Fraley on mon 24 dec 01
I've got a problem with a vitreous engobe I'm using. It was applied over
another much less vitreous engobe given by Alisa Clausen I believe and
it peeled very badly exposing the body in numerous places. It seemed to
happen the worst in a batch that contained 8% red iron oxide (a batch
with 8% zircopax did not peel at all). The 8% R.I.O. batch didn't peel
if it was over only the clay body but it did tend to blister. It seems
the iron oxide is to blame but why? The recipes are below if anyone can
figure it out.
August Vitreous Engobe ^6 Ox.
EPK 30%
frit 3124 20%
silica 20%
neph. sy. 20%
talc 10%
Engobe for dry clay (from Clausen) ^6 Ox
EPK 21%
Ball 21%
Custer spar 26%
silica 32%
Thanks for any help you can give.
Toby Atticus Fraley
Timakia@AOL.COM on wed 26 dec 01
Without going into the technical details(which is for sure not my strong
point with raw materials), I would guess that these two recipes do not fit
each other. That means that the one will shrink more than the other one,
causing the one to pull away. It might also be that the englobe/clay fit is
wrong.
Hope this give some direction.
Antoinette Badenhorst
http://hometown.aol.com/timakia
Alisa og Claus Clausen on wed 26 dec 01
Dear Toby,
I am looking through my glaze books and notes, and I do not see exactly the
Engobe you attribute to my testing for Clayart.
The general dry materials engobe I use is
4 parts Kaolin
4 parts Ball Clay
5 parts Custer Spar
6 parts Silica
that would be an engobe (more flux, less clay than a slip) I use for both
bone dry and bisque.
However, I rarely, rarely use engobe, because I use a slip with additives
(more flux, less clay) on
wet to stiff clay.
I had flaking problems with Zircopax and other opacifiers added to the
slip with additives, due to their highly refractive natures. However, you
have a problem with the iron colored engobe flaking. I think that means
that the two engobes just do not fit together. Also, that it blisters over
just your claybody, could mean that it is oversaturated with the iron
oxide. That is very easy to do, as iron oxide is very potent. When used
in too high percentages, it will go black, metallic or bubble, or all three
on the same pot!
Uscientifically, only literally, looking at your recipes, the first recipe
>August Vitreous Engobe ^6 Ox.
>EPK 30%
>frit 3124 20%
>silica 20%
>neph. sy. 20%
>talc 10%
shows only 30% clay and the second
>Engobe for dry clay (from Clausen) ^6 Ox
>EPK 21%
>Ball 21%
>Custer spar 26%
>silica 32%
has 42%. Plus the first recipe, looking only, has 4 sources for fluxes,
including a silicate. The second recipe has 2 sources. I cannot calculate
these recipes for shrinkage, however, it would be simpler and more
successful concerning fit problems, to layer the same base engobe, coloring
small batches. Such as the following from Mary Barringer:
Dense whtie Englobe
10 Frit 3110
20 Neph. Sye
30 Ball Clay
40 Zircopax
ADD
2 Bentonite
1 Macaloid
I do not add the last two.
As it is, it is a good, covering, white engobe. I have used it on
greenware and it works well for me. I also use my local frit instead of 3110.
To color it, play around with the 40 Zircopax. Add less Zircopax and use
your stains and oxides.
Hope this may help you. Engobes and slips can be really tricky. Tom Buck
told me from the beginning of my problems that trying to put a layer in
between the clay and glaze can really be problematic. But with his
generous help and in general those on Clayart, my slipped pots are hanging
together.
Best regards,
Alisa in Denmark
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