Techno Tip

Creating a Non-Glaze Ceramic Slip or Engobe

Non-glaze slips for pottery and tile deco-
ration have long fostened great fascima-
tion. Almost eweryone has marveled at the
simple beauty of terra cotta ware deco-
rated with white slip and finished with a
transparent glaze, Many potters are adapt-
ing this age old process to stoneware and
porcelain. Industry, especially the tile in-
dustry, routinely applies slips and engobes
{e.2. white engobes are put on darker burm-
ing dry tiles before applying glaze over
top). Slip recipes don't travel well at all
and are often not well understood. Local
materials are typically involved. Slips tend
to be very beoxty specific and there are
different factors to consider depending on
whether slips are applied to leather hard or
dry ware: what temperature they are fired
to, and what plazxe and body they are paired
with, The location-specific factors make it
more practical, even necessary, to under-
stand sone simple principles and create or
tune a ship to fil your purpose,

The major problem with slips is compat-
ibility with the beady. If the slip does not
shrink at the camect rate or amount during
drying and firing, then cracks will de-
velop. No matter what logic or theory
might indicate or what others might ad-
vise, if cracking is occurring then you need
torcact by adjusting the formulation of the
slip in the right direction (or possibly ad-
justing the way the slip is prepared or
applied). Different factors are involwed in
attaining compatibility for firing and dry-
ing and a change that improves one may
als detrimentally affect the other.

At first, the ideal solution might seem to be
a slip made from the same fecipe as the
body (with or without colourants), Horw-
ever you may not be able to achieve good
fired adherence if the body does not vitnify
well and it will not likely be possible to
avoid drying cracks (c.g. slips contain
much more water than the leather hard
clays onto which they are applied so they
will shrink more; slips need to shrink but
the dry bady onto which they are being
applied has already shrunk).

Adherence, Suspension, Hardness
Non-glaze slips have physical working
properties that determine how well they
paint, dip or spray; how well they adhere;

B

by Tony Hansen

how they drain after dipping, how fast they
dry to reach the same water content as an
underlying leather hard body: how thick
they can be applied with each coat; whether
double coats can be applied; how well they
stay In suspension during storage; how
much they tend to gel, etc,

* Normally the clay ingredients act to
suspend non-plastic particles like frit, feld-
spar and silica. They impart adherence
propertics to the leather hard or dry clay
surface and they harden the dry surface
enough to withstand handling (a slip that
dries hard resists smudging and smearing
When pieces are handled; a soft one has a
powdery unstable surface). Bentonite
smectitehectornte type materials are the
best suspending agents. Only 1-2%1s some-
times needed and this has minimum im-
pact on other properties, although drying
time will slow and shrinkage will increase
somewhat Raolins and ball clays will
benefit best working properties and hard-
ness, but they also increase drying shrink-
age preatly, Organic gums and binders can
also be used to improve hardening and
adherence. They will also slow drying and
may introduce glaze pinholing problems if
too much is used as they generate gases of
decomposition during firing.

Fired Adherence

* The fired slips layer may be either
loosely ortightly bonded tothe underlying
body. A slip containing glassy fluxes that
melt and flow will form an interface with
the bexdy to securely stick it on; one with-
out can only hang on mechanically to any
available surface irregularities, A vilreous
body will also supply some glassy fluxes
to adhere to acovering slip. If a slip is mot
glassy and the body is not vitreous then it
is Very important to make sure that the slip
has the same fired shrinkage amd thermal
expansion as the body. Even slight differ-
ences Can Compromise the bond, [tis sim-
ple to test the integrity of and chip resist-
ance of the bond by applying a thick slip
layer bo a test tile,

Dry Shrinkage
* Slips exhibit drying shrinkage just like
clay bodies. The drying and dry bond
between slip and body is fragile at best,

Potters Guild of British Columbia Newsletter

* fit is being applied during leather hard
stage, it is thus important that the slip
shrink together with the body and at the
same rate and magnitude.

* Conversely, if slipis being applied to dry
ware It needs to shrink as litthe as possible,
To achieve this, it will be necessary to
either minimize clay content (especially
fine grained clays), use low-shrinkage clay
or clay-like minerals, or employ a portion
of calcined clays, Kaolins shrink the least,
ball clays much more. Kaolins are avail-
able in many ultimate particle sizes; the
larger the size, the bower the shrinkage will
be (and the less hard the dry glaze will be).
Reducing drying shrinkage by the judi-
enous choice of clay (vs. elimination of
clay) is more likely to maintain fired com-
patibility. Organic binders can be employed
to impart or augment hardening and adher-
ence properties (again, if clay content is
too low the slip may not be compatible
with the fired body; some non-plastic or
coarser particled clavs should be weed).

« Compatibility can be tested by simple
observation. [tis easy to tell the difference
between slips that shell off because they
shrink less (when being applied to leather
hard body) or crack because they shrink
more.

Fired Shrinkage

* Slips have a characteristic firing shrink-
age just as a bexly does, Body and slip must
shrink tagether during cooling in the kiln
in a way that does pot introduce tension
that can weaken the slip body bond. Not
only does the total shrinkage need to match,
but the curves need to be very similar.
Compatibility is simple to test, just apply
a thick layer of slip om a flat smooth
surface of the body; fire it, then try to chip
it off. Lit does come off, determine if the
bond is the problem first. [f not, then it
needs to shrink more or less; adjust in the
night direction and test again,

* The fired shrinkage of a slip can be
increased by using omeore fluxes to impart
greater vitrification of melting, or by using
a finer particle sized clay, In many cases it
15 practical to make a pugged sample of the
slip and make test bars for measuring the
fired shrinkage.

April 2003