Lili Krakowski on sun 30 sep 07
By coincidence there were two queries about slip lately. Let me attempt to
address them.
What is slip and what is engobe? No fixed rule.
SLIP is the word used for clay in dissolved, liquid form. What forms in your
throwing bucket, what is used in poured molds, clay suspended in a lot of
water as part of the making of a clay body.
Slip ALSO is clay in liquid form used for decoration--dipped, sprayed,
poured, painted on.
Engobe--the French word for slip used for decoration-- is used in English
for slip to be used for decoration. As a rule engobes contain more
non-plastic materials than slips.
The reason I stick to "slip" is that it is confusing to speak of "slip
decoration with an engobe", and the books that cover the subject all have
"slip" in their titles.
Slips are very similar to clay bodies. If one is using a very light or white
body one simply can add the colorants to that; if one is using a dark body,
colorants can be added as well, but, obviously, only for dark shades and
only if the color can contain iron (which is what makes dark bodies dark,)
NB: Some dark bodies have added manganese, and some clay naturally contain a
trace of it. Read about safety issues with manganese.)
If /when one uses the clay body it is best to screen/sieve the slip as most
bodies contain some grog or sand, or coarse fireclay, which benefits the
body, but is undesirable in the slip.
Slip for decoration can be made for/and applied at several times in the
creation of a pot. One can use a dark slip as "throwing water" on a light
body. Easily done: take some of your regular slip water, let it get quite
thick, add colorants.. .cheap thrill.
One can apply slip to a pot while it is on the wheel. I "slip" my pots after
trimming.
One can let the pot dry and apply slip then.
One can apply slip after the bisque firing.
Aha! You cry! (Will someone PLEASE cry "Aha!"?) It is evident that a slip
must fit the body just as a glaze must fit the body--and the slip. You
really do not want the slip to peel off, or bubble or cause the glaze that
covers it to craze or bubble etc.! (You do not need to invite disasters!
They will find you on their own!)
Rhodes in "Clay and Glazes for the Potter" neatly lists recipes for slips
(he refers to them as engobes!) for different cone ranges. I will only cite
the ones for Cone 1-6--and remember the end temperature for cones has
changed since 1957!) For damp ware: Kaolin 25, Ball Clay 25, Neph. Sy 15,
Talc 5, Flint 20, Zircopax 5, Borax 5. For dry ware the kaolin and ball clay
go down to 15 each, 20 calcined kaolin is added, the rest remains the same.
For bisque Kaolin goes down to 5, Ball clay is 15, Calcined kaolin is 20, a
leadless frit (no specific mention of which) is at 5, Neph Sy at 20 and the
rest remains the same..
What's this calcined kaolin? Essentially it is pre-shrunk kaolin. The water
has been fired off, so the shrinkage is reduced. Calcined kaolin is way
underappreciated, I think. Replacing some regular clay with calcined clay is
a valid way to address crazing in a glaze. Yes, once the clay is calcined it
no longer is plastic--and something like Bentonite may be needed....But that
is for another day.
To calcine: simply fill a pot--green or bisqued with kaolin and bisque!
Now one inquirer referred to a specific trademarked clay--I guess
trademarked would be correct, it is a clay known by that name--because it
was mentioned in an article about slip decoration. The author pointed out
that one wants the whitest and the finest ground clay one can get--in his
case with 50% or more of particles under one micron. (CM March 1997, Wayne
Bates) The clay he uses is C & C. Looking at Glaze Master ® I found that the
difference in iron content between it and other ball clays is small, very
very small. I would venture negligible as the analysis will change over
different mine runs.
I personally would not go out of my way for a specific ball clay. I called
three beloved and respected suppliers and talked to the Techs. Two said the
micron size would be on the MSDS sheet, the third said one has to ask the
manufacturer. I certainly would not bend myself out of shape over this. I
simply would go with a kaolin/ball clay mix, using my supplier's lowest-iron
content ball clay. NB: Mr Bates work in porcelain. That may affect what clay
he can use in his slips. As I know nothing about porcelain, I cannot speak
to the subject.
Mary Wondraush, the Dowager Queen of slip decoration, refers to slip
decoration as the hardest of techniques. (Quoting from memory here) It is
immensely hard to do it well, and I would urge any beginner to practice with
white slip tinted with food coloring.
To color slips properly requires a large amount of colorant...Mr Bates uses up
to 30% of stain. Buying food coloring at a "craft" store or at a bakers
supply store--buying it in the little pots, not the little bottles--is
feasible, reasonable. These colors are very strong., I not only would
recommend practicing with white slip/food coloring for a while, but using
some food coloring in one's final stain-colored slips as some ceramic
colorants will just be stupid gray in the slip!
One of the reasons I like working with slips is that they allow me to need
only a very few glazes...it really is an easy way to work for someone who
likes the fabrication of the pot best. Before getting into it one must
realize slip decoration is VERY time consuming. If one is going to trail, or
paint or like that, the set-up and cleanup times are long...I definitely can
throw in a couple of hours what would take me a week to decorate.
A last observation. There also are vitreous slips/engobes which essentially
are half way between a glaze and a slip.
While they are worked and applied like slip they vitrify half-way--more or
less-- to glaze. They are technically a thing apart, worth experimenting
with, but NOT what we refer to when motioning slips.
Lili Krakowski
Be of good courage
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