Winter temperatures are upon us here in Kansas. While we still have not seen snow yet, there are plenty of mornings where I have felt the sudden ‘bite’ of frigid air against my face in the morning on my way into work. That got me thinking about thermal shock, controlled cooling, and the ‘other half’ of the firing process. In honor of the onset of cooler temperatures, this month’s Tech Tip Tuesday is about Cooling your Kiln.
We spend an awful lot of time in ceramics worried about how fast we can fire our kiln. How fast is too fast? When can I fire fast, and when do I have to slow down? What temperature do I need to hit (or cone to shut off at). Yet these questions only address half of the firing. Often we are so worried about getting the kiln to ‘fire off’, that we miss some opportunities for getting better glaze results by just letting the kiln drop in temperature. Worse yet, many people will actually damage their work (or the kiln itself) by cracking the lid open in order to speed up the end of a cooling cycle. When deciding on how to cool a kiln load there are 3 factors that I like to consider: Cooling for glaze effect, Cooling for the benefit of the pot, and Cooling for speed of unloading.
Cooling for Glaze Effect:
This factor is really about proper glaze development. My glaze calculation professor in college liked to point out how a glaze recipe from the mid 80s (or earlier) is unlikely to come out as expected in a modern electric kiln. The real reason for this has to do with how kilns have changed in the last couple of decades. Prior to then, many kilns had nearly twice as much insulation as they do today, or used hard firebrick instead of soft IFB. Manufacturers did this for a number of reasons at the time, but what it ultimately resulted in was a kiln that cooled VERY slowly. Glazes that were developed during this period, took advantage of the natural slow cooling of the kiln. As kiln insulation decreased in thickness, kilns started to cool much quicker from the top end of the firing. This means that the glaze recipes from 20 years ago don’t have enough time to develop properly without ‘firing down’.
Ron Roy and John Hesselberth point out, in their book Mastering Cone Six Glazes, that this is especially true with matte and satin glazes. In those kinds of glazes, the glaze surface itself is caused by tiny bubbles that form in the glaze during cooling. If you cool too quickly, the bubbles simply do not form, or do not form properly. Similarly, specialty glazes whose character results from a crystalline structure formed from rutile, titanium, iron, or many of the other glaze colorants, require a slower cooling rate to develop properly.
Cooling for the Benefit of the Pot
There are also structural issues that can be affected by cooling too rapidly. Without going too “in-depth”, clay and glaze go through a commercial heating process known as quartz inversion at around 1063 F (573 C). During that process, all of the silica in the glaze and clay bodies increases in size about 0.45%. Given the mechanical change in the glaze, we have to fire slowly through this process, so that the glaze doesn’t tear itself apart. Something many people forget, however, is that the process goes the other direction during the cool-down. This tends to be a bigger problem for the clay than the glaze. If a pot cools too quickly through quartz inversion, then it can cause the body to crack as it shrinks. This is known as dunting. Other elements that can exacerbate these issues are: Large flat pieces (plates) or pieces that have a large surface contact with the shelf (tiles). Thick pieces, where the outside surfaces of the clay may have a drastically different temperature than the core of the piece. Even kilns that have been loaded VERY empty (one or two pieces only) can cool too rapidly. One of the key elements to determine if a crack happened due to cooling is to look at the edge of the crack itself. Sharp edges in the glaze and the clay happened during the cooling cycle. If the edges are rounded, then the crack happened BEFORE the melt (during drying OR the heating cycle).
Cooling for Speed of Unloading
Now this factor is the one most teachers, students, and production potters are concerned about. Lets face it…there never seems to be enough time before a show, or the end of a semester. How many of you have loaded hot pots into a car the morning of a show….or taken pots directly from a kiln to that semester’s critique? A great number of questions abound on forums and Facebook asking when can you unload a kiln. My general answer is pretty simple….as soon as you can pick up the pots with your bare hands, then you can unload. As a point of practicality, I like to point out that a target temperature is when the pieces are no hotter than 350 F. That temperature comes from a long history of cooking. Most of the ceramic casseroles that use tend to go from a 350 degree oven to room temperature during normal use. Simply put, if the pot or glaze can’t survive that thermal shock on its own, it should NEVER be used for food anyway, right? The tricky part is to remember that the pieces in the kiln are HOTTER than the thermocouple reading during the cooling cycle. So if you have a computer controller, or a pyrometer, then I would target the 200 degree range for the EARLIEST you should unload a kiln.
Naturally, this leads to the question How do I speed up the cooling at the end of the firing so I can unload faster? All too often, I see potters cracking the lids of their kilns. From an equipment perspective, this is one of the worst things you can do to your kiln. Remember earlier how I talked about how clay shrinks during the cooling cycle? Well guess what the kiln lid, bricks, shelves, and posts are made of…thats right…clay. Here we aren’t talking specifically about quartz inversion, however. We are talking about thermal expansion, and thermal shock. When the inside of the kiln gets hot, the parts and pieces expand. You may see this as a glowing line between the lid and main body (or sections) of your kiln. What has happened is not necessarily ‘gapping’, but that the inside has grown in size, while the outside has not grown as much, because it isn’t as hot. The effective wall-thickness is decreased to the point where light is radiating through the thinner wall. When the kiln cools, the inside shrinks back down, and the wall thickness goes back to normal. The problem with opening a hot kiln lid, is that this mechanical shrinkage of the kiln’s bricks must happen SLOWLY, or it will form cracks. There is also the dreaded ‘pinging’ of a glaze that we have all heard when we open the kiln or unload too early. This is also matter of thermal shock, and the pinging you hear is the glaze surface cracking from being cooled (and therefore shrinking) too rapidly.
How do I control cooling?
Slowing cooling down is done in different ways.
- The first step is to properly load and fire your kiln. An evenly loaded kiln will absorb heat evenly on the way up, and will radiate heat evenly on the way down.
- A kiln that has more insulation will cool more slowly. 3″ thick walls cool slower than 2.5″ walls, and Double-walled kilns cool slower than 3″ brick kilns. (These kilns may not need to be fired down, watch your glaze results and adjust as needed)
- A kiln that is fired slower, rather than like a rocket, won’t have to hit as high a temperature to fire off. The thermal mass will be more even in its temperature gradient, and the kiln will cool a bit slower because of it.
- Firing programs can be written with a cool-down segment that slows the rate of cooling to 125-175 deg F/hour until the kiln is back to around 1500 F. Some newer controllers allow a cool cycle to be added to a cone-fire program. Refer to your operator’s manual, or you can contact us here at Bracker’s to find out. (Please have your kiln model and serial number so I can look up which version of your circuit board your kiln has.)
- A kiln cools naturally by radiating and conducting its heat to the air in the surrounding room. This transfer of heat is governed most simply by the number of air molecules that impact the side of the kiln during the cooling process. If you want to cool the kiln faster, you have to have more air hit the kiln during cooling. YEP, thats right, time to get out the old $10 box fan from Wal Mart. Leave the peep hole plugs in and the lid closed. Turn the fan on, and aim it at the kiln in such a way as to create an ‘air flow’ around the kiln. Don’t do this until after the kiln is below 700 F (to make sure the pots have come back through quartz inversion slowly). Then wait til the temp reads 200 or lower, and unload.
What about the vent?
Downdraft vents can change how fast a kiln cools. Generally I prefer to vent during cool-down, but some glazes don’t like is so much. Let your glaze results determine if you should turn it off or not. Remember, however, that if your vent motor hangs directly beneath the kiln, you should NOT turn it off during the cooling as the motor may overheat. If you are lucky enough to have a kiln that controls the vent (Cress Vent System, Skutt EnviroLink, ConeArt), then you can program the vent to turn off when you want it to. Otherwise, you need to remember to shut the vent off when the kiln fires off.
As a final note remember that not all glazes need to be fired down. If the glaze results you are getting out of your kiln are exactly what you want, then don’t change the firing. As in all things “If it ain’t broke, don’t fix it”. Happy Holidays from Bracker’s, and I will see you again in 2015. Stay Cool!!