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and then it
is mixed with the other ingredients.
It is very
necessary to grind very carefully the enamel into which it
enters it
; is besides very useful for diminishing the brittle-
ness of the teeth, and
its colour never changes in the opera-
tion of soldering.
Necessary Observations for the Preparation and Mixing of
Eartlis and Oxides.

The kaolin, which is procured at the manufactories of por-
celain, is not always in a sufficiently pure state.
It frequent-
ly contains much of the oxide of iron, and then it cracks, is
very hard, and badly maintains the transitions of heat and
cold.* Whatever be the purity of the substances employed,
the result is always in subordination to what is used for their
preparation, as well as to the degree of heat to which the
objects are submitted.
The best ingredients, improperly prepared, give results alto-
gether different from that which we look for ; this is so true,
that all the defaults of the teeth proceed from their wrong
preparation.
It is not less important to use pure water for softening the
clay ; that of rain is preferable.
As clay succeeds still better, the older and better tempered
it is, it is more convenient to have prepared a certain quan-
tity before-hand, kept in the cellar, in vessels of porcelain,

* What confidence may be placed in the formulas into which the oxide
of iron enters? I also have coloured teeth with these oxides mixed with
others; but I ought to declare they were of a certain brittleness, which I
renounced a long time from my essays: that which authorizes me at this
time to proscribe the oxides which I have renounced as not being fit for this
work.