'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 manner 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.