Speed is NOT your friend. Your momma told you that very thing when you got your learner's permit as you learned how to drive the family car. The police officer writing your first speeding ticket most likely told you the very same thing as he congratulated you on for doing 75 in a 15 mile hour school zone. You have heard it often enough, but have you ever stopped to think why speed is not your friend? More importantly, do you know why firing fast is not good for you, your wares or your kiln? You don't? Well hang on for a moment while we get started....
Speed = Blow Ups
(Click here for the shortened version)
The classic kiln explosion can generate enough force to destroy all of the wares in the kiln and may even destroy the
kiln itself. What causes those explosions? Water. Specifically the expansion of free water from the physical liquid (water) state into the gaseous (steam) state
will cause those nice explosions. Typically this water is generally classified by ceramists as trapped water but the proper term is free water as the water is
neither mechanically occluded nor chemically bound with other molecules and is free to depart the clay during normal evaporation. In general the majority of
the free water will be driven off (evaporate) while the ware transforms (dries) from the leather-hard to "dry" greenware stage. The remaining free water will
remain within the pores of the ware (the small spaces between clay particles) typically in direct relation to the ambient humidity levels though some
additional free water may become mechanically occluded in small voids of empty space deep within the clay structure. If the ware is rapidly dried from
leather-hard to the "dry" greenware stage there is also a risk of the clay "skimming" over, trapping water underneath the surface because dissolved salts
in the clay (especially due to deflocculates in slip casting bodies) will rise to the surface early in the evaporation process and crystallize on the surface
of the ware trapping the remaining water. Some of this water trapped by salt crystallization can be considered mechanically occluded, but a better term would
be trapped water or bound water even though those terms are not strictly correct in usage. The process of "skimming" over is identical to the process of surface
water evaporating from a lake or pond leaving behind clay chunks which appears dry on the surface but remains quite moist well within.
Much of the free water which remains in the clay structure will evaporate as water vapor as the ware is heated slightly or the ambient humidity level is reduced. Reducing the ambient humidity is difficult due to the size, shape and quantities of our wares, so most clay artists (potters or ceramists) will use the heat of the kiln to "candle" the ware. Candling is the process of slowly raising the temperature within the kiln from room temperature to 212°F (the boiling point of water) to slowly remove any residual water from the clay. Candling is a delicate art due to the complexity of the clay structures, most specifically because water deep within the pores of the clay could potentially reach an excited state before the water actually has a chance to evaporate from the ware. Excitement is not good. The moment a water molecule changes from the water state to the gaseous state the molecule undergoes rapid expansion to a volume 1,600 times greater than volume occupied by a water molecule. Explosive expansion would be a better way to describe the event. Thick walls of clay and other water molecules hanging around will not present much resistance to the rapidly expanding water molecule. The sound of the clay exploding in the kiln has been described "like a flash-bang grenade" going off. Hopefully only the single item will be destroyed, though all too often complete kiln loads of ware can be destroyed by the shock waves reverberating through the kiln. In extreme instances the kiln itself can be destroyed as the insulating firebricks are broken by the shock waves. Even if the shockwave is not powerful enough to destroy the firebricks, the elements in the kiln can be destroyed simply because they become covered with clay debris.
Free water remaining in the clay is not the only issue we are concerned with. Trapped water (water bound in the ware due to the surface of the clay skimming over), mechanically occluded water and water of crystallization will eventually need to be expelled from the clay structure. Of these three remaining water types, the trapped water is usually the only concern. When clay skims over, the trapped molecules of water can have concentrations of dissolved metallic salts which can prevent the water from boiling at the normal 212°F temperature. If the water molecule requires a slightly higher temperature to convert to steam, the conversion process from water to steam will occur at a higher energy level and can be extremely violent. Well above the boiling point of water (approximately 350°F), most of the mechanically occluded water and some of the water of crystallization will finally be driven off as steam, though some water of crystallization may be present above 800°F. Fortunately the quantity of mechanically occluded water and water of crystallization is typically quite minimal AND the steam molecules find easy escape paths because the residual free water has already been removed as have some of the organic materials been oxidized and removed from the clay body.
Ok, here is the simple version:
Clay doesn't ever dry. Sometimes the piece may appear dry but will really be wet within the piece.
When the piece is fired in the kiln, the water in the piece will be converted to steam.
If the steam can not find an easy and quick way to exit the piece, the steam will expand and the force from the expansion will blow the piece up. Might even destroy your kiln in the process.
Most of the water in the piece will evaporate or be converted to steam well below the 212°F boiling of water, but some water can remain above 212°F.
Water being converted to steam at temperatures above 212°F is not good.
Solution to the Problem?
SLOW is our Best Friend.
Fortunately the modern
digital controller has SLOW as a
normal firing operation. Even better than the SLOW setting is the use of PREHEAT. Preheat Mode, found in the
Orton Sentry 2.0 Digital Controller on all
large Paragon Kilns, will cause the kiln to rise at 1°F/minute from starting ambient temperature to 200°F and then hold the temperature for 1 minute (or
longer if desired such as when extremely thick wares are being fired). The use of Preheat Mode will cause the kiln to properly candle the ware such that most
(if not all) free water will slowly evaporate from the ware as water vapor rather than being converted to steam and risking a
Simple, simple solution.
* Preheat Mode ONLY controls the rate of climb from startup temperature to 200°F. At 200°F the Preheat Mode will hold the temperature for requested period of time and then release the Digital Controller to follow the Slow, Standard or Fast firing modes previously programmed.
Isn't all that moisture bad for the kiln? Yep, sure is. Clay products should be "bone dry" before being fired in the
kiln. What is bone dry? The simplest answer is the piece will appear the same color (no surface moisture discoloring the clay) from the top to the bottom and
feel dry to the touch especially at the bottom or any thicker areas.
What can be done about the moisture that is generated from firing wet ware in the kiln? First of all, avoid firing wet wares. But even the driest ware still contains moisture that will need to be removed from the kiln at some point. In the old days, one would crack open the lid (door) of the kiln during the first hour or two of the firing to allow the moisture to escape. Cracking the lid also allows heat to escape and wastes energy. Today, most people use the Orton Downdraft Vent Master
to continuously draw a small amount
of air through the kiln to remove toxic gases as well as water vapor. Obviously the Orton Vent Master eliminates wasteful energy practices, such as cracking the
lid open, and insures water vapor will be removed long after the kiln lid would have been closed in the old days.
Can I do anything to accelerate the drying process? NO. It is always best to let the piece air dry fully. Accelerating the drying process can causing skimming to the surface of the ware which can trap moisture under the clay surface creating complex drying problems. In addition, skimming can create a brittle surface of salt crystals which may less receptive to the application of glazes and underglazes and can lead to a number of glaze defects.
What about the use of a drying room? Any acceleration of the drying process from leather hard to bone-dry greenware should be avoided unless slow, even heat AND air flow is maintained. The same rules apply to the use of dehumidifiers. Slow and Even is always the best.
Isn't the slow firing of a kiln a wasteful energy practice? Technically yes. Technically. In reality the amount of energy wasted is minimal at best. Why? Because at very low temperatures the wares and kiln shelves within the kiln will absorb far more energy than the IFB (insulating firebrick) of the kiln interior because the IFB are refractory (resistant to absorbing heat). A portion of the energy absorbed by the IFB can be lost when heat is radiated to the kiln room, BUT, at very low temperatures, more specifically when there is little difference between the kiln interior temperature and the ambient (room) temperature, the IFB are extremely efficient at retaining heat energy.
In Dogwood's studies of the Orton Preheat Mode versus Standard firing speed without using Preheat, the use of Orton Preheat added a few pennies to the $9 electrical cost computed by the Orton Digital Controller for identical loads of ware fired in the Paragon TnF 27-3. Considering room temperature could not be exactly controlled and is a variable in the calculations, a few pennies in firing costs is within the margin of error for the test.
Follow-up: Dogwood's testing of Preheat versus Standard Speed without Preheat did not reveal any significant costs in firing. However, the use of the SLOW mode for a complete firing cycle would, potentially, be less cost effective than using STANDARD. That said, the amount of wasted energy (energy lost due to heat being radiated by the IFB during the firing process) would be minimal... potentially less than 10% of the total energy consumed... as firing slower does not require the elements to consume full power for as long of periods of time AND much of the additional firing time is generated at very low temperatures where IFB are best at retaining heat. The use of FAST could potentially reduce the energy consumption of the kiln as the amount of heat loss by the IFB is reduced. However, as the elements would be operating a higher percentage of time it is possible that the reduction in heat loss would be offset by higher energy consumption. Please note: even if firing Fast was a breakeven energy proposition, the potential glaze and bisque defects that may arise due to firing fast would offset any energy benefits.
Speed is NOT your Friend, Part II
How fast can kiln reach Cone 04? Oh yea, we just love that question. How fast? With a modest sized kiln chamber found in many hobby kilns, assuming a 3" wall brick kiln for maximum energy retention, a fresh set of elements, big enough wire to carry the juice and the well feed squirrels down at Mississippi Power spinning the turbines as fast as they can... just under 2 hours to reach 1922°F. Of course everything inside the kiln will be ruined, but what the heck ya wanted to go fast.
Remember, Speed is NOT your Friend.
Why is speed an issue? Several reasons. First and foremost, around 1060°F (actually 575°±2°C for those who are technically minded) the clays in the kiln must undergo what is commonly called the "Silica Inversion Point". At that temperature the silica in the clay undergoes a physical transformation in the rotation of the silica molecule. This silica inversion point is a major event. Firing too fast (or cooling too fast) through the silica inversion point and the silica may not properly rotate causing the clay to become stressed and structurally defected (cracked). Fortunately the modern digital controller was designed specifically to slow down the kiln through the silica inversion point such that the silica molecules can properly complete their rotation. Secondly, at the end of the firing, the rate of firing speed must be slow enough to allow gases in the ware a chance to boil off (so to speak) through the glaze surface and the glaze itself to flow and repair the defect caused the gas bubble. This glaze/bubble defect (commonly called pinholes) is more common with thick, viscous glazes and glazes that lack lead in the formula (such as the modern nontoxic Duncan Envision Glazes)
or otherwise do not flow
every well. Again, fortunately, the modern digital controller was designed specifically to slow down the kiln during the last two hours of the firing
specifically so as to allow the proper development of the glazes and minimize glaze defects.
So how long should a kiln firing take? Following the Orton formula for proper firing built into their Sentry 2.0 digital controller
will result in an 8 hour firing time for Cone 04. Add an additional 3 hours to that firing time for Cone 6 or nearly 4.5 hours for Cone 10. Of course you are free to develop your own firing times. Or you can just use the formulas developed by the Orton Foundation and their university research scientists..... because they might have a clue concerning the proper firing of a kiln.
Speed is NOT your Friend, Part III
Can I speed up the cooling time of my kiln? Go fast. Go fast. Shucks darn, everybody wants to go fast. It has taken 8, 10 or even 12 hours to properly fire your kiln. The kiln will now take 16 to 24 hours to cool off to room temperature. Suddenly, everybody wants to shave off a few hours of the kilns cool down time like those few hours will really make that much difference.
Remember the previously mentioned Silica Inversion Point? The silica molecules which rotated during the firing of the kiln must now revert back to their
original state. The modern kiln with 3" wall bricks will naturally be slow enough in cooling... unless the kiln owner does something stupid like trying to
accelerate the cool down time. Cool too fast and the ware can literally shatter into hundreds of pieces. Hundreds of cracks or one crack, doesn't matter much
when even a single crack is one crack is one too many.
Up to this point we have discussed the Silica Inversion Point around 1060°F. There is also a secondary inversion point at about 440°F which affects the silica in clays when the clay is rapidly cooled. Problems with this secondary Silica Inversion Point can be avoided if the kiln is allowed to cool naturally. Unfortunately, way too many kiln users will try to accelerate the cool down process at this temperature because they do not understand the methodology of heat dissipation from a kiln.
During normal firing the air in the kiln is heated by the element, and the air transfers the heat to the wares in the kiln. The ware in the kiln will always be cooler than the air temperature within the kiln because of this heating process. Most importantly the thermocouple always displays the air temperature. At high temperatures the method of transferring heat within a kiln will be strictly via radiation.
When a kiln starts cooling the heating process is reversed. Heat is dissipated by radiation and then later convection. Again, air is the method of transferring heat from the wares. And the thermocouple measures air temperature.
The problem with rapid cooling of a kiln usually occurs when the thermocouple temperature displayed on the digital controller approaches a relatively low temperature (say 300°F) and the kiln owner accelerates the cooling by opening the kiln lid etc. While the air temperature in the kiln may accurately be 300°F, the temperature of the ware can be several hundred degrees hotter within the core of the ware. The reason for the sharp temperature difference is because of air is a relatively poor conductor of heat and is only able to transfer the excess heat to the insulating wall brick (which are also a poor conductor of heat) as the bricks themselves are cooled. This problem with heat transfer is complicated by heavy kiln shelves (which also retain heat) that may be present in excess numbers in the kiln. The cooling of the kiln is accelerated (usually by propping the kiln lid open) and suddenly the wares are rapidly cooled through the secondary Silica Inversion Point and structural defects occur.
However, once the temperature in the core of the ware and shelves has dropped to well below the secondary Silica Inversion Point the cooling rate of the kiln can be SAFELY accelerated by cracking the lid of the kiln... if the ware at the top of the kiln can tolerate the thermal stress of rapid cooling.
The problem with accelerating the cooling of the kiln is that the core temperature of the ware is never accurately known. It is all a guess. From Dogwood Ceramic Supply's perspective we believe every kiln owner should just allow their kiln to cool naturally. Shaving a few minutes or maybe an hour or two of time is just not worth the risk to the wares.
© 2009-2013, Dogwood Ceramic Supply
website last updated: February 2014
Dogwood Ceramic Supply
12590 Dedeaux Road
Gulfport Mississippi, 39503
Telephone (228) 831-4848 Fax (228) 831-3111
Showroom Business Hours:
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Thursday 1PM to 6PM
Closed Wednesday and Sunday