Drying of Clay, thoughts, experience, ideas, dynamics, principles.

Understanding the problems of drying thick work.
It would be easy to assume that drying work that is twice as thick takes twice the time. There are many confounding variables in this, and the simple picture is just not true.

It takes only a little heat to heat water up. It takes 1 calorie of heat to heat 1 gram of water 1 degree celcius. Just to get some comparison, scale in this, a kilowatt hour is 860 thousand calories. Just to avoid confusion, a nutritional calorie is 1000 regular calories.

But to evaporate water, to turn it to steam takes 540 calories for each gram. It takes time, or a big heat differential to transfer all of that heat to the water. As the water evaporates it absorbs heat from its surroundings, cooling them. This is why we sweat to cool ourselves. Evaporation of water absorbs heat.

Clay, especially dry clay is a reasonably good insulator. If you think of that 2 inch thick dinosaur as a bit of water surrounded by insulation, an inch of clay on each side, it is going to take some time for enough heat to penetrate the clay to evaporate the water. Remember, just heating it to boiling is not enough to evaporate it, you have to also get 540 more calories per gram to the water.

Below the boiling point of water at normal air pressure you can only evaporate water until the air surrounding it is saturated, until the relative humidity surrounding the water is 100%. So if you heat clay to say 90˚C or 194˚F and the clay is thick, water inside the clay will only evaporate until the air in the pores is saturated with water vapor. It may not all evaporate until there is time for the water vapor to move through the pores and be exchanged with air from outside the clay.

Explosions happen because the pressure inside the clay exceeds the strength of the clay to contain it. This part of the dynamic creates some compounding factors. As the pressure increases, so does the boiling point of water. This property likely contributes to the wide range of temperatures that we see explosions taking place at. Insulating properties of clay also contribute. the outside of a pot may be above normal boiling, but the inside might be colder from insulation and be at a higher pressure.

Fortunately, not everything makes getting clay dry more difficult. There are a few factors that speed things up. The first is that water wicks through the clay and presents itself, at least in part, at the surface of the clay where heat exchange and drying is easy. In order to understand this well you need to understand three terms, capilarity, surface tension, and viscosity.

Viscosity is the rate at which a liquid will flow. Honey and molasses are much more viscous than water. Acetone has a viscosity that is less than water, but most common liquids have viscosities that are higher. Viscosity of water decreases substantively as temperature increases. This increases its ability to move through clay towards the surface as temperature increases.

Surface tension is a nice term. It describes the tension on the surface of a liquid. When water beads up on a waxed surface the beading is because of surface tension. Without surface tension it would spread out. Surface tension is what holds bubbles intact. In mold making and in bubbly glazes a light spritz of alcohol can cause bubbles to burst. This is because even small amounts of alcohol radically lower the surface tension of the water allowing it to spread out and the bubbles to burst. Surface tension of water also decreases quickly with the rise in temperature. This allows the water to spread across surfaces, say clay particles and present more surface area for drying.

Capilarity, the property of water to up thin tubes or pores decreases slightly with increases in temperatures. The decrease is small enough that in most engineering problems the decrease can be ignored. Due to the increase in speed that this happens due to the decrease in viscosity, in our case it is more ignorable.

The loss of viscosity and surface tension presents us with an opportunity. Clay held at a high temperature maintains a more even wetness because water more easily transfers itself from wet to dry areas. Clay, in general, can be dried more quickly with few problems at high temperatures than at low. The phrase “high heat high humidty drying is used in an old text on brickmaking in the Archie Bray Foundation library and is the place I first encountered the concept. A few years later I needed to dry a thick carved mural quickly and dried most of it at 180˚F in a kiln with the lid propped over night, and some on a table with a fan. The ones on the table all cracked, those in the kiln all did not crack. I was convinced.

In this there are other confounding factors. Almost all electric kilns with the doors open tend to have colder floors. Even with zone control, unless there are floor elements this is likely to be the case. This is because cold air is denser than hot air so it settles pushing the lighter hot air out of the way. The more a kiln leaks, the more trouble there is with cold floors. Drying with the door open is an extreme case of a “leak”.

How wet work is changes the amount of time needed to dry below boiling temps significantly. It conspires with thickness to make thick objects often seem impossible to fire successfully. We have all heard the untruth, “You cannot fire thick work”. Having successfully fired kiln pugs as counterweights, I know this to be an untruth.

While I am still a believer that convection leaves bottoms of kilns colder than tops much of the problem with cold kiln bottoms seems to be the shelf near an uninsulated floor adding to the thermal mass . Work loaded on the shelf with the bottom down adds even more to this. It is not a duplicate of the area near the lid of the kiln. Dispersal of heat at low temperature has to be from convection because radiation is not very effective at the low temperatures.   Since none of these factors are very effective with low temperatures or small differences in temperature the added density at the bottom keeps things wet longer. Keeping thick work off the bottom and when possible placing it rim down vastly improves the situation by getting more of the clay higher in the kiln.

Most dispersal of heat at low temperatures in kilns is from convection caused by the differences in density caused by air temperature. The colder air heats at the elements near the bottom. This often leaves a cone of colder area near the bottom of electric kilns. So when you are preheating at 180˚F the bottom of the kiln, especially towards the center can be several tens of degrees colder. The colder it is, the less heat is transferred to the water and the slower it evaporates. Most often it seems that explosions happen in the bottoms of kilns that are fired with some, but not enough care.

Optimal conditions are unachievable. We have to fire in real situations. But if you had a piece of clay that was slightly wet, you could heat it above boiling for a short time. The water near the surface would evaporate quickly, but being near the surface would not create any pressure within the clay. The evaporation would prevent the water further inside the clay from heating as it would be absorbing so much heat to evaporate. After that surface water evaporated you would need to lower the temperature. The question is what temperature to lower it to? Optimally this might be above boiling. We only need to stay beneath the pressure that the clay can withstand. Under perfect circumstances we could even do this with leather hard clay. I believe that under normal circumstances we almost never achieve perfect drying and some water is always expelled from the walls of our clay under pressure.

Kiln pyrometers, even type S are imperfect. Even a few degrees around boiling could likely create problems with explosions. Because of this I usually used large margins. I started at 180˚F (82˚C) moved to 190˚F and as I got surer to 200˚F (93˚C). As I got close to retirement I started to use a slow rise time through boiling and shorten the hold. I believe that fine tuning this would result in quicker firings. Because there are differences in our many clay bodies and firings are mixed, “optimal” will vary even beyond considering thickness.

Sometime when I first started teaching at Texas A&M University Corpus Christi, The Island University, The only university in the US on its own island, surrounded by salt water, I decided that I needed a goal for speed of bisque kilns. How many pieces was it acceptable to explode in a semester? If you fire too slow you waste student time, and some electricity. If you fire too fast you either have not allow thick work or you blow stuff up. I decided that blowing up two pieces a semester was enough. Five was way too many. I also decided that this was true regardless of thickness. I started holding back thick work for special firings.

I dried kilns at 195˚F roughly 90˚C. How long the kiln was held depended on the wetness of the work, and how thick it was. I avoided loading thick work near the floor of the kiln. As things got busier and there were more classes, kilns were loaded less reliably. Work on the bottom started to explode more. I added time, a slow rise and then a short hold at 20˚F above boiling to try and get the bottom of the kiln to not explode. This was effective.

I started to think about the slow rise and the ability of clay to contain some pressure. I think that the optimal technique for getting work dry might be a short hold below boiling to get the work hot throughout and then a slow rise past boiling keeping the rise slow enough that the water remaining can boil without creating too much pressure. I think that this would be worthy of study. Knowlege of optimization of brick drying could likely inform what we do and save us time, money, and carbon.