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terra sig and calgon

updated wed 15 aug 01


Kevin R. Biles on tue 14 aug 01


I'm new in this field and on this list. I am in the process of making
terra sigillata--had great luck getting layering using trisodium
phosphate as a deflocculator, but see that many recipes call for
Calgon. Can someone tell me which form and where you get it? Our small
town grocery stocks a "laundry booster" which contains phosphate, but I
wasn't sure that was what I needed.

I also see recipes calling for sodium silicate and /or soda ash.

While I'm at it, for the current projects, I don't need a high shine,
just a smooth, satin finish. In fact, since I am depending on others to
fire my pieces for me, I'm looking at cone 04-06, so will proably lose
some shine. The recipe I'm using (equal parts OM-4 and EPK with TSP)
calls for one 24 hour settling period, siphoning off the middle layer
(assuming you get a water layer on top), then letting that layer settle
for 24-48 hours. The middle layer has gradations and I am not sure how
much to take--I'm told 2/3-3/4, but there is quite a range in opacitiy
of the mix between those proportions. I have siphoned off what I
consider to be the best stuff but, as I leave the solutions for longer
periods, I get more settling and wonder if I could take more.

Any suggestions?

Also, how transparent is this stuff? I've read you can use it over
underglazes. Does the terra sig mute their colors? If you use it
(moderately) over a colored clay body, will it give a milky
appearance? Right now, I don't have the opportunity to run a lot of
test pieces.