Techno Tip

Creating a Non-Glaze Ceramic Slip or Engobe

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A Few General Notes About Using Slip

* Slips are more opaque than glazes and
have great ‘hiding power’ (even ina very
thin layer) that enables them to cover the
body and mask its colour and surface char-
acter. This hiding includes smocthing an
otherwise rough or coarse surface, obscur-
ing an undesired colour, hiding specks and
surface imperfections { like those caused
by iron impurities), tightening and sealing
an otherwise porous and soft surface, and
providing a whiter surface to brighten glaze
colours. However a slip will mot prevent
the deposit of soluble salts (from the body)
on its surface as water evaporates and
leaves them behind.

* While slips can act as mediums for stains
and metal oxide Colours most stains do not
develop their colour well in slips because
of the lack of a glassy phase. Colour is
often amatter of chemistry and interaction
hetwoen glaze and colourant but since
slips do not melt as much so the chemistry
docs not happen as spontaneously. Since
beady compatibility concems are s com
plex with slips there is often not enough
flexibility to adjust chemistry for stain
compatibility. A simple solution 13 some-
times available: use a body stain. Chemi-
cally incompatible slip-colour combina-
tions can be forced into working by satu-
tating the slip with colour, but this can be
expensive (and makes little sense when a
slip recipe change would be more effec-
tive). Another aspect to consider is that
since slips are opaque the only stain prarti-
cles that influence colour are those ex-
posed at the surface (this is a waste of
mrienree'y }.

* If you have the facilities to slurry and
dewater a small amount of the slip and
make dry and fired shrinkage test bars, this
is an excellent idea. This way you can
compare the figures directly. [ have found
that slips with 1-2% less drying shrinkage
than the body function well.

* As noted, it is difficult enough to get
glazes to fit properly on a clay, but putting
# slipof unknown thermal and fired shrink
age properties between a glaze and clay
could open areal ‘Pandora's box" for you-

If you are determined to make it succeed,
test the glaze fit on the body first (by hot
waterice water immersion cycling to re-
veal crazing of shivering or by comparing
the fired strength of glazed bars with that
of unglazed bars). Adjust the glaze if nec-
essary and repeat. Next, match the slip to
the beady and deo adhesion and shattering
tests to be sure there is a good fired bond.
Finally, test the combination as you did the
glaze alone.

* The body surface needs some roughness
to give the slip a place to grab on to.
Additionally, contact between the body
and the liquid phase of the slipis important
to establish a good initial bond (take this
into account when spraying). Ifyou would
like a very creamy texture, consider ball
milling the slip thoroughly.

* Slip formulation is normally done on the
recipe level. Ceramic calculations are mot
really applicable because non-glaze slips

don't melt.

* However, be careful about putting metal
oxides in slips that will be exposed to food
or drink. The metals will leach out more
readily than with a glaze since oxide par-
ticles are not dissolved into a glassy ma-
trix.

Creating your own slip is not really all that
difficult, but it will take time, determina-
tion, and a methodical approach with plenty
of testing. Like glazes, tt ts far better to
have one slip you understand and control
than mess around with ten that you don’t
understand und don't work.

Tony Hansen

The Techno Tip column is generously
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Medicine Hat, Alberta, avd its affiliates
in British Columbia, Greenup Porers
Supply Crd, and Vancouver Island
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