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Robert Compton Pottery
Christine Homer Weaving

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Firing Methods and Results

 Salt Glazing


 

Robert loading 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 surface effects.

Vapor Glazing

Sodium Vapor Glazing

Salt Glazed Pottery incorporates a unique method of glaze application.

Salt 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 glaze.
 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.

 

Vapor Glazing

 

Discovery and Process

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

Leeward
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”
Windward

 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”.
 

Salt Texture

Salt Texture

The 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 "orange peel" texture.

 The glaze texture is determined by
the amount of salt,
temperature of the kiln, and composition
of the clay body. 

 "Orange Peel"   "Onion Skin"

Wadding & Patterns

Wadding

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

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

Wadding

Used to prevent pots from fusing to kiln shelves when wood firing and salt glazing.

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 Grounds
Coffee is added to open wadding.  The grounds burn out during firing, making the wads more friable (easier to break into pieces) for convenient removal after the firing.
  Adding Wads
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

Size
The amount of material used in each wad is just one consideration, when preparing pots for the kiln.
Placement
Locating one wad directly under the handle prevents the form from warping when fired.  Three wads are a stable number on small objects.
 Pattern
Choice of size, material, number and placement of wads, makes a visual statement on the fired pot.

Flames, Ash & Salt Paths

Salt - Second Chamber
"Sodium"
Path on Floor
 
Wood - First Chamber
 "Fly Ash"
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 temperature.
 The result is that some cool ash is drawn out of the main firebox into the first chamber.
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 affected.
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
Potters Tears

Droppers
Also Known As
 Potters Tears

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

    

Kissing Pots

When pots 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 section on Wadding Mishaps.
 

Salt Kiln Deterioration

The 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.

 

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