Wood Firing and Salt Glazing

To fuel my kiln with wood and to add salt were easy decisions for me. I have always been intrigued with, and admired and respected the mystical surfaces of pots decorated with ash and salt glazes.

Wood firing and salt glazing are ancient, traditional pottery techniques that date back for many centuries. Ash glazes were discovered by the ancient Chinese. As they fired their pots, wood ash from the fire swept through the kiln and settled onto the pots. In the intense heat of the kiln, the ash melted and was transformed into a glaze. The firebox of my kiln is stirred quite frequently in the early stages to send ash into the chamber where it is deposited on the pots. The melted ash produces a wonderfully rich surface.

Adding salt into the chamber during the firing most likely originated in the 14th century in Germany. The salt produces an actual glaze on the surface of the pottery. The salt decomposes and volatizes; and in the process, introduces sodium into the chamber’s atmosphere. The sodium bonds with the silica and aluminum oxide in the clay body of the pots to form a glazed surface. Salt glazed pots are recognized by their characteristic mottled pin hole surface, which is known as orange peel.

Making the Pots

Stoneware and porcelain pots are turned on a kick wheel in my studio. Literally as soon as my hands touch the clay I am thinking of the effects of the flame and the salt on each individual piece. Rims of serving bowls are left thick to catch ash. Forms are defined on the wheel and incised decorative lines are added to the pots to catch the ash as it enters the kiln and encourage natural ash glaze runs.

Many of the raw (unfired) pots are decorated with slips and shinos. These are mixtures of various clays diluted with water to the consistency of milk, which can be brushed onto the pots, or into which the pots can be dipped. The slips will react with the clay body in the kiln to produce different earthy colors, and help to give the pots a smoother surface. All wares are biscuit-fired in an electric kiln to remove all moisture from the clay body and to prepare them for the intense heat of wood fire kiln. After the biscuit, many of the pots are glazed - especially the forms which will later contain food. (This is not absolutely necessary, as unglazed pots or pots that are merely slipped are also safe for food.)

Packing the Kiln

There are many variables that determine how I pack my wood kiln. I consider which slips and glazes do well in the cooler spots of my kiln (the blues and greens) and which are more desirable in its hot spots (the reds, yellows and oranges). I visualize each piece as it may look when the firing is complete – which side should be facing the flame as it flows through the chamber and blushes or flashes the pot. I visualize the flame flowing through the chamber like the water flowing through the creek beside the kiln, and the pots are carefully positioned to create a flow pattern for the flame.

The Wood Kiln

My wood kiln was designed for me by my mentor and colleague, Bill van Gilder. Bill is a full time studio potter with more than thirty years experience who lives in nearby Gapland, Maryland. He studied pottery in England, Ireland and Africa, and apprenticed with the late Byron Temple. Currently Bill teaches, presents numerous workshops, hosts a ceramics show on television and is a columnist for Clay Times Magazine.

My wood fire kiln is a single chamber catenary arch kiln with an interior preheat fire box and an interior main fire box. The chamber is 45 cubic feet and was designed by Bill so I can stand in it. The preheat firebox is located on the left lower side of the kiln and has channels that run under the floor of the chamber. This kiln design is ideal for the potter who wood fires raw wares; but at the moment, to keep my firing time down to one I can manage, I continue to biscuit fire first.

The preheat firebox does not require the potter to stay up all night preheating the kiln. We start a fire in the preheat box the night before I fire, with very large hardwood logs. The hardwood burns in the preheat box for several hours and needs to be stoked only every few hours. The flames travel through the channels and into the main firebox. The heat spreads throughout the chamber, removing any moisture from the bricks and slowly preheating the pots.

The construction of my kiln took nearly a year to complete. The kiln was built by myself, Kirke Martin, Bill and many friends and family members. Kirke Martin, of Keedysville, Maryland, is a friend who produces sculptural and functional pottery in his own wood burning tube kiln that he fires for up to five days. Kirke was introduced to me by Welsh potter Micki Schloessingk. Kirke was a member of the team that built Micki's wood fire kiln and was also a student of the American potter John Glick.

Firing the Kiln

I start the main firebox very early in the morning. We have carefully stacked an assortment of dry wood, cut to four feet lengths, behind the kiln in the wood shelter. I like to start a firing with hardwood covered with lots of sappy bark, and switch over to pine later in the firing to get quicker heat rises. I do a moderate body reduction at 1700 degrees for three quarters of an hour. During the reduction the kiln is starved of oxygen. This causes a reaction with the iron oxide in the clay body and produces the reddish brown color underlying the glazes and slips.

Near the end of the firing, I begin adding salt into the chamber. I typically add eight pounds of plain table salt into my kiln - a few pounds at a time - over a period of time. Salt is poured onto slabs of wood that are thrown into the main firebox; or is added into the firebox through a salt port (an opening in the rear of the kiln). I continue to stoke the kiln until the kiln reaches nearly 2400 degrees Fahrenheit. At this point we have usually been firing for sixteen hours (not counting the overnight preheat) and the kiln has consumed nearly a cord of wood.

In Closing

The process of wood firing has a certain rhythm to which I am attracted. I fire the wood kiln every five or six weeks. The firing involves a phase of preparation in which wood is collected and stacked to dry. Then there is the phase of making pots that will be slipped, glazed and decorated. Packing the wares into the kiln and the firing follow; and finally, the kiln cools and the door comes down! The kiln is opened, I inspect the work, and a period of rest follows. Each time the door comes down, I am encouraged to venture into new areas of investigation. Each firing is a learning experience. There are still, and will continue to be, new avenues for my pursuit of good wood fired pots. I contemplate the addition of soda instead of salt, the interactions between clay body, slips and glazes, new glaze and slip recipes, quantity of wood ash, type of wood, atmosphere and temperatures within the chamber when I fire, and so on…

…and always the wondering at my kinship with the forces of nature.