Wednesday, March 12, 2014

The Veil Lifted: The Magic of the Bisque

Sometimes I feel like we give the bisque firing the short shrift. Who gets excited about unloading a bisque? Who invites people to watch, or posts pictures? The glaze firing with its shiny surfaces and appealing colors gets all the attention, but really, the bisque firing is when all the magic happens.

The ware is just dried mud when it goes into the kiln. If it wasn't coddled, it wouldn't last a single rainy afternoon. Then by some hot miracle, it becomes permanent ceramic material. Ware exists over 25,000 years old that has undergone this change. Bisqued pottery laughs at rain!

It's enough for most ceramicists to know that it does happen, but did you ever wonder what, exactly causes those changes to occur? I can't say I completely understand the process, but here's as much as I do get:

  1. In the early part of the firing, the remaining physical water is driven off. You may think your ware is perfectly dry when you load it in the kiln, and it may be damn close, but - surprise! - there's always some water left hiding between the clay platelets, because the clay can never be drier than the atmosphere. So unless we have literally zero humidity for an really extended time- in which case your greenware is the least of your worries -  there's always going to be some water to drive off. This is the water that can cause your piece to go boom if too much of it turns to steam while still deep in the walls. That's why we candle below 212° F. But you knew that.
    The next few things happen concurrently, or at least, their temperature ranges overlap. 
  2. The carbon and sulfides begin to burn off around 575°, and continue to do so until 1500° or so. You want all these gone before the glaze firing, because they can cause pinholing and other glaze flaws if they have to push out through a layer of glaze. They are what make the bisque smell so noxious. 
  3. Between 840° to 1110°, the heat breaks apart the kaolinite (clay) molecules into metakaolin and water. Metakaolin lacks plasticity, has smaller platelets, and is more absorbent than kaolinite. At this point the ware could no longer be slaked down; it's a part of history, now. But wait, there's more:
  4. At 1060° F, quartz inversion occurs: the room-temperature stable form of silica - alpha-quartz - changes its crystalline structure and becomes beta-quartz.  It's funny, but more potters are aware of quartz inversion than of the conversion of kaolin to metakaolin, which is the really significant event of the firing. But quartz inversion is a hazardous time for the bisque, because the ware will go through a sudden 2% increase in size. Cracking and splitting (aka dunting) can occur if parts of a piece go through quartz inversion ahead of other parts. All this happens in reverse as the firing cools.
  5. Beginning about 1600° F, the points of the hexagonal-shaped clay platelets begin to fuse whereever they touch. This is why bisqueware is stronger and harder to break apart than greenware. 
I lied, before, when I said all the magic happens during the bisque. There's still vitrification, and all the wonderful events within the glaze. Still, bisquing is a pretty amazing process. All of that Alpha Quartz and MetaKaolin stuff sounds like little, molecular superheroes in the kiln. And they are, for us; if it weren't for them, we couldn't do what we do. And we'd all have to eat off of deer shoulder blades, yuk.

Oh, yeah. I unloaded my bisque today.
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