Salt Glazed Jug
Salt Glazed Jug

Vapor Glazing

Salt Glazed pottery incorporates a unique method of glaze application.   Instead of the potter applying a coating of glaze minerals on to the surface of the pot before firing, during the salt glazing process pots are engulfed in a sea of sodium vapors and fire.  When salt is added into the kiln at 2000 degrees, releasing sodium which acts as a flux on the silica in the pots, the vapors melt the surface of the pottery.  The process is called “vapor glazing”

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 using a piece of angle iron.
Adding salt to the kiln using a piece of angle iron.

Adding Salt to the Kiln

This photo shows a fellow potter 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 Robert adds salt by wrapping it in newspaper creating “salt burritos,” which are added with the wood into the fire box,  as his preferred method.   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 or what is known as an “orange peel” surface.

Applying wadding to a pot before firing.
Applying wadding to a pot before firing.

Wadding

The sodium and fly ash from the wood, create a glaze on the pots, and the kiln shelves, where the pots sit.  Wadding prevents the 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.

After the firing you can see the marks left by the wadding.
After the firing you can see the marks left by the wadding.

In addition to preventing pots from fusing to shelves, wadding acts as a color resist and leaves flame flashing patterns.
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, 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.

 

Unloading the salt chamber you can see how the right side is closer to the fire box and there is less glaze affect on the pots located on the left side of the chamber.
Unloading the salt chamber you can see how the right side is closer to the fire box and there is less glaze affect on the pots located on the left side of the chamber.

After the Firing

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.  The firebox is on the right of this photo and a diminishing amount of glaze appears as it travels from right to left.

Potter's tear is visible on the side of this pot.
Potter’s tear is visible on the side of this pot.

Potter’s 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 this piece, adds an interesting element of deep green glaze, that made it a unique pot.

After the salt firing a silicone carbide shelf looks like this.
After the salt firing a silicone carbide shelf looks like this.

Deterioration to the Kiln & Shelves

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.