search  current discussion  categories  kilns & firing - salt & soda 

salt....or soda

updated fri 28 jul 00


Bruce Gioia on thu 27 jul 00

I found the salt/carbon trap thread very intriguing.
I have some info from CM... which issue I didn't record.
Those interested may find it helpful....

Charting new Paths with Soda
by Gail Nichols >clips<

In the beginning, I faced the same technical hurdles
as many other potters in moving from salt to soda.
Sodium carbonate does not volatilize as easily as salt.
If introduced to the kiln as a solid, they tend to melt
into a lump in the firebox, then melt slowly.

I discovered that the dry mixture of sodium and calcium
carbonates could be mixed with water and made to set like plaster.
(wear gloves) This soda/calcium material contains chemically
combined water. It can be broken into small chunks,
which are then scooped into the kilns fireboxes
with a piece of angle iron.
As the mixture breaks down in the heat of the flame (cone 8-10)
water vapour is released along with the vapourizing soda.
The water vapour helps to carry the soda vapour through the kiln,
enabling good glaze distribution.
The calcium stays behind, giving the added benefit of
protecting the fireboxes from attack by the corrosive soda.

Soda Mix
Calcium carbonate............0.75 kg
Sodium Bicarbonate..........0.45 kg
Light Soda Ash.................0.30 kg
total 1.50 kg

Mix dry ingredients together; to a 1.5 kg batch
ad 800 millilitres water. Wearing rubber gloves,
stir the mixture as it becomes warm and sets,
then break-up into small pieces.

My research revealed that the silica/alumina ratio
and proportion of clay content are crucial factors:
bodies with a clay content of more than 80% and a=20
silica/alumina ratio of 3 or lower were most likely
to promote flashing and colour.

Not only does this high-alumina body promote
flashing and fire colour, it also has some unepected benefits.
Where it is heavily attacked by soda vapour, it developes a
thick crystaline mat glaze that seems to ooze out of the clay,
flowing glacially across the surface, inspiring visual imagery
of snow and ice. The thickness, colour and frostiness
of this high-alumina soda glaze can vary considerably,
offering a range of effects.

Some interesting pots came from a firing in torrential rain.
Water had seeped into the kiln floor.
The kiln insisted on a reducing atmosphere=20
during the soda introduction.
The draw tiles turned black.
The reduction and carbon blacks persisted
through the soak stage and into the cooling.
On opening the kiln, I found combinations of
grays, copper reds and blues, mauves and browns.

It was soon apparent that reducing during different stages
of the firing can alter colour and glaze quality so much
that the results are barely recognizable as having come from
the same source.
Reduction during the soak stage (following the soda introduction
at cone 8-10) produces some particularly strong colours,
but hinders the development of the thick frosty glaze.
Water vapour introduced during the firing stages has a
similar effect, but during cooling produces dry mat glazes
with densely crystallized surfaces.
Reduction during the cooling can bring out a range of colours,
from pale blues to strong reds, enhancing the surface of the frosty =
As a result, I have become a convert to reduced cooling.

I also discovered that, although my frosty matt glaze bears
resemblance to some wood-fired shinos, it is a different sort
of glaze and responds to kiln atmosphere in a very different way,
preferring oxidation during stages when most wood firers
would be heavily into reduction.
Viewing the frosty glaze under an 18x magnifier reveals
significant details.
As expected the mat surface is highly crystalline,
but it seems that is not the only cause of mattness and opacity.
This glaze is also highly crazed, to the extent that the spaces in the=20
fine overlapping craze networks actually approach crystalline size.
It appears to be a combination of crystals and layers of fine dense
crazing that gives the high-alumina soda glaze its mattness and
opacity, as well as the impression of thickness and depth.
The microstructure of this glaze and its method of formation
have become the subject of my ongoing Ph.D. research.

This version is heavily edited from its original foremat...
The photos of the pots were fabulous.
And the whole process sounds so absorbing...
Hence my desire to contribute to the thread.