Leigh Whitaker on fri 2 nov 07
and John)
Okay, nobody fuss at me about testing things first because I thought I had,
but I was mistaken...
I bought some Speedball underglazes. Their website recommends using their
particular clear glaze to go over the top. I figured any zinc free clear
glaze would probably work all right. I tested them out under the Amaco Sahara
HF-9, which works well over Amaco velvet underglazes, and it seemed fine. The
problem was that I tested all the colors except black. I thought I had
tested black, but looking back at my notes I realized I had tested the Amaco
velvet black, and not the Speedball black.
So I fired up some nicely decorated bowls which I had covered with the
Speedball black underglaze on the outside, and different Speedball colors on the
inside. I used the HF-9 as the covering glaze. The clear glaze did not
interact well with the black underglaze at all. It pitted and pinholed like crazy
and as a result developed a very textured surface. I don't think it was the
firing, because I had some other stuff in there that I had covered with the
Amaco velvet black and the HF-9 and they look perfect.
So, obviously I know I need to test some other clear glazes with the black.
I've got some ready to go into the next firing. My question is what sorts
of interactions might cause this to happen? What sorts of things should I
consider when trying to find a clear glaze to cover? Should I just order some
of the Speedball clear glaze, or is it worth trying out some other home made
glazes if the two I'm trying now don't work (I've got M^6 glossy liner and
Tony's Ultra Clear made up and ready to try)?
BTW- for Ron and John... I asked a question earlier about your clear glossy
liner over slips and underglazes. So far it is working fine over all,
except it didn't seem to do well over a slip containing a good amount of iron
oxide and cobalt oxide (I was trying to make a black slip). Other than that it
worked just fine. Thanks for the help.
Leigh
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