Sunday, March 24, 2019

Parting Weighs

My last post was shared on social media among "clay buddies", so I think I'll continue the discussion.

One of the obvious follow-up questions to my previous observations regarding S-cracks in the bottom of pots is:  Why do the cracks occur far more often in a broad-bottomed pot (like a shallow bowl or plate) than they do in a narrow-bottomed pot (like a mug or pitcher)?

You could do an experiment to illustrate why those cracks occur more frequently, but you can probably do that "experiment" in your mind:

1. Make two slabs that measure .25" in thickness, 2" wide, and 2" long. 
2. Butt the slabs together so they meet.
3. Fix the non-meeting ends (opposite where the two slabs meet) in place (use a needle tool to pin the opposite ends to a board).

4. Once dry, measure the distance that the two slabs have shrunken away from each other.

Now repeat the exercise, this time with two slabs measuring .25" thick, 2' wide, 10" long.

Once dry, measure the distance the slabs traveled from each other as they shrunk.

In the case of the first two (small) slabs, you will find exponentially less of a gap than you will find between the longer slabs.

When you follow bad drying practice and you allow the perimeter to dry first, the bigger the circumference and diameter, the greater the force the shrinking is going to have on the center.  Quite often that force is slight enough on a narrow piece that no special care is required in drying.  But most of the time on a broad piece, the forces will pull too hard to get away with any kind of careless drying.


  1. How about the frequency of s-cracks on pots thrown off the hump? This is usually attributed to lack of compression. How do these types of s-cracks fit into your theory , since these are usually smaller pots. Thanks for sharing your thoughts.

    1. That's a good question, darn it.

      I would say that it goes back to my experiment with the pugmill "coins". Any method of creating a pot that aligns particles in a spiral around the center (like throwing or pugging clay) will make a weak spot that bad drying will expose.

      The thing is that the "fix" is still drying it properly, though I will confess that things thrown off the hump are best dealt with by proper drying AND a crackstopper bead -- a bead of clay worked into the center of the bottom to disrupt the spiral clay particle alignment that throwing off the hump accentuates even more -- not because its top isn't compressed, but because its bottom never got slammed against the wheelhead to disrupt that spiral configuration.

      Good question.