From: Marvin Bartel [mailto:firstname.lastname@example.org]
Sent: Wednesday, October 06, 2004 1:22 PM
To: ArtsEdNet Talk
Subject: RE: Sgraffito: how make black slip?
Any suggestions on how to get a black slip?
Black is tricky to make using oxides - easier with stains.
I do not use a black slip, but if I were going to make one I might start
with 1 part cobalt oxide (a strong blue colorant) with 9 parts red iron
oxide (a less powerful brown colorant) and 20 parts clay to make it
stick to wet pots (this is not for bisque). If you want it to look
black when you are using it, just add india ink or black tempera. These
burn out entirely during firing.
If it is not dark enough after firing, use less clay (some clay is
needed for adhesion).
If it is too blue, add iron or subtract cobalt.
If it is too brown, add cobalt or subtract iron.
You will note that these colors are opposites on the a color wheel - so
it is a matter of neutralizing opposites to get black.
If you put glaze over this type of black slip, it may turn blue (this is
the tricky part).
I have a good black glaze that is simply a medium dark brown glaze with
6 percent cobalt added to make it black. When I overlap or dilute it
with another glaze, it turns blue. I use it for a night sky background
for my dream pieces.
One part black nickel oxide with two parts clay might might also work
for black slip but probably just grey. It may turn a bit green, so some
red iron oxide might kill the green. This black could be combined with
the cobalt and iron black to make slip that is less likely to deviate
Using a color inclusion stain (like Mason stain), one should be able to
mix black stain with just enough clay to make it stick to the wet pots.
Probably 1 part black stain and 2 parts clay. Stains cost more because
they are oxides that have been prefired with silicates and then ground
back to a powder for our use. This stabilizes the color so it does not
change much when we fire it. What you see is what you get.
All this is simply off the top of my head educated guesswork based on
many years of working with the stuff. Please tell me what happens if
you try it.