[Clayart] Cracks in Bisqueware - help!

Snail Scott claywork at flying-snail.com
Fri Dec 11 10:38:33 EST 2015


Every crack is caused by stress, and every stress has a cause. It can be 
the stress caused by parts that didn't have equal moisture levels at the 
time of assembly or stress caused by uneven drying (these are the Big 
Two), or stresses caused by rough handling while too stiff to visibly 
distort, uneven heating in the kiln, or even stresses induced during 
fabrication of the parts. Throwing and handbuilding can both cause their 
own latent stresses in the clay. These can include the 'twist' imparted by 
the wheel, damp or thick bottoms while throwing, slabs that are under 
tension on one face and compression on the other due to rolling or 
bending, pulled handles that get bent further after partially stiffening 
and wanting to 'spring back', etc.

Drying (even slow, perfectly even drying) can cause pre-existing latent 
stresses to manifest. Other times, the clay hangs together through drying, 
but the stress of the bisque firing is enough to kick it loose. Sometimes 
this will release all the stress, but we know that this is seldom the case. 
A crack in bisque is sometimes stable, and filling it in will be successful. 
Most often, however, some residual stress is still present and the crack 
simply gets bigger in the vitrification firing. Clay softens when it nears 
its vitrification point, but the patch materials won't have the exact same 
response to heat as the clay itself, usually softening more, and will offer 
little or no strength to the repair at the peak firing temperatures. They 
may continue to act as a filler if fill is all that's required, but if there are 
still stresses present (as there nearly always are), the repair materials 
can't help much. When this occurs, the effort in repair is wasted. 

Worse, sometimes the vitrification firing doesn't release all the stresses, 
either, and it can crack even after firing. Inducing new stresses by 
pouring boiling water into an object already known to have had stress 
cracks in the past seems like a very bad idea. People get hurt that way. 

I know you are frustrated, but personally, I would not risk repairing any 
cracks in a teapot or mug. During use, these are subject to some of the 
worst stresses of all ceramic forms, only excepting flameware, and that's 
because even experienced professional potters tend to shy away from 
the huge stress issues that stovetop cooking induces!

If pottery does break, it will do so while in use.  A plate breaks, you 
might get pot roast in your lap and go hungry, but it likely won't cause 
injury. Pour boiling water into something like a mug or teapot, though, 
and if it cracks from the stress, it will release boiling water all over the 
user. Teapots, since they are heavier, more structurally complex, and 
hold more water than a mug, are at greater risk for failure. People 
aren't being cruel or insensitive when they tell you to scrap it and 
make another. Quite the opposite - they are concerned for you and 
your customer and they want you both to be safe, happy, and free of 
injury and lawsuits.

The famous 'McDonald's Coffee Incident' occurred just a few blocks from 
my house. A small business owner and an ordinary customer, doing an 
ordinary thing - selling and buying coffee. No harm intended or ill-will, 
but that doesn't keep accidents from happening. The woman with the 
coffee needed skin grafts, but not because the franchise owner was evil, 
or knowingly negligent. Nobody else had thought of the possibility either. 
Selling a teapot with a pre-existing crack, even a 'fixed' one, IS negligent. 
It's not a bookend or a paperweight, it's a thing meant to be filled with 
scalding hot water. Photograph its good side, keep it on a shelf, fill it with 
mementos or loose change or a potted plant, but don't let it out into the 
world to be used.

           -Snail


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