Snail Scott on wed 14 aug 02
At 09:22 PM 8/13/02 -0400, you wrote:
>I wish someone would put out a book on slips and engobes for cone 6
>electric.
James Chappell's recipe book has many engobe recipes,
with colorant suggestions too. Other books have some
as well. In general, engobes are much less fussy than
glazes, and many of the tricky issues surrounding glaze
development are only slighly relevant to engobes.
>Recipes for both clays for application to soft leather, leather hard and
>greenware and bisque...
For most recipes, simply modifying the proportion of
calcined materials to 'regular' clay will be sufficient
to cover this full range of applications.
>What makes matte, satin or glossy surfaces...
In short, the amount of flux.
>Use of the usual oxides for a variety of different colors and learning
>*why* such an such oxide combination does this color...
Because engobes do not reach a level of full melt, the oxides
do not combine or shift to the same degree as with glazes.
Mixing of colors is much more direct and predictable.
>Use of layering techniques if possible with slips and engobes...
Some formulas have more opacifiers than others, and
are more melted at your temperature, increasing
transparency slightly. But as mentioned above, lack
of a full liquid melt means that the result is much
more predictable and controllable than with glazes.
>Firing information such as regular cooldown, slow cooldown, etc...
Scarcely makes any difference at all.
>My latest experiment applying an engobe on greenware turned out awful.
>Horrible dry very matte surface...
Underfired, or not enough fluxes.
>color much too dark...
Careful with that cobalt! It can be hard to control
the amount when making very small batches. Stains
can be much more controllable. For certain colors,
stains are a more expensive way to get colors similar
to oxides. But if you are trying for pale blues,
stains are much easier to measure, since you aren't
trying for very fine precision with tiny amounts.
And compared with the cost of cobalt, stains aren't
as big a cost increase either. (Not that you'd be
using tons of it anyway.) ;)
>cracks and flaking off after firing...
Applied to too-dry clay. Or, not enough
(uncalcined) clay in the recipe. If all the
clay in the recipe is currently uncalcined,
look for a recipe with a higher percentage of
clay. Or, try adding about 1% of bentonite.
(Or try your current recipe on bisque - might
work!)
As with any ceramic process, testing first will
save much finished work. A few experiments will
quickly fine-tune your results!
-Snail
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