We’re talking about clay of course!
I’ve discovered that while some people have no idea what the word bisque means, to others it draws images of biscuits (or cookies if you socialise with Americans), soup (think of a smooth, creamy, crustacean-based soup) or the porous result of an 1000 degrees Celsius firing (give or take a few degrees, depending on the clay-body).
For potters, a bisque firing can reveal those sneaky S cracks in the bottom of vessels, it may hint at future slumping of a vessel and even leads us to tossing distorted or defective vessels before the glaze firing.
The benefit of a bisque firing is that chemical and physical changes occur to the clay body which makes it porous and readily able to absorb glaze. In more technical terms – the claybody sinters without vitrifying.
To give you a bit more of an idea about what happens behind the door, I want to explain some of the big moments in the bisque… (note: because I am an Aussie, I use degrees Celsius)
As you would know, water boils at 100 degrees so potters have to make sure that their pots are bone-dry with all water evaporated before the kiln reaches this point – otherwise *crack, bang, boooooom* and hello shattered pots 😦 Clay can be very deceptive so potters often choose to warm the kiln very slowly, often overnight at a low heat to ensure that the physical water has evaporated before increasing the heat.
Between 300-800 degrees, the kiln temperature raises slowly and steadily. With the physical water long gone, the chemical water also disappears but at much higher temperatures – between 480-700°C. Carbonaceous materials (impurities in the clay along with paper, wax, etc) are slowly burnt away and they need to be gone before reaching 800°C. Otherwise the clay surface will start to seal off and sinter (where the clay particles cement themselves together). Upon reaching these temperatures, the clay body will have permanently changed.
*WARING: THIS SECTION MAY ONLY BE OF INTEREST THOSE WORKING WITH CLAY – WE’RE GETTING A BIT TECHNICAL HERE. IF THIS IS NOT YOU, I’VE PROVIDED SOME ALTERNATE MUSIC IN THE FORM OF THE FRIENDS TV THEME:
‘So no one told you life was gonna be this way [insert shakey shakey sound here and carry on with the lyrics…]
TO THOSE INTERESTED: Also of note are two important inversions. The quartz inversion occurs at 573°C and demands special care during the glaze firing (essentially quartz crystals change from an alpha (α) crystal structure to a beta (β) crystal structure). This conversion creates stresses in the clay so temperature increase and decrease must be slow to avoid cracking the work. The second inversion, occurs at 226°C when cristobalite, the crystalline form of silica suddenly shrinks. Fast cooling at this temperature will cause wares to crack and its un-creatively known as the crystobalite inversion (Potters call it as they see it after all) 😛
‘I’ll be there for you, coz you’re there for me too’ – AH, ALL BACK TOGETHER AGAIN.*
Once you have reached the desired bisque top temperature (anywhere between 942 and 1063°C [cones 08-04], depending on clay body and desired porosity of your bisqued wares), the kiln can be soaked for a period of time (holding the end/top temperature, increases the amount of fused matter and chemical action) and turned off.
Then the waiting game begins. You’ve gotta wait for the kiln to sloooooooowly cool down. Don’t even think about even cracking open the kiln until after 200°C (remember that crystobalite inversion?).
All in all, a bisque firing can take anywhere from 8 to over 16hours – and that’s not including the cooling time!
Today for example, I loaded up a bisque and I won’t be able to get at the vessels out until Wednesday arvo at the earliest. Probably won’t touch them until Thursday to be honest.
There’s so much that goes into the firing side of ceramics – there’s heaps more we could talk about in terms of stacking the kiln and the actual firing cycle (what temperatures and for how long) but I think that can be another day’s effort!
Big thank you to Stephanie Outridge Field for her help with this bisque firing too 🙂
And while I’m thanking people, to those wonderful people who congratulated me by leaving comments on the Clunes Update post thank you very much! Its a very exciting opportunity thats for sure!!
Have yourself a great week!!