Techno Tip

Creating a Non-Glaze Ceramic Slip or Engobe

Thermal Expansion

* To function together as a fired object
body and slip must have a similar thermal
expansion, This means that when the ob-
ject is heated or cooled quickly during use
the minute amount of expansion and con-
traction that occurs must be similar in slip
and body. The covering slip on a ceramic
picce is a separate entity and has its own
coefficient of thermal expansion independ-
ent from the underlying body. If sufficient
incompatibility exists, sudden heating or
coming will overcome the slip body bond
and the slip can flake off,

* Ifa glaze is applied over a slip it is amore
complex situation. The slip is acting as a
buffer layer and it must be expansion com-
patible with both the underlying body and
the overlying glaze. Normally it is best to
adapt the glaze to the body first and test it
well to assure compatibility. Work on
adapting the intermediate slip layer can
then proceed on the assumption thar body
and glaze are already matched. However
note that glazes normally need to be under
compression to achieve fit and an interme-
diate layer will have to be well adhered to
the body to hang on despite compressive
forces from the overlying glaze.

Other Factors

* Slips should exhibit a degree of maturity
or vitrification in keeping with their in-
tended surface character. While slip-glares
melt like normal glazes, non-glaze slips
normally have the flux content tuned to
develop a surface character somewhere
between body and glaze. As noted above,
a slip that is more vitreous than the under-
lying clay body will adhere better.

* Slips develop characteristic fired surface
qualities. They can be smooth, silky, glassy,
textured, dry, stippled, etc. You control
this by the degree of melt, slip makeup and
application methods,

* Aslip’s flow characteristics, water con-
tent, stickiness, eclling properties, and
drying speed enhance or degrade its appili-
cation properties (therefore ware quality).

As noted, the above factors can be divided
between physical and fired considerations,
If you need a ship for a particular body, the

April 2003

by Tony Hansen

first step is to determine its purpose, then
propose a simple recipe and begin testing.
It is usually best to concentrate on match-
ing physical properties first, then fired,
then come back to fine tune physical again.

Let's make a slip for dipping and painting
on leather hard stoneware surfaces that
may or may not be glazed over. We need
good covering power to hide dark brown
body color where necessary and the sur-
face should have just a slight sheen so that
it gives a pleasant feel to the touch,

A Slip Recipe for Cone 10

The most fired compatible slip would ob-
viously be the body itself made into a slip,
however this is not the case for drying
compatibility. We can view a basic slip as
a mix of clays with enough feldspar to
impart the necessary maturity and the rest
filled out by silica. The real trick is to
choose the right mix of clays (e.g. ball
clay, kaolin, bentonite) to provide a dry-
ing-shrinkage compatible material. Here
is what | would start with:

30-35% feldspar or Nepheline Syenite

This is more feldspar than a porcelain
body would have and will thus provide
adequate melting to produce a surface
having aslight sheen, Fora cone 6 slip, this
would need to be about 50% or more if no
boron frit is weed.

1-2% 325 mesh Bentonite

This will help harden the dry surface to
make if resistant to smudging during han-
dling. Bentonite has an incredibly small
particle size and it is very effective to
increase drying shrinkage, suspend the
slurry, and harden the dry surface. This
smal] amount will not impact fired proper-
ties. This ingredient can be reduced or
increased to allow easy day-to-day fine
tuning to compensate for tendencies to
flake or shrink off,

20-25% Ball Clay

This will have the biggest single effect on
the slip’s physical properties. It will sus-
pend and harden it and give it a sticky
nature. [t will, however, move the colour
toward off white since it contains some
iron impurities.

Potters Guild of Bntish Columbia Newsletter

20-25% Kaolin

Its larger particle size will complement the
ball clay well and impart a creamy texture
to the slurry. The two materials together
can be thought of as the ‘clay comple-
ment’. Their amounts can be traded off
one against the other to tune the slips
physical properties during development.
Plasnc, non-plastic, and calcined kaolins
are available to give you considerable con-
trol over the slip's hardness, suspension,
and shrinkage properties without impact-
ing fired results.

20% Silica

This is @ filler and helps to give the slip
thermal expansion properties more com-
patible with typical bodies that contain
lots of silica. Lf glazes tend to craze over,
the slip or the slip weakens the body; flint
is Important.

3% Zircopaxr

This will help give the shpextra whiteness
and opacity so that it covers well. Its fine
particle size will enhance the slip’ s creami-
ness and working properties.

3% Gum

This will impart extra dry hardness or
make it paint better (but it will slow down

drying).

To challenge the slip-body bond, apply a
thick layer of the ship to the side of some
test pieces that are at the correct stage,
Allow to dry. Ifthe slip shrinks more than
the body and forms little islands with curled
up edges, then it's too plastic (take out
some bentonite or trade off some plastic
clay for bess plastic). If the slip has not
shrunk, allempt to scrape it off with a
knife. If it falls away easily, it is not
shrinking as much as the body (add ben-
tonite or trade off a less plastic clay for a
more plastic ome}.

Neat, fire the ware and break it into small
pieces. Examine shards closely to see if
the slip is flaking off. If it is, add more
feldspar to develop a better slip-body in-
terface of try to match the body's firing
shrinkage patterns better by adjusting the
kaolin-ball clay mix up and down,

Techno Tip continued on page I]

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