Chris Schafale on sun 29 aug 99
Helpful friends,
Here's a problem I haven't encountered before, and I hope some of you
can explain it. This is the first summer since I got my kiln, and to
my dismay I discovered that the kiln, located in the basement, heats
up the house enough that I can't bear to fire it when the
temperatures are above 90 outside (we don't air-condition, and with
*inside* temps in the mid-90's much of this summer, this transplanted
New Hampshirite has been wilting enough as it is!)
Anyway, this has meant that a lot of greenware has been sitting
around for weeks, waiting for the temperatures to break. My
studio is in an enclosed room in the basement. At one point, I
rigged some makeshift shelves in the outer part of the basement so I
could relieve the gridlock in the studio. After a couple of weeks, I
noticed that the bone-dry pieces in the outer area were
changing color, turning an ugly brown wherever they were exposed to
the air. They stayed their normal color on the bottoms and even
where "shadowed" by overhanging rims, etc. Pieces with lids stayed
normal color inside, and pieces that were covered with plastic,
even loosely, didn't change color until they'd been uncovered for
several days. All these pieces are made of brown or reddish cone 6
stoneware clays. Anyway, I finally got to bisque some of these
pieces, and was disturbed to find that some of the discoloration (not
all) remained, especially near rims, after bisquing. On the bisque,
the discoloration is a faint grayish-brown tinge, except on some
sharper edges where it is whitish.
My questions: What is this? Is this the "scumming" or
"efflorescence" of soluble salts that I've read about? (I thought
that was supposed to be white?) Can someone explain the
mechanism of what's happening? What are the consequences? Should I
expect problems in glazing these pieces? And what problems would
those be? Is there anything I can do at this point? For instance,
the discoloration doesn't seem to go much below the surface -- would
it help to sand the surface of the pieces before glazing (wearing a
respirator, of course)?
Thanks for any help or insights on this.
Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@intrex.net
Mike Bailey on wed 1 sep 99
In message , Chris Schafale writes
Dear Chris,
Re your comments about pots turning brown. I think this is from one of
the iron salts - iron sulphate is the usual one that afflicts us in the
U.K.
Sometimes we actually see this in the bagged clay - after the clay has
been sweating in its bag the iron sulphate migrates to the outer edge of
the clay. This shows up when you slice through the end of the clay as a
brown outer ring. In some instances potters don't notice it and just
give the clay a few turns to knead it into a ball ready for throwing.
The result that this outer edge clay is in fact very poorly distributed
in the clay and shows up as a brown swirl in the body of the pot.
In some other situations the evaporation takes place on edges, even on
the ridges left by ones finger prints, here the iron salt produces brown
finger prints.
In some situations a cure could be rather difficult - especially if you
are using a bright shiny transparent glaze which would tend to enhance
the colour difference. One would expect that opaque glazes would be
rather better at hiding it.
As you say, a bit of careful sanding down could help.
Cheers,
Mike.
--
Mike Bailey. Bath. U.K.
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