|Monthly Tech-Tip |
At the Medalta Potteries (in Medicine Hat, Alberta, Canada) during the 1920s they made stoneware crocks up to 60 gallons. These monsters weighed more than 200 pounds and had walls between one and two inches thick. These crocks were made from the same clays that are employed in Plainsman H550 functional stoneware today and the glaze was typical of the feldspar recipes we still use. Even if you could fabricate one of these and figure out how on earth they dip-glazed a 250 pound unbisqued vessel, it would certainly crack into many pieces as it split during firing heat up and dunted during cooling. What was the firing secret? Simple. Energy was cheap, huge beehive kilns the size of a house could be fired for less than a dollar a month! Kilns were hard brick and massive and the firing cycle was one week. That's right, seven days. To me the moral of this story is that firing needs to be tuned to the ware.
Consider another case that the average potter would find equally mystifying. In industry today it is common for roller kilns to fire stoneware in a 2-3 hour cold-to-cold cycle. My experience tells me that this is impossible. How do they do it? Obviously, they can only do this with ware that is lighter. But still, it is amazing, there must be much more to it. Here are a few factors:
First, they use a body that employs low lignite, large particle, lower plasticity clays in the lowest possible proportion so water and gases can be vented out quickly.
Second, they fire smarter. There are firing consultants in industry that do nothing but design custom firing curves for manufacturers. In the firing of any body there are periods when temperature rise can be much faster and others when it must be slower. But how much faster and where? A thermo-gravimetric analysis (TGA) test weighs a clay sample during firing to determine when it expels the most gases. A differential thermal analysis (DTA) test reveals the periods of firing where the body is exothermic and where it is endothermic. With this and other information one can design a firing curve that provides for the shortest possible firing time.
Third, commercial kilns fire very evenly, some expose ware to less than a degree of gradient. Draft is one factor, many kilns have burners that double as blowers and create a kind of 'hurricane' within the kiln that exposes every part of ware to heat.
A fourth factor is not directly related to cracking but I will mention it. Modern glazes are formulated to be fast-fire. They are low in boron and remain unmelted until just before the end of the firing. This makes allowance for easy channeling of gases of body decomposition before the glaze melts.
How long should a firing be? If ware is cracking and you don't want to get into complicated analysis of your firing curve then it should just be longer, it is a relative thing. However it does not take brain surgeons to fire smarter also. Hold at boiling point as long as possible (over night candling is best) and go up (and especially down) through quartz inversion slower (1050F, 570C). In electric kilns there is no draft, this is a real problem in avoiding gradients; you have no choice but fire slower in the hopes of getting a more even firing.
The heat-up was not slow enough. This kiln has no air flow. This piece was very thick. What would it take to fire this? Possibly days of holding at 250F to drive off pore water. Then days of slow heat up.
The cracks happened on heat up (since they have opened up wide). A combination of issues contribute. The kiln shelves heat-sink the wide flat bottoms, vessel walls are thick, there is some unevenness of wall thickness and only a 30-minute hold at 220F to remove glaze water from the bisque (that could have left dampness in thicker sections). Factors like these combine to produce temperature gradients within the piece. The firing schedule rose rapidly from 250-2100F (400F/hr) amplifying these gradients as it climbed. At quartz inversion these gradients produced a wave of volumetric change moving through the bisqued piece and this likely initiated a crack where stresses met at a combination sharp-contour and thickness-change, the bottom corner.
The cross section of a bowl. For the best success in drying and firing, it is advantageous to have as even of a thickness as possible. But it is also important not to have sharp concave angles. It would have been possible to make the section outside the foot ring thinner by creating a more abrupt concave contour, but that contour, if too sharp, could offer a point of weakness where a crack could start.
The rim was allowed to get ahead of the base. A thinner section (that happened during throwing) was exploited by the stresses and a crack appeared during heatup, likely during the bisque.
It started at a sharp angled indent on the outside (that coincided with a thin wall section) and grew around the perimeter (not visible). From there it branched to the base.
Cracks are happening during heatup (we know this because these have widened, cooling cracks are hairlines). Assuming the cracks were not there after drying, the piece likely could have been fired successfully by employing a slower heat-up. How to solve: Smooth, compress and round those sharp concave angles. This denies cracks a place to start. Even if the concave corners could have been rounded and compressed to a tiny radius (e.g. 2mm), it would likely have helped. And rounding and compressing the sharp edges would help (there are a couple of cracks on the straight sides).
The firing crack from the rim down has released the stresses produced by uneven thermal contraction during cool-down in the kiln. Any factor that contributes to a temperature gradient within a piece will contribute to the likelihood of dunting. Cooling too quickly through quartz inversion, for example, can cause this in almost any piece. Pieces that are thick and heavy, or have uneven cross section (with thick foot and thin walls, for example) will certainly suffer gradients, even in slow cooling. A wide, flat bottom (that is heat-sunk by the a heavy shelf) will also increase the temperature gradient between the outer walls and the inner foot. If that wide piece has vertical walls that get direct radiant heat, especially if one part is more exposed to the elements, it will start a gradient during the up-ramp in the firing. And, on the down-ramp, it will "come back to bite you" with a crack.
The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.
Some Keys to Dealing With Firing Cracks
Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.