by ronlang » Apr 23, 2012 10:45 am
Hi Randy
We are just back from MABS this weekend in Harrisburg. Thanks I appreciate your comments. Yes, larger pots just seem to create larger problems with added likelihood of warping, cracking in the drying, bisque and even glaze firing stages. The same physical principals are at play in smaller work but because larger pieces are heavier with thicker walls, with longer areas of unsupported floors (assuming raised feet), drying and firing will have special problems.
Your choice of clay is the first thing to consider. Courser clay bodies with grog (crushed fired clay, sand, etc) will help as it both lessens shrinkage and provides a structural matrix of varying particle sizes to strengthen and decrease warping and cracking. I mix my own clays so I don't know the particulars of the Laguna blends. But if you choose a body that is used for sculpture with grogs then you are on the right tract. My clay body for larger pieces is between 25 and 30% course material.
Next be aware of the moisture level of the component parts as you assemble them. They should be as close as possible to each other. If slab building, I tent the clay slabs together under plastic and even thrown parts like the thrown round wall compressed into the oval shape with it's floor slab for a day or better two, before joining them.
Next be sure of your joining technique. I score all parts that will come together thoroughly and deeply with a needle tool. And add only water to it and score again. I personally think that using a thicker paste of slip to join slabs can lead to cutting corners in really breaking into the joint areas (scoring) and "welding" them together. I even finish with a small coil pressed into the joint as a added "weld".
If you are adding feet be sure to support the floor in enough places so when flipped back upright it is well supported during drying. ( You have, of course, supported the floor from the inside of the pot when it was inverted it to add the feet?!) I use clay balls just a little stiffer then normal working clay so they won't shrink away from their job. I also lay a piece of canvass on top of the inverted pot and clay ball supports, then a level board on top. I tap the board to compress the balls to the level of the feet and then flip the pot upright. Importantly... you will notice as the pot drys slowly under a tent of plastic, the canvass sheet will pucker up to show that the pot is being allowed to move (shrink) freely. This is where a lot of larger heavier pots fail because they are not given a way to slide as they shrink. A non absorbent surface like plastic, metal, even painted wood would be the worst offenders. You have to let the piece move freely when drying. And dry them as slowly as possible to allow components to equalize in moisture.
Bisque fire slowly with a preheat cycle if possible. I know this is tough if you don't have your own kiln. I still support the floor during bisque with the same balls but be careful that the kiln shelf is flat or that you sand the dried supports so that you are not putting pressure points on the floor. All this is tough if you are in a community situation.
Stresses to the clay sometimes don't show up til the bisque. Sometimes the location of the crack gives a hint. Let me know if this is of any help or if I can answer any other questions.
Best,
Ron
