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deflocculants and flaking in terra sig

updated thu 31 jul 97


Vince Pitelka on tue 22 jul 97

I received the following question from Lori Wilkinson, and thought that the
question and answer might be of interest to the list.
- Vince

>I want to ask you what you are using in your sig formula instead of calgon
>these days. I have used your recipe but still am having lousy luck with it
>sticking and flaking off after firing. Tia
>Lori Wilkinson

Lori -
I use 1/4 of 1% sodium silicate, and 1/4 of 1% soda ash, both dissolved in
hot water, and added to the bucket before adding the dry clay.

As to the flaking, I never have my sig flake off, at all. Actually, that is
not quite true. When the sig is applied to very smooth surfaces, I have
occasionally had a little flaking in a long lo-fire reduction firing in a
gas kiln.

The primary things that promote flaking of sig are:

1) - Excessive reduction in firings above ^010 - makes the sig shrink.

2) - Excessive smoothing of the surface before applying sig - I like to sand
the surface smooth, but this leaves a very toothy surface which the sig
sticks to beautifully. Surface should not be burnished before applying sig.

3) - Excessively thick sig - currently I like to mix the finished sig to
about 1.14 specific gravity. In my post on sig which has been repeated on
Clayart a number of times, I suggest that different applications might
requre finished sig varying from 1.15 to 1.2, but I have come to believe
that even 1.15 is usually too thick.

4) - Excessively thick buildup of layers of slip on the pot - even if the
sig is very thin, if you build up too many layers, it will promote flaking.
Also, some people like to polish between layers, and I think this is a
mistake. I apply repeated layers as soon as the moisture has soaked in from
the previous layer. I build up the sig until the sanded texture begins to
dissappear, and then burnish or polish.
- Vince

Vince Pitelka -
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166