I've done a bit of this.
there are 2 main methods that I've used with success. +/- to both
plaster drying tables
**DO NOT USE PLASTER OF PARIS!!!
it's too soft and will get into your clay and wreck what you make.
it is what Lowes/Home Depot sells
you gotta find No 1 Pottery Plaster
try asking at a pottery/clay store near your closest major college campus
for me that is Columbus Clay in (obviously) Columbus OH. I live 200 miles away.
I think a 50# bag was less than $20
it's is worth the shipping cost
follow the mixing directions (water is added per pound of plaster)
this method works quickly and, if left unattended, can rocket your clay back to too stiff overnight.
I put handfuls of gloppy clay on about 2-4" thick.
depending on how fast I will get back to it
the plaster table is best in a sunny well ventilated area so that IT can be dry to start with
the other method is hanging canvas frames
Imagine a window screen turned on it's side, only it's make of canvas.
secure the canvas all the way around a frame
1x4"
should do it
sort of like stretching a canvas for a painting.
only you put the super dooper soupy liquid clay slurry into the frames.
this allows a lot of the water to seep out the bottom (at first) and then to dry more slowly by evaporation, much like the plaster
i've seen photos of these frames set in racks, hung under deep eaves on a pottery barn or shed.
Plaster is expensive and heavy but it dries thing out quick
this is best if there is little or no processing needed in terms of particle size in the clay.(how clean it is)
canvas (cotton duck fabric is the good heavy stuff, not thin cotton sheets) is not cheap either $6.00/yrd but not terrible
but the frames are only as heavy as the
wood they are made of when empty.
this is a slower process but better if you need to slake the clay down to a slurry to get out all the larger particles.
as for river banks and clay colors:
when it comes to heating clay above 1000 deg C, the end color, most likely won't match the wet one.
my
local clay is either blue or yellow
I think it has to do with the oxidation of the iron as it got exposed to water over the eons.
the yellow stuff sits on top of the blew and tends to follow cracks into the blue.
Because I live on Lake Erie, I can see the exposed bluff and exactly how the clay layers stack up for the first 30-50' of our area.
large clearer areas are apparent from the side view, digging down into it and finding a clean area will be some work
on a river though, I would wonder where the clay came from
maybe investigate upstream or uphill from where you are digging for an exposed hillside with no (or very very little) vegetation hanging from it.
once you find the clay source, you can avoid much of the gravel and organics
if not, you are on the right track
just keep going
the clay will tell you what it can do
this can help, once you learn how to use the system and decipher the images
http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
this can point you to the soil types in your area
USDA put out
books by county once upon a time
mine county's Soil Survey was issued in May of 1973
the data haven't changed
the books take a while to get the hang of too.