TECHNO TIPS

“Troubles overcome are gocd to tell.” - Yiddish Proverb

Ten years ago when | first started making majolica glazed
earthenware, ‘Murphy's Law’ quickly came into effect (i.e,
what can go wrong will go wrong - and if did). Most
problems were solved by developing a suitable glaze and
by developing my own clay body that was appropriate for
strong, durable,utilitarian ware. One problem persisted: |
could not seem to make any Sense out of the white spots
that would break out on the decorated areas during a glaze
firing. | could live with a few spots, but, after some finngs
there could be so many spots that the decoration was
essentially ruined. These spots were not blisters or
pinholes where there was an actual hole in the glaze
surface. This spotting was not a glaze flaw but rather
evidence of the passage of gas through the glaze and
decorated area which in turn caused the white spots where
the gas had exited.

| had seen or experienced this problem using high fire gas
firing and the fact that there was virtually no technical
information available on the cone of 04 firing range left me
with no other altemative than to come to an understanding
as to what exactly was happening in the kiln. One clue, in
the beginning, was that those pots that were glazed only

on the inside surface had no spotting at all. The obvious
culprit had to be gas coming from the clays in the clay
body and glaze. | am sure many of you have experienced
bisque firing clay bodies that produce clouds of blue, acrid
smoke. These gases are produced by the burning out of
carbon and sulphur. Most of {hese gases have bumt out
by 900 C. (1952 F). So this should not be a problem given
that cone 04 is 1940 F. However, some sulphur can
require a temperature as high a5 1150 C. (2102 F) to
completely burn out. So, why not bisque to 2105 F and
then glaze fire to 1940 F. so that there are no more glaze
flaws? With my clay body, this would be like trying to
glaze a stone, obviously not a practical solution.

Now back to those pots which were glazed only on the
inside and thal never had any spotting. Why was this so?
Well, the foul little gas bubbles are lazy and they take the
pat of least resistance (i.¢., via the relatively porous clay
body, mot through the thick syrupy melting glaze }). Now if
| had glazed everything on the inside surface, this would
have been enowgh - but | did not do this. However, at this
time | bought two new (used) kilns that took a long time to
fire (i.e., old elements, dense settling). The spotting
disappeared. In retrospect, this is what | think was
happening - given that all the carbon and sulphur did not
mysteriously disappear out of the clay. The slower bisque
firing was allowing more of the gas to burn out and even
more important, the slower glaze firing was allowing the
gas to bum out in a less violent way. A more rapid firing
would cause larger bubbles of gas to be released over a
short period of time with enough force to create an
opening or spot in the overglaze color.

The slower firing was burning off the gas slowly enough to
prevent the force of the escaping gas from causing large
escape pockets in the overglaze stains. I! think it is
important to remember when we use the term “Kiln Firing”,
what we really are doing is firing the clay and glaze, not
the kiln. Turning the limer and switches on and walking
away just will not work, electric kilns require as much if not
more attention than gas kilns, particularly when firing
majolica. | fire my bisque kilns to cone 06, slowing the kiln
down for the last 200 F, (1 use a $100 analog pyrometer to
keep track of the kiln temperature rise). The glaze kilns
are fired to cone 04 in the setter with large cone 04 visible
through the peephole and the analog pyrometer. | try to
give the kiln at least three hours between the bisque (cone
06) and glaze (cone 04) temperature.

Try to become as aware as possible of the character of
your kiln (no two are alike). Place large O4 cones
throughout it (the kiln) during a glaze firing to find out its
hot and cool spots. Leam to adjust the switches to slow or
speed up different sections of the kiln. Learn to use your
eyes and nose when firing, they can tell you much and
remember. If you can fire majolica successfully, those
copper reds and other glazes will come easily.

- DARCY MARGESSON