[Clayart] Geology of clay

[Robert Harris]

Wikipedia is always a good place to start
https://en.wikipedia.org/wiki/Clay
https://en.wikipedia.org/wiki/Fire_clay
https://en.wikipedia.org/wiki/Ball_clay

Also use Digitalfire. Tony Hansen has a bunch of good articles (you'll have
to dig for appropriate ones).

An ordinary potter's clay is a mixture of clay minerals (e.g. kaolinite,
mica etc), feldspathoids, and silica. (Plus things like iron oxide). Clays
in general are the result of hydrothermal weathering of feldspars (mostly
granite etc.)

Clays will collect at the bottom of granite mountains. They also collect at
the bend in rivers where the slowing water deposits sediment (and this can
be from dried up rivers, or rivers that have shifted course). The further
from the source, the more contaminated and likely, the lower their maturing
temp will be.

Fireclays (high temp clays) are named such because they're found at the
boundaries of coal seams.

For testing ...

What technology is available to your colonists? How are they firing? Solar
panels can provide electricity, but you'd need highly resistive wire (i.e.
Kanthal). Wood, of course. Dean McRaine (lightwave potttery) who used to be
a member of this list, has a great series of blog posts on making a waste
vegetable oil kiln/burner. A high temp kiln could be made from scavenged
re-entry heat shields (if their entry vehicle remains with them ... or if
there is a bumpy landing a few might have conveniently fallen off). For
simple but efficient kiln designs (not bottle (updraft) kilns!) that use
solid fuel, look at some of the more recent kilns built in India and
Thailand (both stoneware and earthenware). The Thai kilns often have
extremely long fireboxes because they banned chainsaws to reduce illegal
logging. These kilns prioritise efficiency over ash which most western
built wood kilns are designed to maximise.

For testing purposes I would make a small kiln with a (very large) peephole
that I can pull various draw rings out of. (Also put bars of a specific
thickness across tiles etc and watch for bending (and pull them out). Do
they have access to a pyrometer? I'd imagine you could make or scavenge a
simple one from their ship. (Depending on what they've been left with).
Making your own cones is relatively simple. (Also look at the way Wedgwood
developed his various pyrometric devices, he basically invented the
concept). For eathenwares, bloating is the biggest problem besides melting.
Also lime pops can be a problem with some scavenged clays.

Food safety ... depending on how straightened their circumstances are ...
who cares. Pretty sure that we all died of horrific diseases well before we
were poisoned by pottery. The only thing that's really nasty is lead, and
if your potter knows his stuff, either he'll fire to high temps or he'll
use terra sig or something else (boron, zinc) instead of lead. (And finding
and mining lead is probably harder than finding fireclay or kaolins and
figuring out how to high fire). The biggest reason we carried on using lead
glazes was a lack of knowledge of efficient kiln design to reach high temps
(plus the ubiquity of low fire clays). In general lead was always more of a
problem for workers than for users.

Hope that's of some help.

Robert



On Mon, 3 Jan 2022 at 14:48, laughinglion <laughinglion at comcast.net> wrote:

> Hi folks.I have a question, one of my incarnations as a crafter is as a
> romance writer.  I have dumped some folks on another planet, that has clay
> but does not utilize it.  One of my characters is, of course, a potter
> (elsewise why ask here).Can someone point me towards the geology of clay
> and how to test it, in an unknown venue, for proper firing temps and food
> safety postfiring......I want to play with this, but introducing clay to
> the previous colonizers i need a solid source of information.I own I am
> guilry of NOT knowing the science behind finding, digging and amending my
> own clay and worse not knowing where to find the resource information
> itself.  Glazes we have wonderful sources.....But i truly dont understand
> how a clay pit became a clay pit.  So would you folks who do understqnd the
> geology, the digging and amending of clay llease send me to your favorite
> resources.  Thus, at 60 I may learn more about my favorite medium.Sent from
> my Galaxy
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