Firing Methods and Results
the salt chamber of his large Noborigama. Many of the pots, like the
large urn to his left, have no glaze applied to them before firing. These pieces are entirely dependent on the sodium vapor for their
Sodium Vapor Glazing
Salt Glazed Pottery incorporates a unique
method of glaze application.
glazing is a process where pots are engulfed in a sea of sodium vapors and
fire. Salt is inserted into the kiln at 2000 degrees, releasing sodium which
acts as a flux on the silica in the pots. This action creates a unique
The entire process is called "vapor glazing"
The Pottery is actually glazed by sodium vapors
in the kiln, rather than by a coating of glaze minerals applied to the
surface of the pot before firing.
The amount of salt used, depends on the intent of the potter.
Adding a pound of salt per cubic foot of kiln volume
is usual for potters seeking deep texture.
Salt firing is a process where salt is introduced to
the kiln towards the end of a firing. The extreme heat volatilizes the salt
freeing the sodium which has a fluxing action on the surface of clay.
The technique dates to 15th century Germany. The discovery
of the process likely resulted from using salt soaked drift wood or brine encrusted staves of barrels as fuel in wood fired kilns.
Adding Salt to the Kiln
A fellow potter (right) adding salt to Robert's
earlier Noborigama. This method utilizes a length of angle iron filled
with salt that is slid into the firebox and dumped.
In his new larger Noborigama (show above) Robert
uses "salt burritos"
(packets of salt that are wrapped in newspaper), with the fire wood,
as the preferred method of adding salt.
Salt Texture & Effects
The rear of this pot faced away from the fire, in the
flame shadow or "leeward side"
. The resulting subdued glaze surface is referred
to as “onion skin”
The front of this pot was facing the source
of fire, which could be thought of as the “windward side"
and received a greater amount of salt glaze. The textured salt glaze pattern
is referred to as “orange peel”.
intense heat breaks the chemical bond
in salt, freeing sodium vapors which then react with silica in clay.
The resulting interaction
melts the surface of the pot, creating a unique texture.
The surface is
referred to as having a "onion skin" or
The glaze texture is determined by
the amount of salt,
temperature of the kiln, and composition
of the clay body.
|| "Onion Skin"
Wadding & Patterns
Bare areas, on the foot of
pots, shows where they were supported on "wadding" (a mixture of clay and alumina).
Wadding is also used in
Wood Firing. however
the process of
salt glazing requires a higher alumina content in the wadding.
Besides the aesthetic value of these marks, wadding prevents
the salt glazed pot from fusing to the kiln shelf
on which it is fired.
Cord Cuts & Wad Marks
Sodium (salt glazing), and fly
ash (wood firing), create glaze on the pots, and on the kiln shelves, on which
pots sit. Wadding prevents pottery, in a wood firing or salt glazing, from
fusing to the kiln shelves.
The size, shape and materials used for wadding,
are important aesthetic considerations for the potter. In addition to
preventing pots from fusing to shelves, wadding acts as a color resist and
leaves flame flashing patterns.
Cord Cut Shell Pattern
The unusual texture on the
underside of this pot results from the method Robert used to cut it free of
the potters wheel. Utilizing a twisted cord (one he made for this process)
Robert drags the cord under the freshly thrown pot as the wheel is slowing
rotating. This action separates the pot from the wheel head, and
produces a vibrant pattern.
Application of Wads
Used to prevent pots from fusing to kiln shelves when wood firing and
The receipt Robert uses for wadding is 60% Alumina Hydrate and 30%
Kaolin with coffee grounds mixed in, to suit the needs of each piece.
Coffee is added to open wadding. The
grounds burn out during firing, making the wads more friable (easier to break into pieces) for convenient
White glue makes sticking on wads both easy and secure. The
glue burns out in firing and has no effect on the pot or wads.
Application of Wads
The amount of material used in each wad is just one
consideration, when preparing pots for the kiln.
Locating one wad directly under the handle prevents the form
from warping when fired. Three wads are a stable number on small objects.
Choice of size, material, number and placement of wads, makes
a visual statement on the fired pot.
Flames, Ash & Salt Paths
Salt - Second Chamber
Path on Floor
Wood - First Chamber
Pattern on Floor
|Vapor Path in the Salt chamber
The pattern on the floor results from
sodium vapors glazing the bricks. There is a tendency for the gases to be pulled toward the center
of the chamber, as seen in the image above.
This unevenness can be countered with the design of the bag wall and
placement of pots and posts.
Note: The firebox is on the right of this photo and a diminishing amount
of glaze appears as it travels from right to left.
|Ash Pattern in the Wood chamber
When the first chamber of
Robert's Noborigama reaches temperature, stoking ceases in the main
firebox (at right). The second chamber (salt chamber- left)
continues to be fired until it also reaches
The result is that some cool ash is drawn out of the main firebox into the
Since the first chamber is no longer being stoked, the glazes stiffen
and any ash drifting onto the pots does not melt, leaving a dry
stone-like finish. A small number of pots in the firing will be re-fired
later, to melt the ash and improve the surface quality of the pots
The largest amount and size of cool ash is
nearest the bag wall. This only occurs on the floor of the kiln, pots
located on shelves above the floor are not affected.
Salt Glazing Effects
Also Known As
Salt that is introduced
to a firing, creates a unique glaze, but also acts as a flux on the
interior of the kiln itself, and all the kiln furniture.
Silicon carbide shelves, on which the pots are loaded, can and
will melt from this fluxing action. Salt reacting with the silica in
the shelves may
rain glass droplets, off the bottom of the shelves, onto the pottery
below, creating what are know as "potters tears" .
The effect may enhance a pot, and individuals who enjoy the wabi
sabi aspect of the process, will prize such pieces. However, droppers
falling on a the galley of a lid will ruin the pot.
The Potters Tear, visible on the piece shown to the right, added
an interesting element of deep green glaze, that made it unique to
this individual pot.
Additional Details on Processes
are fired in a Salt Atmospheres, any piece that comes into contact with
another pot could be fused together.
These occurrences are normally prevented by placing the pots carefully in
the kiln and using wadding. More about this is explained in the
Salt Kiln Deterioration
salt used in a kiln to make the unique glaze also acts as a flux on the
potters kiln and kiln furniture. The Kiln shelves above have been glazed and
eaten by the sodium and require cleaning before the next firing.
More about the actions of wood firing and salt glazing are described in
the section on
Wood and Salt Firing Details.