tammy brown on mon 17 may 04
I have an excerpt from one of my books that I would
appreciate some feed back on if you will. Here is the
excerpt and then I will throw out my questions.
I think this pertains to some of the advice that Snail
gave me regarding Impervious decoration..
Here goes:
Impervious decoration:
Vitreious slips are slips fired to a temperature that
bonds the materials into a very hard, impervious
layer. Unlike ordinary slip, which allos water to soak
into the pot, slips that ar evitrified are usually the
final decoration of a pot, with no covering glaze
applied. here are two simple recipes for vitreous
slip
Earthenware slip
1830-2040 degrees F (1000-1150 degrees C)
Dry porcelain 100 parts
Borax E. Frit 35 parts
Stoneware slip
2040-2300 degrees F (1150-1260 degrees C)
Dry porcelain 100 parts
Feldspar 35 parts
Okay my first question...If the directions say to use
it as a final decoration...do they mean that I should
bisque fire with no finish and then apply this to
bisque?
If I add vitrified slip to bisque ware as I am
guessing this suggests...will the shrinkage rate not
apply as it is bisque and not greenware? I have tried
applying slips at different stages...when I applied it
on very dry the piece blew up ( not sure if this is
why but I think possibly). Have not had a piece blow
when I applied the slip to a wetter surface...leather
hard maybe..
Is this what you were referring to Snail when you
mentioned vitrified slips?
Has anyone tried these and do these two recipes sound
like a decent ones?
If not can anyone suggest one..for I would really like
to try this...
Thanks
Tammy
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Snail Scott on sat 22 may 04
At 07:47 PM 5/17/04 -0700, you wrote:
>Okay my first question...If the directions say to use
>it as a final decoration...do they mean that I should
>bisque fire with no finish and then apply this to
>bisque?
Sorry to be so long in reponding - I was out of town
for a few days. Anyway...
The way to tell if a recipe is intended for bisque
or greenware is to look at the raw (uncalcined)
clay content. These recipes are basically three
parts clay to one part non-plastic materials (i.e.
almost anything other than raw clay). So, the
shrinkage will be fairly high, and it will do best
if applied to greenware rather than bisque. Thus,
you could apply it and then bisque, but if this is
intended to be the final surface treatment, you might
as well just single-fire it to your final temperature.
You don't need the bisque first unless the piece is
too fragile to be coated while green. If you do want
to apply it to bisque, you need less raw clay in the
recipe. Simple calcine about half the clay before
adding it to the recipe; that'll reduce its 'green'
shrinkage.
By the way, the 'stoneware' recipe you mentioned will
probably only be vitreous at ^10 or so. 1/4 feldspar
is not going to be able to flux it sufficiently at
the so-called 'mid-range' (^4-6) to be considered a
vitreous surface. This is an assumption which you
will encounter widely in older books, written when
mid-range stoneware was very rare and almost all
stoneware work was being done at or around ^10. For
mid-range stoneware recipes, plan on using frits or
borate compounds for a better melt.
>If I add vitrified slip to bisque ware as I am
>guessing this suggests...will the shrinkage rate not
>apply as it is bisque and not greenware? I have tried
>applying slips at different stages...when I applied it
>on very dry the piece blew up ( not sure if this is
>why but I think possibly). Have not had a piece blow
>when I applied the slip to a wetter surface...leather
>hard maybe..
Shrinkage is a basic factor whenever you fire, and
matching the shrinkage of the clay to the shrinkage
of the unfired coating on top of it is always a
necessary consideration. The slip (engobe) itself is
just a sort of clay, after all, and you're applying
it as a wet coating, so it will shrink. Clays (the
plastic component of the recipe) are what result in
shrinkage, and the more clay in the recipe, the more
shrinkage. If the work underneath is already 'half-
shrunk' by a bisque firing, then it will need a
coating that also has less shrinkage still ahead of
it. It doesn't call for serious precision; just a
general 'ballpark' proportion of plastic ingredients
(clay) to non-plastic ingredients (everything else).
Shrinkage, however, has nothing at all to do with
things blowing up. Only water makes things blow up.
If it was still damp from applying the engobe,
or was fired too quickly for its thickness, then
that is why it blew up. Remember then you apply a
coating to the work, it is adding more water to the
piece, and it needs to be allowed to dry again.
When you put the engobe on the wet piece, you
probably gave it a decent amount of time to dry
since you knew it was wet. Perhaps the 'dry' piece
was put in the kiln too soon after applying the
engobe, and was not given enough time to re-dry.
>Is this what you were referring to Snail when you
>mentioned vitrified slips?
Yes, these are basic engobes. They'll do fine as a
staring point, but feel free to test variations:
different fluxes, different proportions, different
clays, different colorants, etc. Many recipes are
more complex than these, you may have noticed. That
doesn't mean that these very simple recipes won't
work. They might be just what you want, but tinker
anyway.
-Snail Scott
Reno, Nevada, USA, Earth
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