Stephani Stephenson on sun 17 nov 02
Dan
The scumming comes from soluble alkali minerals (salts) in the clay,
As the clay dries, the water in the clay carries these alkaline minerals
with it , up through the clay.
When the water evaporates from the clay, the minerals are left behind,
on and near the clay surface.
If you live in an area where water itself has a lot of alkali, or lime
content you know have seen how readily deposits and 'scum' forms in
sinks and showers. I would venture that water itself can contribute to
the alkali, as well as 'dry' ingredients in the terra cotta.
Now the annoying thing , my mind has gone blank on exactly what the
alkali is., and I'd like to know too.., I think calcium, possible
magnesium , would it be sodium also? And I wonder, you chem freaks out
there , would these be sulfates of the above?
And you are right , Dan , in your observations. The reason the scumming
shows up in certain places has to do with where the water leaves the
piece in the drying process. I use to make a lot of figurative sculpture
in Terra Cotta, and , if left to dry in an upright position, the Face
would get scummed! My temporary solution was to dry them with another,
less noticeable part as the high part!
Also, I noticed that if the piece was refired higher, the scumming would
actually begin to flux, and become shiny like a glaze.
Certain Terra Cotta claybodies are prone to scumming...I have heard it
said that redart contributes, due to impurities, organic matter in the
clay, but I never could verify that ..Also , small amounts of Barium are
added to claybodies to prevent scumming.
Application of vinegar would neutralize it. I have never tried that
personally. Sanding won't take care of it, neither will waxing, for the
most part...sometimes it will bloom right through it again....
From a simple consumer's viewpoint, I have found some commercial terra
Cotta bodies are really problematic , but many others are not. If you
are not formulating your own clay, but purchasing your clay, you may
want to try another terra cotta body, as an excessively scumming
commercial body is a hassle, not worth babying in my opinion....
I wonder if anyone on the list who makes their own clay can talk about
the difference between Terra Cotta bodies using Redart v. Carbondale
vs. Neuman. Carbondale gives a beautiful red to red brown at cone 5,
Redart seems to flux at a lower temp... have never used Neuman Red. Are
any of these more prone to scumming or is it a matter of the water
used, the other ingredients, balancing the ph?, etc.?
(oi yoi yoi !Everytime I think I know something I just realize how much
I don't know......)
here's to your scum free zone
Stephani Stephenson
steph@alchemiestudio.com
Autumn Downey on sun 17 nov 02
Stephani is correct about the barium carbonate being the antidote to scumming.
However, and hopefully I remember this right - it isn't aded to all
claybodies, particularly those in use in schools, so you may have to shop
for a body where it has been added commerically - probably easier and safer
than adding it yourself.
Ask your supplier about this. He ought to have an answer.
Autumn Downey
Yellowknife, NWT
>Certain Terra Cotta claybodies are prone to scumming...I have heard it
>said that redart contributes, due to impurities, organic matter in the
>clay, but I never could verify that ..Also , small amounts of Barium are
>added to claybodies to prevent scumming.
>
Yellowknife Guild of Arts and Crafts website:
http://users.internorth.com/~downeya/index.html
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