Episode: 2213
Title: HPR2213: Clay Body
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr2213/hpr2213.mp3
Transcribed: 2025-10-18 15:47:38

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This in HPR episode 2,213 entitled Claymodded, it is hosted by Brian and is about 11 minutes
long and currently next visit flag, the summer is basic clay theory.
This episode of HPR is brought to you by AnanasThost.com.
Get 15% discount on all shared hosting with the offer code HPR15, that's HPR15.
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Hello, heck of public radio, this is Brian again.
I think after listening to the community news, I should probably discuss some clay.
I'm a potter, can give me some friendly ribbing about having not submitted a pottery show.
I recorded in the middle of my beeswax rant while the heat gun was rolling, what I want
to do as my first clay project for anybody, no matter who they are.
And it's very simple, but I figure we should do some background because in the world of programming,
I consider myself to know basically nothing.
And I realize that every tutorial that I ever find, any book that I pick up and begin
reading, they always begin with defining what programming is before they have you even
write a hello world.
So what is the goal of this programming that we're doing?
And I'm going to kind of apply that to my episode structure here and do a basics.
What do we mean when we say pottery?
What we mean is this mineral deposit and clay when we make a pot is only one component
of what we would call the clay body.
The clay body is what colloquially is called clay.
If you buy clay at the store for whatever firing temperature, it will be a clay body.
Perhaps that clay body comes straight out of the ground as it is.
Most likely it does not.
Most likely the clay body is formulated from a number of different minerals that are pulverized
to aid the clay in its holding structure, workability, the plasticity, which is what is
pretty self-explanatory, in clay, the opposite of plasticity we call short.
We say the clay is short or the clay is plastic.
And longer the clay ages in a workable form in its consistency in that moisture content.
If it's too dry, then it won't actually age.
And if it's too wet, then it won't actually age.
Because what's happening in the aging is that all of these tiny, tiny particles of clay
are aligning there because it's basically alive as much as the earth is alive.
The clay particles are broke down, mountains, ground dust.
So bottom of the mountain, you end up with this super-pure white clay, a kaolin of sorts
or that would be considered a primary clay, it is exactly where it was formed.
And that's why it's so pure white.
The secondary clays, more of the ball clays, they're finer in particular size, which
aids to their plasticity.
They've collected a little bit of mineralization as they've been moved.
And then we have the tertiary clays, our common surface red clays that are really
un-pure as far as the actual clay content.
Most of the tertiary clays are clay bodies right out of the ground.
So in the clay bodies that we're looking for, we commonly have low-fire earthenware,
and we have high-fire stoneware.
And the porcelains are more of the purest of the clays, but there is nowhere that I know
of where porcelain as a clay body comes straight out of the ground.
Even in China and the birthplace of porcelain, it is this kaolin deposit that's mixed with
a felt spar and silica, and I'm not sure exactly what the recipes are, but that's usually
the process.
Your basic porcelain recipe is 25% of your four main ingredients.
Your kaolin, your ball clay, your silica, and your felt spar.
And 25% of each will give you that classic diner porcelain.
It's very short, it's not very plastic, so it's not easy to work on the wheel.
But it is a nice, durable basic porcelain.
If you want to formulate your own porcelain, what you do is you would begin with that in
mind, and now you have those percentages, the best thing to do for your clay body is
to vary because you're using mineral deposits from the ground, they will vary.
So if I'm only using one kaolin in my recipe, and something happens at that mine, and
my clay changes just a little bit, it could have huge impacts on my pottery.
So it's always nice to space them out, maybe use two different kaolins, maybe three
different kaolins, maybe two, three different felt spars, and that's where you end up with
these complex recipes, and a lot of that is just a guard against flaws in the minerals.
Because they stay extremely pure and very consistent for a really long period of time,
but there are these tiny changes, and those tiny changes will introduce bugs into the
code in your pot.
And what we're working with in the clay body, once we structure everything together, we
now have clay, and we're going to turn it into ceramic.
So to do that, we have to heat it through the temperature point that's called quartz
inversion, and it's in a very layman's term where all of the structures of those molecules
destabilize, and the crystalline quartz is no longer in a crystalline structure.
And the point that that happens is just before your red heat.
So if it's glowing, you're through quartz inversion.
When it goes through quartz inversion, the pots have a drastic thermal change in size,
so it's real important to do that real slow so they don't crack, and also on the way down.
Other than that, you can basically go as fast as you want.
There are other issues with Cristobal light, where upon cooling, from that quartz inversion,
when you come back through, you end up with free silica that's still not crystallized
into quartz.
And that can be forming these micro-silica crystalline structures called Cristobal light that can
be forming all the way down into your oven temperature.
So on your first high firing of these pots, you want to really make sure you don't open
that door until they're good and cool.
So we're working with the alignment of the clay particles in the building of these
wares, and if we have the clay in a good, soft, workable state, and we let it age, those
particles will tend to align magnetically, and that's actually an interesting side note
on the historical value, because when we fire the clay, all of that alignment happens
geometetically toward magnetic north.
So in archeological sites, in a fire pit in the ground, if it was fired up through quartz
inversion, through red heat in those clay particles in the ground, because they fused into ceramic
in the fire pit, they can tell by knowing our calculations of plate tectonics, the pretty
darn close age of the last time someone had a fire in that fire pit, because the particles
realigned toward where magnetic north was at that site, at that point.
So we're working with these particles, and their alignment has a memory, and it has a
memory wall you're making it.
And while you're making it, if you have a flat slab of clay, and you grab one edge and
you bend it up, it's going to remember that.
If you don't take action to try to reset the memory of that clay, even though you don't
see that, it might present itself in your firing or in your drying.
The clay bodies tend to shrink, tend to 14% from wet workability to final second glaze
firing, because first you fire it to a bisque, to a biscuit fire, so that it can be handled
real easily, and you know that if you had a flaw in your making, that it would survive
your glaze fire, because flaws in the making can create a bomb in the kiln that basically
blows up and throws stuff everywhere.
And if you're firing a glaze, all those pieces will stick to your glaze pots, and for most
purposes be ruined.
So you want all that stuff to get out of the way in a preliminary firing, where if anything
breaks, it doesn't matter, because nothing is going to stick, we're not forming glass.
And the glaze is nothing but glass, stabilized with clay, and we use a little bit of different
metals for the different colorings and opacities, and that's a whole real interesting thing,
maybe that'll happen, in a show here, one day, many moons from now.
So let's just do this first episode.
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