Unfinished.
There are a lot of oversimplifications. In some ways any statement can seem one. Sometimes oversimplifications contain truths but fail at conveying understanding.“Air bubbles cause explosions” is an example of an oversimplification common in my field of clay work. Often this is explained as “just untrue”, “air bubbles do not cause explosions”. Unfortunately, to me these examples both are partial truths and partial falsehoods. It makes me uncomfortable to write them down.
In order to understand how air bubbles encourage explosions you have understand what an explosion is, and when it takes place, why. I am going to leave out explosions in air, in many ways they are a separate case.
Clay explodes not because of steam. Steam is part of the picture in most clay explosions. The clay explodes when the wares ability to contain the pressure of the gas inside is exceeded. Strength is important. It is the most important reason why it is harder to explode bisque. Bisque is stronger and can contain, hold, withstand more pressure. Porcelain may be easier to explode because of less pore space, but it is also because it tends to be weaker as greenware. Paper clay may explode less because of pores letting the pressure dissipate, but also because the paper fiber vastly increases the tensile strength of the greenware.
If you think of a quarter inch slab of clay 4 inches square, you can also think of it as two 1/8 inch slabs connected where they touch. Each of these slabs is stronger because of the other. When you think of them disconnected they are less strong. The reason that air bubbles preferentially explode is because of the weakness in the wall that the air bubbles create.
Things that decrease pressure from steam
- open pores from grog, course clay, pinholes, other inclusions.
- slower heating
- dryer clay
Things that increase strength
- lack of air bubbles or other strength reducing imperfectionsor inclusions
- fibers
- stronger clays. Some ball clays are chosen in industry to increase dry strength.
Clay can also explode because of organic inclusions and water of hydration of clay and other minerals. Colemanite chunks in your clay can cause explosions. So can plaster. Likely there are other theoretical causes.
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— additions in response to questions
-Evaporation in a closed vessel (or one with small holes) is a complicated dynamic. I will try and explain it all in a way that can easily be understood. I have not tried before.
-Standard pressure is the pressure of air at sea level. It varies some,,, but it is a good place for a standard.
-I am going to leave out discussions of partial pressures. They do not seem needed at this point. But really understanding this might require it. One step at a time.
As water evaporates or turns to steam it absorbs energy. The molecules in water become more energetic and “bounce around more” so they act as a gas rather than a liquid. This is a substantial amount of energy. So, it takes a while to increase the heat past boiling to evaporate much water. The temperature of a pot of water in air essentially does not raise above the boiling point until all the water is gone. This is why your pot of water on the stove does not all instantly evaporate when you get it boiling. You have to add more heat to it. To make things simple, it takes 70 calories to bring 1 gram of water from 30 degrees to 100 (One calorie per gram per degree C). But it then takes 540 calories, at room pressure to evaporate it. Until you add the extra heat it will remain a liquid. So this process is not instantaneous. In a kiln at that temperature you are mostly transfering heat to the ware via convection. It is a slow process. This is part of why you can have explosions when the kiln is at 250˚C. The water has not had the heat added to it yet in order to evaporate. Convection is slow unless the temperature difference between the ware and air is high.
At sea level water boils at 100˚C (212˚F). As you go down in pressure, as happens as you gain elevation, the boiling point goes down.
As you increase pressure the boiling point goes up. This is most of the reason pressure cookers cook faster. They cook at higher pressures and hence the water in them gets to higher temperatures. The pressure comes because the water has enough energy to become a gas but is constrained by the volume.
So as you make steam in your clay by heating above 100˚C the pressure increases. If it is small enough to not explode the work, this increased pressure slows the evaporation because you now need to climb in temperature as well as add the heat of evaporation. Hopefully the steam leaks out through the pores fast enough that the pressure does not get greater than the clay can hold.
I hope that this is clear enough to make sense.
In glazed near vitreous ware that has gotten water into nearly closed cells traditional drying below boiling is not very effective.
Consider a long very narrow tube connected to a small container of water where the tube is the only opening to the outside. Just for an example lets chose 95˚C . My numbers are approximate.
Lets also assume that the tube starts completely saturated with water vapor. This is about 80% steam.
There is no great force moving the steam out of the tube. The way it leaves is via diffusion into the atmospheric air. As the water is creeping out via diffusion the air is creeping in. In a long narrow tube this happens very slowly. Until the percentage of steam over the water drops, no more can evaporate.
Bringing the temperature to just under the temperature of boiling does not help this dynamic. It just raises the percentage of water in the gas in the tube closer to 100%
Two things do change as you raise the temperature. The viscosity drops and the surface tension drops. These two properties do change the dynamics of drying in clay. As they drop the water is more able to spread out on surfaces providing more opportunity to dry, and also to spread more evenly through an object. But with really small pores it likely does not help enough to make much of a difference. Very little water will close off the pore.
It is my opinion, I do not treat this as fact, and I am not responsible if you blindly accept the ideas of some retired clay dude and blow up refires, that the best way to dry out refires is to get them near boiling, hold them at that temp until they are uniformly hot and then slowly raise the temp past boiling. I use 113˚C as the top temp if things are very thick and have soaked in water. I have used 2˚C per hour,, I suspect it is excessive.
The problem is that thermocouples and meters are not particularly accurate, even type R pr S unless you calibrate them and spend big money. Consequently you need a buffer for error.
As you add heat above 100˚C (assuming sea level) you will boil some of the water. This will increase the pressure in the container and force water and steam out of the tube. So long as the container, your clay body can withstand the pressure (better not be to high) the water will be expelled. Just before I retired I used a slow rise rather than a hold at ~110˚C to finish drying stubborn items and take care of cold spots.
The last couple of years I have thought that a long dry below boiling was a waste and that trying to get just above boiling would be more productive. The problem here is that kilns are not even in temperature and as they get older and leakier they get less even.
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This discussion parallels high temperature drying. Once your ware is leather, unless it really is close to not being able to hold itself up, drying at a high temperature is superior. The low surface tension and viscosity of water at higher temperatures allows the clay to remain more evenly wet. How slow it dries is based on how fast you bleed humdity out of the container holding it. This might not be particularly good for kilns, but I think that its problems are over-rated and the real problem with corrosion is from the organic acids from organic matter. Somewhere I have something written on this process.
[answers to questions and clarifications added]