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: shrinkage of terra sig on bisqueware

updated thu 11 apr 02


iandol on wed 10 apr 02

Dear Jeff Tsai,

I suppose that once again we are dealing with a complex relationship =
between dissimilar materials.

Shrinkage of any clay during drying will depend on the volumetric =
relationship between the mass of water and the mass of clay in the in =
the Terra Sigilatta fluid. If deflocculants have been used during =
preparation of the original clay and evaporation has been used to =
increase the viscosity to a level where the fluid flows rather than =
runs, then the clay volume to water volume will be high and shrinkage =
will be low. It can be less that the relationship between water and clay =
in a plastic body.

A second point relates to the structures of the materials. Bisque has an =
open porous structure, determined to some degree by the size =
distribution and proportions of the particles employed to make the clay =
body. Since Terra Sig is composed of the finest fractions of clay, these =
may well be drawn into the interstices of the bisque by capillary =
action. Subsequent evaporation of water will increase the mechanical =
bond strength of the coating.

There may be other factors which might be considered.

Best regards,

Ivor Lewis.