Slips and engobes - Alfred's Clay Store

Slips and
engobes
Lecture #11

Slips and engobes
• Historically used in Europe to make red clays look like
imported white porcelain
• Slips and engobes are used for many different reasons
– Brighten overlying glazes on darker colored clays
– Create a dense surface over an otherwise porous body
– Allow for a vast pallet of color and texture
– Cover up surface imperfections/roughness/even iron spots
– Improve glaze fit without adjusting the glaze or the clay
body (create a buffer layer)
• WYSIWYG
– Don’t run
– Don’t pinhole (during firing)

• Traditionally
– Slips are made exclusively of clay
• For wet to leatherhard application
– Engobes always contain fluxes and fillers and
usually contain clay
• Often used for bisque-fired ware
• Usually, though not exclusively, denser than slips
• Today we call both categories “slips”
• Slips are distinguished by the state of the
ware to which they are applied and by the
extent of vitrification
– I.e. Vitreous or non-vitreous slip
• for wet, leatherhard, green, or bisque application

• C.f . Latin – “sealed earth”
Terra sigillata
– 1 st century B.C. (spread by the Roman empire)
• Historically used to seal a porous clay
– Also used in drain tiles to make them waterproof
• Usually applied to leatherhard-dry clay
• A true slip – Contains just clay
• Made of extremely fine particles (colloidal in size)
– Will not settle or cake over time in the bucket
• Much more opaque than regular slips
– Requires a much thinner coating than regular slips

Terra sigillata
• Dense, silky-smooth, sometimes glossy
– Gloss is temperature dependent based on the clay used
• Generally retained up to C. 04; some can go to C. 2
• Gloss is enhanced by burnishing (re-orients clay particles
parallel to the surface? The jury is deciding on this one)
– Burnishing tools are typically smooth (spoon, river-bed stones);
cloth also works well (t-shirts, cheesecloth, dry-cleaning plastic)
• Historically red clays were used but any plastic (fine) clay
can be used
– White sig. can be prepared from ball clays or kaolins
• Basic Formula
Water 70%
Dry Clay 30
Darvan #7 0.1-0.3%
Destroys thixotropy (i.e. helps
non-clays settle)

• Ball mill ingredients
and place in jar,
undisturbed for three
days
• After three days,
layers should be well
defined
• Top layer is siphoned
off
• Sig. is then carefully
siphoned off (careful
not to include bottom
layer)
Procedure for
making terra
sigillata

Drawbacks to terra sigillata
• Glazes cannot be layered overtop of terra
sigillata
– Glaze tends to dissolve terra sig. layer
• Terra sig. is too thin to withstand glaze fusion
• A lot of clay is wasted in preparing terra
sig.
– 50 lbs. of ball clay yields roughly 2 gallons of
sig.
• This can be somewhat improved by ball milling
• Preparation is somewhat tedious
• Limited firing range

Formulating slips
• A slip can be made by using the clay body itself,
minus any large fillers (i.e.. No grog)
• Advantage
– Compositionally very similar to body
• CTE is almost identical to body
• Drawbacks
– Slip must be applied to very wet clay to avoid cracking
– Slip can’t be lighter in color than the body without
radically changing the sources of clay
• To improve on these limitations and to tailor the
slip to our needs, we often include other ingredients

So…
• Slip ingredients usually include:
– Clays
– Fluxes
– Fillers
• A slip may also require:
– Hardeners
– Opacifiers
– Colorants
• These additions depend on the composition of the slip

Clays
• Chosen for their relative color and shrinkage
• Light colored clays are necessary for a wide pallet of
colors
– Especially true when developing light colors
• Combinations of kaolin and ball clay are normally
used to adjust shrinkage
– More ball clay = more shrinkage
– More kaolin = less shrinkage
– Total clay is usually between 40-80%
• Improve slip’s adherence to the body in dry state
• Help keep slip in suspension
• Improve the brushing qualities of non-plastics

• Amplified by
the presence
of clay
• Can be
exaggerated
by adding salt
Thixotropy
• Retains brushstrokes (can
be good or bad, depending
on your needs)

No salts Salts
Grolleg 48%
Frit 2110 52%
Darvan 811 0.14%
+ Epsom Salts

Fluxes
• Help fuse the slip to the body
• Selection is temperature dependent and
reflects what we already know for clay
bodies
– High temp-Feldspars/+auxiliary fluxes
– Mid temp-Neph Sy/Talc/Frits
– Low temp-Frits/Talc
• Color response to oxides or stains will be
enhanced or impeded by our choice flux
– Similar to a glaze in this respect

Hi Soda
3110
Hi Calcium/Boron
3124
Hi Calcium/Boron/Alumina
3195
Hi Magnesium
Talc

Fillers
• Silica is the most commonly used
– Promotes hardness (depending on the amount of
vitrification)
– Reduces drying shrinkage
– Promotes whiteness
• Pyrax can also be used (though not as white as
flint)
• Talc (color killer)
• Wollastonite
• Calcined clay (i.e. glomax)
• Filler content usually ranges from 15-30%

Hardeners
• Used to improve hardness of slip after drying (anti
smudging)
– Slips high in clay usually don’t require additional hardeners
– Required in low clay-content slips (esp. when ball clay is absent)
• Inorganic
– Borax is soluble
• Recrystallizes when it dries, forming a tough finish
• Can be used up to 5% but other fluxes may need to be decreased (borax is
a boron source so it acts like a flux)
– Bentonite/Vee Gum
• Fine size hardens upon drying (up to 2% Bentonite /up to 1% Vee Gum))
• Organic
– Sugar (dissolved in water first!)
– Gums (CMC, Gum Arabic)
– Spoil with time/can migrate to the top of the container

Opacifiers
• Help ensure whiteness and allow for thin application
without loss of opacity
• Clay is opaque; recipes low in clay depend on opacifiers
– Not as necessary when dark stains/oxides are used
• Tin Oxide
– Traditional opacifier (600+ years); gives warm whites
– very expensive!
• Zircopax (zirconium silicate)
– Cooler whites (can be compensated with small rutile additions);
more affordable than Tin
• Opacifiers tint colors
– Don’t use if black/dark colors are what you want
– May impede development of saturated colors (I.e. bright yellow,
red etc..)

Colorants
• Can be added as either oxides/carbonates or stains
• Oxides/carbonates
– Cheaper than stains
– Limited pallet / and can also produce bubbling (esp. Carbonates)
• Developing strong colors in slips requires larger amounts of
colorants than for glazes
– Glass in glaze provides depth and brilliance
– The drier the slip, the more difficult strong color response is (i.e.
vitreous slips are better candidates for strong color than dry, non-
vitreous slips)
• Addition amounts
– Vary with stain/oxide used; lighter colors require more
• I.e. Yellows require 10-15% stain
• Blues require just 3% cobalt carb. (even .25% is noticeable)

Colorants
• Mottled Slips
– Additions of granulated materials (I.e. rutile, illmenite,
manganese, homemade aggregates)
– Oxides – More mottled (and more concentrated)
– Carbonates – Less mottled/more homogenous
• As in glazes, oxides present in slip will push or pull colors
– Some commercial stains will not give the intended color in the
presence of certain oxides, especially zinc
• Mason color chart identifies these (Esp. greens, purples and pinks)
• Cautionary note:
– Many metal oxides are not food safe in slips (because slips lack
the glassy structure found in glazes)
• If its not safe in the raw state, chances are it won’t be safe in a fired slip
either

Slip compositions
• Note changes in flux (for temperature
ranges) and clay (for application ranges)

Comparing bodies to slips
V.C. HF 4 All purpose white base slip
Cone 9-12
Grolleg
30
EPK
15
XX Sagger
25
Kaolin
Kaolin
Ball Clay
Nepheline Syenite 15 Soda Spar
Flint 10
Wet to leather hard application
V.C. Off White Throwing Porcelain
Tile #6
Cone 9-10
30
Kaolin
EPK
C&C
15
15
Kaolin
Ball Clay
Kona F-4 20 Soda spar
Flint
10
Pyrax
10
Macaloid +1
• More
similarities
than
differences
between the
two
• Could we use
a slip as a
clay?

Wet vs. Bisque
• Wet application slips
– High clay = creamy quality perfect for painterly
qualities
– Clay and slip can be carved as one (sgraffito)
– Ideal for slip trailing/thick, textural layering
• Bisque application slips
– Timing is irrelevant (ware can be slipped at leisure)
– Application can be washed off and reapplied
indefinitely
– Can be applied over glazed ware
– Work undergoes distortion from drying and firing
before slip is applied (great for precise work)

V.C. AT 1 White Vitreous Slip
Cone 6-10
Velvacast
Calcined Kaolin
XX Sagger
Custer Feldspar
Nepheline Syenite
Bisque application slips
10
10
5
25
10
Flint 30
Borax 5
Zircopax 5
C19-1 Dense Bisque Slip
EPK
Cone 04
4
Kaopaque
5
Glowmax
15
Wollastonite
9
Frit 3124
41
Flint 6
Molochite 200 Mesh 20
CMC
+1
• Val’s slip uses calcined clay
• Not creamy compared to
wet application slips
• For thick applications
– Best applied through spraying
– Uses very little raw clay
– Glowmax is calcined
– Molochite is added as a filler

The shrinkage myth
• Many bisque-application slips call for large amounts of
calcined clay
– Myth… During drying, raw clay shrinks / calcined clay
doesn’t so loading up on calcined clay is the answer
– Truth… Water separates particles in both cases…even if you
use just calcined clay, you will get shrinkage (not as much as
with raw clay, but still enough to cause problems)
• Including plastic clay in the recipe increases the strength
of the slip during drying and can help it overcome
cracks as the slip is placed in tension during drying
(calcined clay has no strength to overcome this tension)
– Adding too much plastic clay, however, will ultimately
increase shrinkage, tension, and cracking…A balance is
required!

Spackle
• For very thick applications on bisqued ware
• Inert
• Lots of calcined materials
– Esp. clays
• Raw clay is used sparingly (10-15%) to suspend
mixture
• Relies heavily on a wide particle size distribution
– Screened material
• Empirical testing (trial and error)

• Sudden thickness change from top (thick)
surface to the sides (thin) causes separation

Adding salt to the slip ensures a move even
thickness on both the top and sides of a tile
• Without
Epsom
salt
• With
Epsom
salt, the
slip “gels”

¼” thick per
firing without
cracks

Multiple firings,
then cut on a
brick saw
Sanded prior
to firing

Spackle
Example
Raw Claw
Cone 04
10
Calcined Kaolin 10
Calcined Ball Clay 10
Calcined Fireclay 5
Frit 3110 25
Grog (screened) 30
Wollastonite
10
CMC
+1
Suspension
Low Shrink
Low Shrink
Low Shrink
No shrink
No shrink
• Epsom salt is added to prevent settling
– Added as a saturated solution to the water before adding all
other dry ingredients
– Creates Thixotropy (gelling effect when left undisturbed/coarse
material won’t settle);
– Epsom salt can be added to any slip, provided it contains clay
• Salt affects the raw clay, not the non-plastics
• NOTE: Salt dissolves plaster molds over time!

Using spackle
on bisqued
work

Spackle and
bisque slip

Vitreous slips
• Halfway between a glaze and a slip
– Less clay/shrinkage than regular slip
• 10-20% clay for suspension and green strength
• Higher non-plastics
• Higher fluxes
• Can be applied to dry or bisque ware
– More difficult for wet applications because of low clay/low green
shrinkage (can’t keep up with a drying, shrinking clay body as
easily)
• Can be used alone as a functional surface
– Very vitreous / almost glaze-like
• Think of these as slightly underfired opaque glazes
• Can also be used under glazes to influence texture and
color of glaze

Vitreous vs. Non-Vitreous
V.C. 5 vitreous slip
EPK
Cone 04
20
Frit 3110
20
Nepheline Syenite 20
Whiting
5
High amount of fluxes
Flint 15
Zinc Oxide 20 Opacity/whiteness
Steve’s non-vitreous slip
EPK
Cone 04
9
Om4
37
High plasticity
Frit 3124
11
Lower fluxes, especially
Nepheline Syenite 9
frit
Talc
28
Flint 9
Zircopax +5 Opacity/whiteness
• Both are for dry
to bisque
application
• V.C. 5 vitrifies at
C. 04
– Not much clay
– High amount of
flux
• Steve’s is quite
porous at C. 04
– More clay and
less active flux
to melt it

Cracking
• Does not occur randomly
• Every slip has a limit as to how thick it
can be applied before cracking occurs
• Cracks begin where there is a sudden
change in thickness
• This can be used effectively to create
designs
– Brushstrokes are never even
• Bristles create ridges and valleys (cracks start
in valleys)
• Direction of strokes can be used to one’s
advantage
– I.e. Grids vs. spirals vs. random patterns
– Scribing into still wet slip will influence
where cracks occur
Slip surface
Crack

Robert Sperry Steve Heinemann

Cracking in wet to leatherhard applications
• Slip cracks, forming little islands sometimes with
curled-up edges
– Slip is shrinking too much
• Apply slip when clay is wetter
OR…
• Remove some bentonite or substitute ball clay with kaolin
• Slip doesn’t shrink enough - Slip separates in large
sheets from body
– Slip is not shrinking enough
• Apply when clay is drier OR…
• Add finer clays

Solutions to cracking in drying
stage
• Cracking can be minimized by changing
how the slip is applied
– Dipping vs. pouring vs. brushing vs. sponging
vs. spraying
• High water content can lead to cracks
– More water = more shrinkage
• Deflocculating the slip reduces water, and
by extension, shrinkage
– Adding salt can help control viscosity of a
deflocculated slip (I.e. Stick-up slip)

Crow’s Foot cracking

Cracking due to trapped air
• Slips that are thick can trap air during mixing
• Air bubbles don’t allow slip to shrink (tension is formed)
– Bubbles become nuclei for crow’s foot cracking
– Three cracks propagate from a point
• Sometimes the offending bubble is seen at the surface,
sometimes it is below the surface
• Solution… remove bubbles!
– Tapping the container
• Bubbles will rise and pop
– Tilt and rotate the container to expose bubbles
• Bubbles at the surface will stretch and pop
– Increase water content
• Bubbles will rise and pop

Cracking after firing
• Slip goes into kiln defect free but comes out
cracked (very annoying)
• Can be due to minute drying cracks (invisible to the
eye); you have exceeded the maximum thickness
for your slip
– Cracks open up during firing
– Common in high zinc slips (zinc shrinks!)
– Solution - Apply a thinner and more even coat of slip
• Can be due to large difference in firing shrinkage
between body and slip
• Solution - Compare fired shrinkage of body with that of slip (do
shrinkage test bars for both the slip and the body)
– If slip shrinks too much - decrease flux
– If slip doesn’t shrink enough - increase flux

Cracking after firing
• Can also be due to a mismatch in CTE between
body and slip
• Sharp transitions and corners are usually the
first place this happens (the first place to look)
• Usually a case of shivering, where the crack
runs through the body and separates the slip
from the surface
• Remedied by increasing the slip’s CTE

Shivering slip

Witness the awesome
destructive force that is…
Shivering

Measuring
slip
thickness
• Easy to do
and more
reliable
than
measuring
water
content

Tape
Using a pin
tool as a
thickness
gauge
• Useful for slips
and glazes applied
on dry or bisque
ware
• Stab the slip/glaze
before it hardens
• Tape serves as a
visual reference

END