Slip Casting - Alfred's Clay Store

Slip
Casting
Lecture #10

Plaster
• Earliest use of plaster
– Impressions dating back 9000 years
from Anatolia and Syria
• 1545 is the first historical record of
plaster use in ceramics
– (Piccolopasso’s “Three books of the
Potter’s Art”)
– Before this date we have many examples
of clay molds but not plaster molds
• If plaster was used, it was too soft to survive

From Gypsum to Plaster
• Gypsum mineral consists of hydrated calcium sulphate crystals
• Gypsum is mined form the earth in rock form
• It is calcined at about100-120°C and goes through the following
reaction…
Gypsum
→
Plaster
CaSO 4 ·2H 2 O → CaSO 4 ·0.5H 2 O + 1.5H 2 O (released as steam)
• During the calcination process, water is driven off
• After calcining, the material is ground into powder and bagged

Alpha vs. Beta Plasters
• Beta plaster:
calcined under
atmospheric
pressures
• Casting Plaster (AKA Moulding
Plaster)
• #1 Casting Plaster
• Dental Plasters
• Industrial Gauging Plaster
• Industrial Plaster
• Metal Casting Plaster
• Moulding Plaster
• Potter Plaster Regular
• No #1 Pottery Plaster
• Puritan Pottery Plaster
• Alpha plaster:
calcined under
elevated pressures
• Cerami Cal
• Dura Cal
• Fast Cast
• Garden Cast
• Hydro Cal
• Hydro Stone
• Tuf Cal
• Tuf Stone
• Ultra Cal

I.e. #1 Pottery Plaster
I.e. Hydro Cal
lbs. of water for
100 lbs. of
plaster
Beta Plaster
Alpha Plaster
65-160 22-45
Hardness Soft Hard
Strength Weak Strong
Porosity High Low
Intended use
Casting/Press
Molding
Ram
Pressing/Model
Making

The Role of Water
• Calcination of gypsum makes plaster hygroscopic
(“thirsty” for water)
• When we add water to plaster, we are restoring the
water that was driven off during calcination
– Plaster converts back to Gypsum
• Because of plaster’s affinity for water it has a shelf
life
– Recommended shelf life: 4-6 months
– Shorter shelf life under damp conditions

Crystallization
• During hydration Gypsum crystals grow, creating
a strong interwoven structure
– It is important to not over-mix the plaster as it starts
to set, as doing so will destroy the integrity of the
structure
• I.e. pour the plaster before it starts to set
• Always use clean buckets when mixing new
plaster
– Old plaster acts as a seed for crystallization of new
plaster
– This will create uneven setting of plaster in the new
batch

SEMs of Gypsum crystal

Factors affecting the setting of
plaster
• Water temperature
– Plaster sets quicker in warm water than in cold
• Mixing time
– Long mixing time speeds up the set
• Amount of water
– The more water used, the more porous and weaker
the mold will be
• The amount of water, known as the Use Consistency
Ratio, varies greatly depending on the type of plaster
being used

Ideal mixing condition
• By the end of the first minute
– All plaster is sifted into water
• By the end of the second minute
– Plaster is fully soaked in water undisturbed
• By the end of the third minute
– Plaster is mixed
• By the end of the fourth minute
– Plaster is poured
• Beyond the fourth minute mark, the plaster
starts setting

Plaster Calculator
• Courtesy of www.claystore.alfred.edu

Why Slip Cast
• High detailed duplicates
• Allows us to experiment with recipes that
lack plasticity
– I.e. bone china, talc or spodumene bodies etc…
• Can create very consistent results
– Equal density during forming

Equal density during forming
• In slip casting,
particles align
perpendicular to
the mold’s faces
– As water is
sucked into the
faces of the mold,
clay particles are
forced against the
mold’s walls and
align accordingly
• Very low density
gradients =
consistent results

The Ideal Casting Slip
• Does not settle
– Extremely important!
– If you use a slip that has settled
without remixing it, you are not
using the entire recipe in your
casts
• Gels enough to add strength to
the cast, but not so much that it
coagulates inside the mold
– Drains well from the mold
Clay
Non clay
Over time, heavier
materials settle to
the bottom of the
container. This is
not good for slip
casting!

Casting bodies
• Require minimal plasticity compared to throwing
bodies
– Generally require some ball clay to improve dry strength
for handling and for trimming
• Also improve thixotropy (gelling) - But must not be overdone!
– 50% clay or even less can be used
• Remainder made up of non-plastics/silica/ and fluxes
• Non-plastic recipes are more fragile in the green
stage than plastic recipes
– May have problems releasing from the mold (some ball
clay in a recipe can help release due to higher shrinkage)

Casting body considerations
• Avoid large non-plastics (grog/sand) which tend to settle
quickly
– A non-uniform body will result (grogs on bottom/clays on top)
• Casting rate is influenced by water passages
– Some large particle kaolins are specifically graded for casting
(I.e. Velvacast)
• Relatively large size allows water to migrate to mold surface during
drying
• Excessive ball clay will slow down casting rate
• Clays with high iron or alkali can be more difficult to
deflocculate
– Also, adding Red Iron Oxide to a body can cause casting to
stick to the mold; select clays that naturally contain iron instead
– Use Darvan 811 for iron-bearing clays

Clays for casting
• We are trying to help water escape to the walls of the
mold
• Coarse clays (kaolins)
– Open up passages for water / speed up casting rates
– Generally decrease strength
• Fine clays (I.e. ball clays)
– Tend to slow down casting rates as they clog the passages
which water needs to escape
– Increase gelling
– Increase strength
• Bentonite or Veegum should be avoided

Non-plastics for casting
• While clay will remain suspended for long
periods of time, non-plastics won’t
• Grogs/sand will tend to settle quickly
– This is why recipes generally lack them
• Flux considerations
– Similar to throwing/sculpture bodies

Reeves
34% F-4
40% Grolleg
26% Flint
40% clay
(coarse clay)
Wally’s Wonderslip
26% Custer
1% Whiting
21% Goldart
12% EPK
10% Tenn. #10
30% Flint
43% clay
(finer clays)
• Quicker casting Vs. • More plastic

Casting body examples
Reeves Porcelain Cone 7-10
Grolleg 40
Kona F-4 34
Flint 26
Water
Darvan #7
Clay 40%
SSA 5.65
35%
0.35%
Spleth Porcelain Cone 8-10
Grolleg
30
Velvacast
12
Ball Clay
8
Custer 18
Flint
20
Pyrax
12
Water
Darvan #7
Clay 50%
SSA 8.09
40%
0.35%
• Translucent but
slumps
• Adopted from a
throwing body
– Similar to classic
porcelain (50/30/20)
• Stands up better
during firing
• Less flux
• Higher clay content
(especially ball clay)
• Easier to
release/handle

• Traditional casting bodies have low surface
areas, allowing them to cast quickly
• Casting bodies with finer material will be
more plastic after casting but will also cast
more slowly
• Converting sculpture/throwing bodies into
casting slips
– You may want to reduce plastic materials
– Consider reducing/eliminating grog

Obstacles to casting
• Clay in its natural state is flocculated
– Strong attraction between clay particles
– This strong attraction is counter-productive to
making a fluid slip
• Clay is also naturally thixotropic
– Gels when not agitated
– Too much thixotropy makes it difficult to drain
slip from the mold

Thixotropy
•Video 1 - Gelled
•Video 2 - Non Gelled
• Note the gelled
areas around the
sides of the
container
– These areas
aren’t circulating
because the clay
is thixotropic (a
deflocculated
clay doesn’t do
this)

Deflocculation
• Deflocculation destroy thixotropy and flocculation
• Deflocculants are better known as “Dispersants”
because they disperses clay flocks
• The dispersant has an overall charge (either + or -)
• Dispersants completely coat clay particles
– Clay particles act as if they were all of the same charge;
clay particles no longer attract
• This repulsion results in low viscosity as clay
particles repel each other
– Slip drips off your hand for a long time

- - - -
- -- - - - - - --
- - - -
- -- - - - - - --
Flocculated
Dispersed
• Dispersed clay packs more densely than non
dispersed clay
– Clays take up less space because they are more ordered
Less void space/higher density
• A.K.A. Higher Solids Loading

Dispersed vs. Flocculated systems
Property Flocculated Dispersed
Viscosity High Low
Solids loading possible Low High
Suspension element Particle floccs Individual particles
Particle structure 3-dimensional network Independent particles
Packing behavior Low density High density
Compressibility Compressible Incompressible

So we add dispersants because…
• Flocculated clay - 60% water to make the slip fluid
• Dispersed clay - 40% water to make the slip fluid
– Flocculated clay takes longer to cast/the mold gets
saturated more quickly (less casts per day)
– And even with a lot of water, the slip gels prematurely
(due to flocculation)
• Difficult/impossible to drain from the mold
• Dispersants also help make our casts more dense
– Higher green strength

Dispersants
• Old School
– Sodium silicate and Soda Ash (Sodium Carbonate)
• Soda ash tends to be thixotropic, changes over time, and
has a short range for over and under deflocculation; also
readily dissolves plaster
• Sodium silicate is more forgiving than Soda ash (still tends
towards thixotropic slips)
• Both are usually used together to minimize their
shortcomings
– Many older recipes call for these two materials

Pretend this
is clay
Polymer
(Darvan)
• New School
– (Sodium based) Polyacrylic acids
– Negatively charged polymer chains
• A polymer is a very long molecule
– Designed to attach/wrap around each individual clay
particle

Darvan
• Benefits over conventional soda ash/sodium silicate
• Longer casting range, denser castings, more stable over
time, increased mold life
• Darvan 811 (short chain polymer)
(Sodium Sodium polyacrylate)
– More concentrated part-for-part than Darvan 7 (43% vs.
25%); Ideal for red clays, but can also be used in white
bodies too
• Darvan 7 (long chain polymer)
(Sodium polymethacrylate)
– If the recipe doesn't specify, I use Darvan 7 (its more
universal)

Darvan affects viscosity of clay
Note that viscosity
increases when too much
dispersant is added

Dispersant test
• Weigh out dry materials for the batch in 10 separate
containers (200 g minimum for each)
– Dry mix
• Prepare 10 containers
– 75 grams of water in each (37.5%)
– Add Darvan in 2 drop increments (use a pipette for this)
– Mix water and Darvan well (use small Jiffy mixer, not handheld
blender)
• Sift dry material into water and Darvan
– Mix well using the mini-jiffy mixer
• Add progressively more Darvan until the most fluid
point is reached

Sequence for testing a casting slip
2 4 6 8 10 12 14 16 18 20
Add 20
drops
2 4 6 8 10 12 14 16 18 20
22
4 6 8 10 12 14 16 18 20 22

• Keep adding Darvan to the initial samples until
you don’t notice the samples getting any thinner
• Add Darvan to enough samples so that the point
where the samples reach a minimum viscosity
happens in the middle of the set
Fluid
Minimum
Viscosity
Same apparent viscosity as #22
14 16 18 20 22 24 26 28 30 32

Thick
Viscosity
Danger of settling
Thin
18 22
Drops of Darvan
Fluid
Choose
this point
Minimum
viscosity
Very fluid
14 16 18 20 22 24 26 28 30 32
Gelling
Settling

Testing for Gelling
• Mix all points with an electric mixer
to destroy any previous gelling
• Let samples sit for X amount of time
– X could be 10 minutes, 30 minutes or 2
hours, depending on how long you
expect to be casting for
• Tilt each container at a 45º angle
– For the points that gel, the slip will
cling to itself and won’t move as a
liquid
– This gelling effect is the result of
thixotropy; it should diminish as you
increase your Darvan
Fully gelled
Somewhat gelled
Not gelled

Gelling effects (increasing Darvan from left to right)…

Testing for Settling
• After testing for gelling, swish the container back and
forth a few times; then again, tilt it at a 45º angle
– Wait a few seconds for the slip to sheet off the bottom
– Scrape the bottom of the container with something
preferably non-porous (i.e. a plastic utensil works well)
– This will give you a visual cue as to whether settling has
occurred

Won’t settle
Settling effects…
Probably won’t settle
May start to settle

If settling occurs…
• If settling has occurred, it is the non-plastic part of
the recipe (i.e. flint/feldspar/filler)
– This will occur as we get closer and closer to the
minimum viscosity
• If all points settle
– Try starting with less water next time
– Encourage thixotropy by using finer clay materials (i.e.
add more ball clays)

Before finishing your tests…
• You need to figure out what the weight of 1
drop of Darvan is…
– Weigh 50 drops of Darvan, then divide by 50
i.e. 50 drops weigh 2.63 grams
So 1 drop = 0.0526 grams
• If my chosen point used 30 drops, for 200g
of clay, then…
– 30*0.0526=1.578g for 200g of clay
– Or 0.789g for 100g of clay which is 0.789%

Keeping notes…
Viscosity Test (200 gram samples)
0 drops (no Darvan) –Thick Paste
2 drops (0.0526%) -Paste
4 drops (0.1053%) -Paste
6 drops (0.1579%) -Paste
8 drops (0.2106%) -Paste
10 drops (0.2632%) -A little wetter but still paste
12 drops (0.3158%) -Noticeably wetter but still not fluid
14 drops (0.3685%) -Same
16 drops (0.4211%) –Fluid when mixing/almost pourable
18 drops (0.4738%) –Much more fluid when not mixing/thick syrup
20 drops (0.5264%) -Same
22 drops (0.5790%) –Bubbles rise on their own when not agitated
24 drops (0.6317%) -Same
26 drops (0.6843%) –Consistency of thick cream
28 drops (0.7370%) -Same
30 drops (0.7896%) -Same
32 drops (0.8422%) –Very little resistance to stirring
34 drops (0.8949%) –No noticeable change
36 drops (0.9475%) - No noticeable change
38 drops (1.0002%) - No noticeable change
Conclusion
Gelling Test After 45 Minutes
18 drops (0.4738%) –Gels heavily
20 drops (0.5264%) -Same
22 drops (0.5790%) –Gels but starts to move
when container is tilted
24 drops (0.6317%) –Same
26 drops (0.6843%) –Same
28 drops (0.7370%) – Very little gelling
30 drops (0.7896%) - Same
32 drops (0.8422%) – No gelling
34 drops (0.8949%) – No gelling
36 drops (0.9475 %) - No gelling
38 drops (1.0002%) - No gelling
Settling Test After 1 Hour
18 drops (0.4738%) –No settling but coagulates at the bottom
20 drops (0.5264%) -Same
22 drops (0.5790%) –No settling/Starts to sheet nicely
24 drops (0.6317%) –No settling
26 drops (0.6843%) –No settling
28 drops (0.7370%) - No settling
30 drops (0.7896%) - No settling
32 drops (0.8422%) – No settling
34 drops (0.8949%) – No settling
36 drops (0.9475 %) - No settling
38 drops (1.0002%) - No settling
Use 32 drops (.8422% Darvan) because this point seems to be the most liquid, and it
doesn’t gel right away.

Fine tuning the slip
• Once you’ve figured your Darvan additions,
its time to fine tune
1 st - Test specific gravity
– Do this test after the slip has been thoroughly
mixed
2 nd - Test viscosity
– Do this 12-24 hours after mixing

Specific Gravity
• This measures the amount of solids in a
given volume of slip (i.e. the slip’s density)
• Slip will always weigh more than a
comparable volume of pure water
1cc of water = 1g
1cc of slip = more than 1g
• If the slip is not dense enough (i.e. if it has
too much water), the casts it produces will be
weak

Specific Gravity
• Measure the specific gravity of the slip
– i.e. 100cc of water = 100g
– I.e. 100cc of slip = 187.5g
• Specific gravity = Slip weight/water weight
• Most casting slips fall between 1.7 and 1.8
(1.75-1.78 is ideal)
If your slip weighs more… add water to decrease
the weight
If your slip weighs less… add dry materials to
increase the weight
A bottle that
measures
exactly
100cc

Viscosity
• This measures the viscosity
of the slip
• Pour a known volume of
slip (usually 500cc)
through a ¼” hole
– Record the amount of time it
takes to drain
• Should take about 100 –
130 seconds to drain
– If too long, add dispersant
– If too short, remove
dispersant
Corporate
DIY

Specific gravity
bottle
Viscometer
Hydrometer
For measuring
density
For measuring
flow
Not good for slip
Good for thinner
materials like glazes

Calculating dispersant
level…
• The amount of dispersant needed to
disperse a body depends on the surface
area of the clay body (we are really
only interested in clays)
– We are looking to fully coat each
individual clay particle without adding
excess dispersant
• A spreadsheet has been written that
adds up the total surface area within a
body
• Based on a known amount of darvan
needed to coat a given surface area (
.1 mg/m 2 ), it then calculates how much
Darvan we need for our particular
body…
m 2 /g Material
21.99 Tile Kaolin #6
27.09 EPK
12 Grolleg
36.41 Helmar
9.97 Kaopaque
11.4 Velvacast
15.6 Peerless
21.2 SSP
21.28 Tenn #10
22.5 XX Sagger
27.51 OM #4
24.32 C&C
1.1 G-200
1.14 NC-4
1.39 Kona F-4
1.76 Custer
4.65 Cornwall Stone
1.1 A400 Neph Sy
1.44 Flint
2.62 Pyrax
31.7 Bentonite
98.66 VeeGum/Macaloid

Dispersant Calculator for Batches
• Courtesy of www.claystore.alfred.edu

• In general, more plastic bodies have a
higher surface area, and therefore require
more dispersant…
A Traditional Porcelain
55 Grolleg
25 Custer
20 Flint
#570 Stoneware
19.13 EPK
28.55 Goldart
28.55 Hawthron 35
14.27 OM-4
9.5 Custer
7.33 m 2 /g 21.8
.2923% Darvan .8643%
More surface
More dispersant
required

Is the dispersant calculator perfect
• NO!
• You still need to do a 10-point test to fine
tune viscosity
• Some clays (i.e. Hawthorn Bond) bring in
salts which counteract dispersion
– This calculator doesn’t account for these cases
– The calculator was developed for white ware
bodies (i.e. Porcelains); as we stray further
away from these, its predictions are less and
less reliable
• It is great for predicting future tests with
what you’ve already tested

Stick up slip
• Start with a casting slip with minimal water
• Add a saturated solution of salt one drop at a time
– Never add dry salt!
• Effects of salt will occur much more rapidly than
effects of dispersant (one drop at a time means…
one drop at a time!)
• Resulting slip can be tailored for viscosity
– Gelling behavior can be controlled (salt brings
thixotropy back to the slip)
• Salt also creates a flocculated system which is
more compressible than a dispersed slip
– Relatively plastic compared to dispersed clay
– Can absorb the stress of compression better (i.e.
attaching pieces together)

• Adding salts reflocculates
a
dispersed slip
• Increases
viscosity
• Use Epsom Salts
(Magnesium
Sulphate) or
Calcium Sulphate
• Not used in slip
casting
Apparent Viscosity (1.0 s -1 , Pa.s)
V
i
s
c
o
s
i
t
y
10 2
10 1
10 0
10 -1
10 -2
Adding salt…
10 -2 10 -1 10 0 10 1 10 2 10 3
Divalent Cation Concentration (mM)
Salt

Salt for suspension
• Salt also helps control settling in slips and
glazes
• When the slip/glaze remains undisturbed it
gels because of the salt
– The gelled structure freezes non-plastics (i.e.
flint, feldspar, colorants etc. can’t settle)
– Salt will only affect the clay portion of a glaze
• Clays with high surface areas respond better
• Will not work if glaze contains no clay

Slip poured over a tile…
• Without
Epsom
salt
• With
Epsom
salt

Procedure for mixing a casting slip
• Weigh water into a bucket
• Add dispersant; mix well
• Add a small amount of the recipe’s clay + all
non plastics (I.e. flint and Feldspar)
– This allows all non plastics to be coated with clay
in a very thin system (very efficient mixing)
• Add remaining clay (the mixture will thicken
up to its final viscosity
• Mix thoroughly

Solid Casting Super-Flat slabs

• Slip gels too
early
– Seals off fresh
supply of slip
– De-lamination
void spaces can
occur

• Solid Casting on
a vibration box
• Vibration
disrupts gelling,
which allows
slip to feed into
the mold for the
duration of the
cast (1 ½ hours)

• Non-Vibrated
• Vibrated

• Fired slab is as flat as the board it’s dried on (and
the shelf it is fired on)
• Fired dimensions: 22 x 15 x 3/8”
References

My references…
Any questions

References
Cushing’s Handbook
by Val Cushing
Third Edition, 1994
The Potter’s Dictionary of Materials and Techniques
by Frank and Janet Hamer
A&C Black Publishers Ltd., London, England
Third Edition, 1993
Clay and Glazes for the Potter
by Daniel Rhodes, revised and Expanded by Robin Hopper
Krause Publications, Lola, WI
Third Edition, 2000
Mold Making For Ceramics
by Donald E. Frith
Krause Publications, Lola, WI
1992

Glazing over slips
• Great way to achieve bright, glossy surfaces while not
worrying about colors running or bleeding into each
other
• Clear glazes applied over slips
– Physical depth of clear glass can show off the textural qualities
of the underlying slip (difficult to achieve with glaze alone)
• Glaze composition influences the color of the slip
– Lead glazes tend to dissolve slips, blurring edges (Slipware)
– Alkaline glazes produce brilliant results
• Semi-opaque glazes over slips provide many options for
rich, mottled results
– Especially when glaze thickness varies
– Dark oxides (iron, manganese, cobalt etc.) in the slip bleed
through glaze

From
Val’s book
• Alkaline
• Calcium
• Boron
• Magnesia
• Zinc
• Barium
• Lead
Chappell’s book is
a great source for
specific oxide
amounts

Terra sigillata
• C.f . Latin – “sealed earth”
– 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 – Contain just clay
• Made of extremely fine particles (colloidal in size)
– Will never settle or cake
• Much more opaque than regular slips
– Consequently 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%

• 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

Slips,
engobes
and slip
casting
Lecture #9

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 (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

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 need, we often include other ingredients

• 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

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
• 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 (add up to 2%)
• Organic
– Sugar
– 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. yellow, red
etc..)

Colorants
• Can be added as either oxides/carbonates or stains
• Oxides/carbonates
– Cheaper than stains
– Limited pallet
• 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
– 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
Kaolin
EPK
15
Kaolin
XX Sagger
25
Ball Clay
Nepheline Syenite 15 Soda Spar
Flint 10
Wet to leather hard application
V.C. Off White Throwing Porcelain
Cone 9-10
Tile #6
30
Kaolin
EPK
15
Kaolin
C&C
15
Ball Clay
Kona F-4 20 Soda spar
Flint
Pyrax
10
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
– Wet 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
– Can be applied over glazed ware
– Work undergoes distortion from drying and firing
before slip is applied (great for precise work)

Bisque application slips
V.C. AT 1 White Vitreous Slip
Cone 6-10
Velvacast
Calcined Kaolin
XX Sagger
Custer Feldspar
Nepheline Syenite
10
10
5
25
10
Flint 30
Borax 5
Zircopax 5
• Val’s slip uses calcined clay
• Not creamy compared to
wet application slips
C19-1 Dense Bisque Slip
EPK
Kaopaque
Glowmax
Wollastonite
Frit 3124
Flint 6
Molochite 200 Mesh 20
CMC +1
Cone 04
4
5
15
9
41
• 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
– 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
needed!

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

¼” thick per
firing without
cracks

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

Spackle
Example
Cone 04
Raw Claw
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 don’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!

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
– Low clay/low shrinkage
– More difficult for wet applications because of high flux/low
clay/low green shrinkage
• 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
Cone 04
EPK 20
Frit 3110
Nepheline Syenite
Whiting
20
20
5
Flint 15
Zinc Oxide 20 Opacity/whiteness
High amount of fluxes
Steve’s non-vitreous slip
Cone 04
EPK
Om4
9
37 High plasticity
Frit 3124
Nepheline Syenite
11
9
Lower fluxes, especially
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
• Similar problems to glaze fit (but for entirely different
reasons)
• Slip shrinks too much - Slip cracks, forming little islands
sometimes with curled-up edges
– Slip is shrinking too much
• Apply when clay is wetter OR…
• Remove some bentonite or substitute ball clay with kaolin
• Slip doesn’t shrink enough - Slip releases 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
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
(shrink-test bars for both)
– 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

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

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