Tom Buck on wed 3 mar 99
Ms Francoise Bazard:
Your information on the origin of the word "engobe" has the ring
of truth ("it sure sounds French"). Yet, authors of books in English
(US/UK) seem to have chosen to make a distinction between "slip" and
"engobe" ... one runs across the notion fairly often that a slip is for
wet-ware and an engobe is for air-dried raw-ware.
The authors make the point that the pot and the add-on coating
must shrink to the same extent otherwise the coating will show open cracks
or in the rare case, flake off. Since a slip contains a lot of clay it
will exhibit considerable shrinkage going from wet to air dry and, later,
shrink more in bisque and glost firings. A typical value for air-drying is
approximately 6%. Kaolins and smectites will shrink more. For comparison,
recall that a pot off the wheel will usually shrink 12-15% in size before
it is ready for sale.
An engobe, as the term is used, means a mixture with less than 50%
clay content, and so it doesn't shrink as much. That the engobe meshes
with the very dry pot successfully probably arises from the fact that as
the dried pot is rewetted, it swells somewhat, and then shrinks again in
step with the engobe.
Both slip and engobe are often modified in fired colour by use of
common oxides of iron, copper, cobalt, others, and the amount of these
colourants required runs to twice of that in a glaze for bisqued-ware.
Once the slip-coated pot, or engobe-coated pot, dries and is
bisque-fired, it should accept glaze (dip, spray, brush) in the same way
as an uncoated biscuit would, and meet the criteria generally accorded
"good pots" -- our Guild demands no crazing, no pinholes, no unsightly
glaze runs, smooth feet/bottoms. Hence, slips and engobes must be
formulated to ensure the fired pot meets these criteria.
BTW, Francoise, if you get nervous with the male/female feuds in
France, pick up wheel, kiln and all, and join us in North America where
the pottery population is dominated by the ladies.
A bientot. Paix. "Toe-mah".
Tom Buck ) tel: 905-389-2339 & snailmail: 373 East 43rd
St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of Ontario,
Canada).
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