control either to allow the pilots to keep burning (in which case I leave a slight crack in the dampers) and to be able to observe the temperature inside the kiln in order to guage the time to open the kiln and observe results.
KILN OPENING: The opening of the kiln is often rushed, by cracking dampers, and even the door. You'll know if you did it too fast - pots will ping loudly, some may even break. Best to allow a full 36-48 hours to allow the kiln to cool completely and slowly. You'll save your pots, and your kiln!
SUPPLEMENTAL Materials:
Mark Potter's Alpine Bisque Firing Schedules
ClayArt Resource Database on Alpine Kiln Firing
Alpine Glaze Firing Schedules
CAW (Creative Arts Workshop in New Haven CT) Notes on Firing:
CAW (Creative Arts Workshop) Bisque Firing Schedule:
David Bolton, Central Michigan State University, Ceramic & Firing Notes
Vince Pitelka's famous Notes on Firing the Alpine:
Unloading a well-loaded Bisque:
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Guide to Chaga Harvesting and Preparation
I've already posted on the positive benefits of Chaga for the health. Other sites on the web go into detail about this bounty of th...
