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.

Thursday, March 21, 2019

No Part Of Nothin'

I'm gonna get all kinds of controversial in this post.

I thought about posting an image of Glenn Close submerged in the bathtub-- you know the one? ...one of the scariest moments ever committed to film?  ....even gives the Psycho shower scene a run for the money?  You think Glenn Close is finally dead, just before she comes back to life and jumps up from the water.

Some things won't die.

I was going to post that image, but it was just too ugly for my blog.  I didn't want to look at it.

But like Glenn Close in Fatal Attraction -- the idea of "compressing" the bottom of a pot to keep it from developing an "S-Crack" won't die. 

But (to borrow from Bill Monroe of bluegrass music fame), "Compression ain't no part of nothin'"

Seriously.  Compression isn't a thing.  I don't know what potter first came up with the idea of "compression".

Smoothing the bottom and shaping its contour is a thing.  A good thing.  Making the bottom appropriately thick or thin is a thing.  A good thing.  Even changing the alignment of the particles of the bottom is a thing.

Compression isn't a thing.

The issue of S-Cracks is a simple problem of physics.  The thing that causes S-cracks is that when clay shrinks, if all the clay that surrounds the center is already dry, the center (where the S-crack develops) clay will be physically pulled apart as it shrinks.  And it will be pulled apart at its weakest point.

The idea behind "compression" is the supposition that you can make a bottom without a weak point to be pulled apart.  It's not bloody likely.  The particles are aligned the moment you slam the clay to the wheelhead.  What you do to the clay above that contact point is not going to change that very clay that meets the wheelhead.

Now, if you're talking about compression as Richard Aerni does it in the bottoms of his monumental molded pieces, we're talking about a bit of a different problem he's solving.  And when he's doing that compression, he is pounding the bottom with a wooden post because he knows he embedded a very weak spiral at the bottom of that pot.  He enclosed the top of a cylinder, inverted it, and made it the bottom of the pot.  He has a particular weakness he needs to repair by pounding the clay into a more homogeneous mass.


But that's nothing like "compressing" as it's described by those who wish to prevent S-cracks.

Sometimes what we do that we call "compression" does solve the S-crack problem.  But that's not because we compressed the bottom.  That's because when we did what we call "compressing", we actually simply made the bottom thinner.

And a thinner bottom will crack less frequently -- but not because thick bottoms are inherently weaker or more prone to cracking.  They're not.  In the end, they're stronger.  A too-thin bottom isn't really good for a pot either. 

No, the thinner bottom cracks less frequently than a thicker bottom for one simple reason -- and that reason is the only thing one can do to minimize S-cracks.  That reason is that a thinner bottom is more likely to dry first -- or, at least, to dry before the surrounding clay becomes rigid and pulls the bottom apart.

The only meaningful way to address clay that is prone to S-cracking is to make sure the center of the bottom isn't the last part of the pot to dry.  If the center of the bottom dries first, it simply will not S-crack.  It will already be at a fixed, fully-shrunken state when the clay around it dries and tries to pull it apart.

Don't believe me?


Here's an experiment that should convince you:

1.  Cut ten "coins" from the end of your pugmill.  As the clay is extruded, cut discs that are about as thick as you imagine the bottom of your pot should be.

2.  Let those "coins" dry.  


If your clay is like mine, almost every one of the coins will develop an S-crack.  10 out of 10 with S-cracks will probably be the result.

3.  Now cut ten more "coins" and, rather than simply letting them dry, wrap the perimeter of each coin with plastic in a matter that allows only the center of each coin to get air. 

4.  Allow the ten wrapped coins to dry.  You will find that if there are S-cracks, they will be very small.  But you should find most, if not all of them will be virtually crack free.


(that should also be a hint:  Don't be tempted to use the extrusion as it comes off the pugmill as an "already centered" ball of clay to begin with.  That shortcut to centering will almost certainly invite whatever S-cracking you are still going to experience -- even if you do dry the pots properly.)

The experiment with the pugged coins should amply demonstrate that compression isn't a thing.  It isn't the solution to S-cracks.  Proper drying is.


Saturday, March 2, 2019

So, I'm Like...


Something I enjoy is taking a day trip – a ride in a car with a good conversationalist. I have several conversationalist friends. Three such friends spring immediately to mind: Kevin, Greg, and Jim. A drive through the countryside with one of them is one of life’s great pleasures for me.

Kevin and I share a theological background and can talk almost endlessly about the intersection of theology, science, faith, and knowing.

Greg is interested in history in a way I enjoy discussing it – to imagine this Hoosier countryside and what it must have been like in the distant past. He’s knowledgeable about the people and events that made Indiana look the way it does today.

Jim taught archeology for 40 years and has an academic’s perspective on so many subjects I find interesting. And one such drive-through-the-countryside conversation with Jim sprang to mind today.

I just spent an hour listening to artists talk about their “aesthetic”.

Jim once told me about a book he found interesting in his last years of teaching. I don’t remember the title of the book, but I do remember a concept Jim shared from his reading of it. It seems the author hypothesized that art actually pre-dates language in the human experience. Certainly art predates written language. And maybe it does predate formal language.

But the interesting conclusion the author drew was that as language developed it began to eclipse art as a means to communicate. Language became better and more accurate than art at communicating so much of what we needed to communicate in our human experience.

But there remained important things that language never did sufficiently convey. There are still things art conveys better -- more accurately. Those things remain the exclusive purview of art. We express with art what we cannot express with language.

That may be why many of us bristle at the disconnect between art criticism and the works themselves.

And it may be why today as I was listening to artists struggle to talk about their “aesthetic” I mused that it might be because they’re attempting what is impossible in the first place. Their attempts were as useless as describing a taste to one who has never tasted it, or a color to a blind person, or a song to the deaf. 

If there were words, we wouldn’t have reached for the method of first resort – art.