When did soda firings replace salt firings, and why? Take a closer look at the history, fired characteristics, and practical considerations of both firing styles for contemporary studio practices.
Defining the Terms
Crazing: A series of fine lines in the fired ware caused by the glaze being under tension when cooling.
Draw Ring: A clay ring pulled out of the kiln as the sodium compound is introduced. In stages, rings are withdrawn, indicating the buildup of sodium on clay body surfaces.
Liner Glaze: A glaze designed to coat the inner surfaces of functional pottery. Liner glazes must have a smooth surface for cleaning, a durable finish, and be free of any surface defects.
Maturing Range: The action of time and temperature upon a clay body or glaze to develop the required physical properties for vitrification.
Orange Peel: The pebble-like surface on clay formed when sodium vapor reacts with the alumina and silica in a clay body.
Salt Firing: The introduction of sodium chloride (salt) into a kiln, reacting with the exposed clay to form a sodium/alumina/silicate glaze. Hydrochloric acid and chlorine gas are released into the atmosphere as byproducts.
Soda Firing: The introduction of sodium carbonate or sodium bicarbonate into a kiln, reacting with the exposed clay to form a sodium/ alumina/silicate glaze. The byproduct releases carbon dioxide and water vapor into the atmosphere.
Target Brick: A refractory placed in the kiln to disperse a sprayed water/ sodium carbonate solution evenly during the firing.
Salt-Firing History
Present-day soda firing is derived from salt firing, as both use a sodium compound with somewhat different results. Salt firing is an efficient method of glazing pottery originating in the 12th century in the Rhineland region of Germany.¹ Salt-glazed pottery was produced in England during the 17th to 18th centuries. During this time, the process migrated to the northeast and southeast regions of the US. Traditionally, sodium chloride (NaCl)—table salt—was thrown into the firing kiln to produce a sodium vapor that reacted with the clay and glazed the pottery. Past and current ware is characterized by a gray to brown fired color with distinctive cobalt oxide decoration or marking resulting in a blue color.
Salt-Firing Process
Temperatures near or above cone 6 (2232°F (1222°C)) are needed for salt (sodium chloride) to separate into sodium oxide and hydrogen chloride in the kiln. However, the glaze effect is dependent on the vitreous quality of the clay body. When introducing salt into the kiln, sodium vapor is forcefully released and forms with alumina and silica in the clay body. The sodium/alumina/silicate combination produces the distinctive orange-peel surface texture, but immature clay bodies will not promote this effect. Care must be taken during the salting process that only sodium vapor reaches the pottery and not the actual salt. Areas that are exposed to thick layers of salt-fired vapor can show crazing and discoloration. When salt enters the kiln, its vapor has a greater intensity than sodium carbonate does in soda-fired kilns. Interior surfaces of bowls, cups, and containers are less likely to receive the vapor, leaving dry areas of exposed clay.
The byproducts of salt firing are chlorine gas and hydrochloride gas, which travel up the kiln stack as they meet moisture in the atmosphere. Anyone who has observed a salt firing has seen the white cloud that covers the immediate area with sodium chloride and hydrochloride vapors. These pollutants directly, and quite dramatically, affect the bricks and metals associated with the salt kiln in an adverse way, and, in a more subtle fashion, also affect the local ecology. For these reasons, many salt kilns have been shut down in urban settings or areas that have stringent pollution laws. Expensive anti-pollution devices can be installed, but they are generally not practical for the studio potter.
Soda-Firing Process
One of the first places to research and document soda-vapor firing was New York State’s College of Ceramics at Alfred University from 1971 through 1974.2 A three-year study was initiated comparing the effects of salt and sodium carbonate on clay body and glaze formulas, after which soda-firing kilns became part of the ongoing ceramics program.
There are over 255 sodium-based compounds. However, only sodium chloride, sodium carbonate, and sodium bicarbonate are suitable for vapor firing. The other sodium compounds are either too expensive, have undesirable handling components, or are dangerous when introduced into a kiln.
Sodium bicarbonate—baking soda (NaHC03)—decomposes in the kiln at 1562°F (850°C) to form sodium carbonate and soda ash (Na2CO3). Both sodium compounds release water and carbon dioxide and can be used interchangeably with no difference in the glaze result.
Both sodium carbonate and sodium bicarbonate can be substituted for salt. However, consideration must be given to creating a more turbulent kiln atmosphere by moving the damper in and out during the introduction of either the sodium carbonate or sodium bicarbonate, which will increase the movement of soda vapor in the kiln. Spraying a water/sodium carbonate or bicarbonate solution into the kiln onto a target brick is an efficient option for dispersing the material. Care must be taken that the sprayed solution does not directly hit the pots, resulting in over-glazed rough areas on the clay body and glaze surface. While sodium carbonate or bicarbonate will not release toxic materials into the atmosphere, the sodium vapor can land on kiln metal and brickwork, causing corrosion.
Ideally, a better method of dispersion would be to introduce sodium carbonate on an angle iron repeatedly in small amounts at the highest viewing port directly over the firebox, thus allowing for maximum volatilization of the material. A less effective method is making small packets of the material and throwing them into the kiln. In both salt and soda firing, the dispersion of the material in small quantities over time allows for maximum distribution of the vapor. This guideline is especially important as sodium carbonate and sodium bicarbonate both separate at lower vapor pressures and can reduce complete coverage of the pots as compared to sodium chloride. With kiln design and dispersing sodium carbonate in small quantities over the fire box, comparable vapor coverage equals sodium chloride firings. While this flashing effect, orange-peel regions with random drier surface areas might be aesthetically pleasing, it was not the goal of the Alfred study.
Soda/Salt Interesting Facts
Pyrometric cones melt prematurely in salt or soda firings as the sodium atmosphere acts as a flux. Shielding the cones from the sodium vapor is recommended.
Pyrometer probe readings can be inaccurate due to the repeated sodium kiln atmosphere.3
Once sodium carbonate or sodium chloride is introduced into the kiln, it can settle into the bricks in the walls. In subsequent firings, residual sodium can vaporize from the kiln walls. This re-volatilization of sodium is often observed when pulling the first draw rings before salting is introduced.
The atmosphere created by salt or soda firings can cause defects such as dry areas in glazes and clay-body formulas, which contain any form of MgO, found in talc, dolomite, or magnesium carbonate.
Colors appear brighter in soda firings, possibly due to the formation of the glaze in the absence of chlorine gas and hydrochloric acid during the firing.
Rock salt (sodium chloride), due to its large particle size, will produce larger orange-peel textures as opposed to finer-grind table salt.
Silica sand at 60 mesh in the clay body (2% to 8%) will increase orange-peel textures.
Clay-body formulas with high levels of alumina and silica are more reactive to any kind of sodium vapor.
Small test draw rings of the same clay body can be placed throughout the kiln. After the introduction of salt or soda, they can be pulled out periodically during the firing to determine the amount of orange-peel coverage. Significantly, the interior surface of the ring will indicate an accurate indication of vapor coverage on the pottery. Draw rings can show a slightly glazed surface due to the buildup of sodium on interior kiln brick walls from past vapor firings.
The introduction of too much salt or sodium carbonate can act as a strong flux when in contact with glazed and unglazed surfaces. An excessive buildup of sodium carbonate or sodium chloride within the lower part of the kiln and firebox can cause the fluid material to contract upon cooling, dislodging bricks. A vapor-firing kiln is like a large glass tank and is subject to the same contraction forces when cooling.
the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
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When did soda firings replace salt firings, and why? Take a closer look at the history, fired characteristics, and practical considerations of both firing styles for contemporary studio practices.
Defining the Terms
Crazing: A series of fine lines in the fired ware caused by the glaze being under tension when cooling.
Draw Ring: A clay ring pulled out of the kiln as the sodium compound is introduced. In stages, rings are withdrawn, indicating the buildup of sodium on clay body surfaces.
Liner Glaze: A glaze designed to coat the inner surfaces of functional pottery. Liner glazes must have a smooth surface for cleaning, a durable finish, and be free of any surface defects.
Maturing Range: The action of time and temperature upon a clay body or glaze to develop the required physical properties for vitrification.
Orange Peel: The pebble-like surface on clay formed when sodium vapor reacts with the alumina and silica in a clay body.
Salt Firing: The introduction of sodium chloride (salt) into a kiln, reacting with the exposed clay to form a sodium/alumina/silicate glaze. Hydrochloric acid and chlorine gas are released into the atmosphere as byproducts.
Soda Firing: The introduction of sodium carbonate or sodium bicarbonate into a kiln, reacting with the exposed clay to form a sodium/ alumina/silicate glaze. The byproduct releases carbon dioxide and water vapor into the atmosphere.
Target Brick: A refractory placed in the kiln to disperse a sprayed water/ sodium carbonate solution evenly during the firing.
Salt-Firing History
Present-day soda firing is derived from salt firing, as both use a sodium compound with somewhat different results. Salt firing is an efficient method of glazing pottery originating in the 12th century in the Rhineland region of Germany.¹ Salt-glazed pottery was produced in England during the 17th to 18th centuries. During this time, the process migrated to the northeast and southeast regions of the US. Traditionally, sodium chloride (NaCl)—table salt—was thrown into the firing kiln to produce a sodium vapor that reacted with the clay and glazed the pottery. Past and current ware is characterized by a gray to brown fired color with distinctive cobalt oxide decoration or marking resulting in a blue color.
Salt-Firing Process
Temperatures near or above cone 6 (2232°F (1222°C)) are needed for salt (sodium chloride) to separate into sodium oxide and hydrogen chloride in the kiln. However, the glaze effect is dependent on the vitreous quality of the clay body. When introducing salt into the kiln, sodium vapor is forcefully released and forms with alumina and silica in the clay body. The sodium/alumina/silicate combination produces the distinctive orange-peel surface texture, but immature clay bodies will not promote this effect. Care must be taken during the salting process that only sodium vapor reaches the pottery and not the actual salt. Areas that are exposed to thick layers of salt-fired vapor can show crazing and discoloration. When salt enters the kiln, its vapor has a greater intensity than sodium carbonate does in soda-fired kilns. Interior surfaces of bowls, cups, and containers are less likely to receive the vapor, leaving dry areas of exposed clay.
The byproducts of salt firing are chlorine gas and hydrochloride gas, which travel up the kiln stack as they meet moisture in the atmosphere. Anyone who has observed a salt firing has seen the white cloud that covers the immediate area with sodium chloride and hydrochloride vapors. These pollutants directly, and quite dramatically, affect the bricks and metals associated with the salt kiln in an adverse way, and, in a more subtle fashion, also affect the local ecology. For these reasons, many salt kilns have been shut down in urban settings or areas that have stringent pollution laws. Expensive anti-pollution devices can be installed, but they are generally not practical for the studio potter.
Soda-Firing Process
One of the first places to research and document soda-vapor firing was New York State’s College of Ceramics at Alfred University from 1971 through 1974.2 A three-year study was initiated comparing the effects of salt and sodium carbonate on clay body and glaze formulas, after which soda-firing kilns became part of the ongoing ceramics program.
There are over 255 sodium-based compounds. However, only sodium chloride, sodium carbonate, and sodium bicarbonate are suitable for vapor firing. The other sodium compounds are either too expensive, have undesirable handling components, or are dangerous when introduced into a kiln.
Sodium bicarbonate—baking soda (NaHC03)—decomposes in the kiln at 1562°F (850°C) to form sodium carbonate and soda ash (Na2CO3). Both sodium compounds release water and carbon dioxide and can be used interchangeably with no difference in the glaze result.
Both sodium carbonate and sodium bicarbonate can be substituted for salt. However, consideration must be given to creating a more turbulent kiln atmosphere by moving the damper in and out during the introduction of either the sodium carbonate or sodium bicarbonate, which will increase the movement of soda vapor in the kiln. Spraying a water/sodium carbonate or bicarbonate solution into the kiln onto a target brick is an efficient option for dispersing the material. Care must be taken that the sprayed solution does not directly hit the pots, resulting in over-glazed rough areas on the clay body and glaze surface. While sodium carbonate or bicarbonate will not release toxic materials into the atmosphere, the sodium vapor can land on kiln metal and brickwork, causing corrosion.
Ideally, a better method of dispersion would be to introduce sodium carbonate on an angle iron repeatedly in small amounts at the highest viewing port directly over the firebox, thus allowing for maximum volatilization of the material. A less effective method is making small packets of the material and throwing them into the kiln. In both salt and soda firing, the dispersion of the material in small quantities over time allows for maximum distribution of the vapor. This guideline is especially important as sodium carbonate and sodium bicarbonate both separate at lower vapor pressures and can reduce complete coverage of the pots as compared to sodium chloride. With kiln design and dispersing sodium carbonate in small quantities over the fire box, comparable vapor coverage equals sodium chloride firings. While this flashing effect, orange-peel regions with random drier surface areas might be aesthetically pleasing, it was not the goal of the Alfred study.
Soda/Salt Interesting Facts
the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
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