Understanding and Use of Biological Databases
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Understanding and Use of Biological Databases


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Understanding and Using Biological Databases Francis Ouellette Lecture 1.4
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Slips and
Lecture #11Slips 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)
– 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 applicationTerra sigillata
• C.f . Latin – “sealed earth”
– 1 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 slipsTerra 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%
Destroys thixotropy (i.e. helps
Dry Clay 30
non-clays settle)
Darvan #7 0.1-0.3%• Ball mill ingredients
Procedure for
and place in jar,
making terra
undisturbed for three
• After three days,
layers should be well
• Top layer is siphoned
• Sig. is then carefully
siphoned off (careful
not to include bottom
layer)Drawbacks to terra sigillata
• Glazes cannot be layered overtop of terra
– 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
– 50 lbs. of ball clay yields roughly 2 gallons of
• This can be somewhat improved by ball milling
• Preparation is somewhat tedious
• Limited firing rangeFormulating 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 ingredientsSo…
• Slip ingredients usually include:
– Clays
– Fluxes
– Fillers
• A slip may also require:
– Hardeners
– Opacifiers
– Colorants
• These additions depend on the composition of the slipClays
• Chosen for their relative color and shrinkage
• Light colored clays are necessary for a wide pallet of
– 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