COLUMBIA

Techno Tips: Reducing the Firing Temperature of a Glaze

Moving a cone 10 high temperature glaze
down to cone 5-6 requires major surgery on the
recipe or the transplantation of the colour and
surface mechanisms into a proven cone 6 base
glaze.

It is amazing how many potters push their
clay body or kiln (and utility bill) to the limit
(e.g. Cone 11) to get a special reactive glaze
to melt the way they want. It would be rare
to find an industrial manufacturer who could
afford or would have the desire to do this. It
only makes sense to fire to the lowest possible
temperature. It takes much more energy, for
example, in a typical poorly-insulated pottery
kiln to increase the temperature by 1 degree
at 1000C than it does to do the same at
500C. Obviously, lower temperatures mean
less wear and tear on kilns, especially electric.
Not-so-obviously, it can be easier to keep
the temperature even throughout the kiln at
lower temperatures and thus easier to soak at
the end. In addition, lower temperatures are
easier to fire to a programmed schedule and
easier to keep consistent firing-after-firing.

At the same time, it does not make sense to
fire too low either since quality and strength

Discovery.
avel |

Art Tr

2008-2010

Turkey
Morocco
Burma

Oaxaca, Mesica
ACLS

By Tony Hansen, Digitalfire

issues come into play below about cone 1. It
is important to realize that high temperature
firing is not a requirement for strength and
quality ware. The range of achievable color
is actually better, in most cases, at lower
temperatures. Lower-fired bodies can also
be very dense and strong. Dental porcelain,
albeit an extreme example, completely melts
into a white pool of glass far lower than
any typical functional porcelain would even
begin to densify. Admittedly the mullite-
glass-silica microstructure characteristic of
high temperature porcelains is not as highly
developed in middle-fire bodies, but even
with the simple glass-bonding-of-aggregates
mechanism you need make no apologies for
cone 5-6 or lower porcelain strength.

‘The question is: Can you juggle the amounts
of ingredients in a typical cone 10 glaze recipe
to make it melt at cone 6? The answer is
almost always no. Low fire and cone 10 glazes
are fundamentally different in the sense that
they employ different fluxes. All low fire glazes
contain lots of boron, the principal flux. Cone
10 glazes contain none. Additionally, oxides
that behave as fluxes at cone 10, like MgO

Potters Guild of BC Newsletter : October 2008

and CaO, act as the opposite, refractory
matting agents, at low temperatures! Cone
5-6 is a bit of an in-between land in this sense.
Some boron is almost always needed because
common cone 10 fluxes like feldspars, calcium
carbonate, talc and dolomite do not melt well
at all at cone 5. Thus, increasing the content
of existing fluxes will not be sufficient, you
must introduce new more powerful (better
melting) ones and learn a new set of dynamics
and tradeoffs. There are some special purpose
middle-temperature glazes that will melt
without boron (e.g. high zinc ones) but these
are the sole domain of middle temperature
and they are difficult to manage, I will not
discuss them here.

Cone 10 glazes employ raw materials instead
of frits and these individual materials do not
really melt at one temperature, they usually
soften over a wide range. Microscopically, raw
glaze powders are mixtures of a wide variety
of mineral particles that fall into groups that
have very different melting temperatures and
behaviours. Many particles actually do not

Continued on Page 10, Glaze