Imagine pulling pottery from a kiln, not coated in a pre-applied, uniform glassy layer, but kissed by fire and chemical vapor, resulting in surfaces alive with texture, flashing, and unpredictable beauty. This is the realm of atmospheric firing, specifically vapor glazing techniques like salt glazing and soda firing. Unlike conventional glazing where a liquid suspension is applied before firing, vapor glazing introduces the glaze-forming elements into the kiln’s atmosphere at peak temperatures. These vapors then circulate and react directly with the clay surfaces, creating integrated, often directional, and uniquely textured effects that are impossible to replicate precisely.
A Glimpse into History: The Origins of Salt Glazing
Salt glazing boasts a rich history, emerging quite serendipitously in the Rhine River Valley of Germany sometime during the 14th or 15th century. The story often told involves potters throwing common salt (sodium chloride) into their wood-fired kilns towards the end of the firing cycle. Why? Perhaps initially to clear fly ash, or maybe noticing the effects of salt-rich wood or seawater spray near coastal kilns. Whatever the exact trigger, they observed that the intense heat vaporized the salt. The resulting sodium vapor reacted chemically with the silica and alumina present in the clay bodies, forming a thin, hard, translucent sodium-silicate glaze directly on the pottery surface. This discovery revolutionized stoneware production.
Early German salt-glazed stoneware, often decorated with cobalt blue, became highly prized and widely exported. Think of the iconic Westerwald pottery or Bellarmine jugs. The technique migrated to England by the late 17th century, most notably taking root in Staffordshire. Here, it fueled the production of vast quantities of domestic ware, from tankards and dishes to intricate figurines. Its durability and impervious nature also made it ideal for industrial applications like drainage pipes and chemical storage vessels well into the 19th and 20th centuries. The characteristic surface is often described as having an “orange peel” texture, a result of the molten glaze pitting slightly as it forms. The color can range from warm browns and grays to tans, heavily influenced by the clay body composition and the kiln’s atmosphere – typically a reduction (oxygen-starved) atmosphere enhances warmer tones.
The industrial use of salt glazing eventually waned due to environmental concerns, specifically the release of hydrochloric acid gas as a byproduct of the sodium chloride reaction. However, the technique experienced a significant revival within the studio pottery movement in the mid-to-late 20th century. Artists were drawn to its unique aesthetic potential, the direct interaction between flame and surface, and the historical resonance of the process.
Understanding the Salt Glaze Process
Achieving a successful salt glaze requires specific conditions. The kiln must reach stoneware temperatures, typically around Cone 8 to Cone 10 (approximately 1260-1300°C or 2300-2380°F). At this point, common rock salt or table salt is introduced into the kiln, usually through specific ports. The intense heat instantly vaporizes the sodium chloride (NaCl).
The crucial chemical reaction then occurs within the kiln atmosphere. The sodium vapor (Na) reacts with water vapor (H2O) present from combustion or the clay itself, and importantly, with the silica (SiO2) and alumina (Al2O3) in the clay body surfaces. This forms a durable sodium-alumino-silicate glass directly bonded to the clay. The chlorine component typically combines with hydrogen to form hydrochloric acid (HCl) gas, which exits through the chimney – the primary environmental drawback of traditional salt firing.
Key characteristics of salt glazing include:
- Orange Peel Texture: The hallmark pitted surface, varying in intensity depending on the amount of salt used and firing conditions.
- Directional Effects: Glaze thickness often varies, being heavier on surfaces directly facing the flame paths and salt introduction points.
- Clay Body Interaction: The final color and texture are highly dependent on the underlying clay’s composition. Higher silica content generally promotes better glaze formation.
- Atmospheric Influence: Firing in reduction (limited oxygen) tends to produce warmer, richer browns, grays, and tans, while oxidation (ample oxygen) can yield cooler tones.
- Wadding Marks: Pots must be separated from kiln shelves using wadding (small balls or pads of refractory clay mixed with alumina hydrate) to prevent them from being fused by the glaze. These wads leave distinct marks, becoming part of the piece’s visual record of the firing.
Soda Firing: A Cleaner Alternative Emerges
Concerns over the HCl emissions from salt kilns, coupled with a desire for different but related atmospheric effects, spurred experimentation in the latter half of the 20th century. Potters began exploring sodium carbonate (soda ash, Na2CO3) and sodium bicarbonate (baking soda, NaHCO3) as alternative sources of sodium vapor. This practice became known as soda firing.
While related to salt glazing in that it relies on sodium vapor reacting with silica, soda firing offers distinct characteristics and is generally considered more environmentally benign, as the primary gaseous byproduct is carbon dioxide, not hydrochloric acid. Pioneers like Gail Nichols and Ruthanne Tudball, among many others, extensively explored and documented soda firing techniques, pushing its aesthetic boundaries.
The Nuances of Soda Firing
Introducing soda into a hot kiln requires different methods than tossing in rock salt. Common techniques include:
- Spraying: Dissolving soda ash or baking soda in hot water and spraying this solution into the kiln through ports using a garden sprayer or purpose-built injector. This allows for more controlled application.
- Introducing Powder/Paste: Mixing soda with water into a paste or introducing the powder directly, often wrapped in paper or placed in combustible containers, into the firebox or kiln chamber.
- Using Resist: Applying wax or other resists to pots can block the soda vapor, creating contrasting unglazed areas.
Soda firing often results in surfaces that can be smoother than traditional salt glaze, although texture can still be built up with heavy application. The effects are highly dependent on the method of introduction, the kiln atmosphere, placement of the pots, and the clay and slips used. Soda vapor is generally considered less mobile than salt vapor, leading to more pronounced directional effects – often called “flashing.” This refers to the blushing or coloration that occurs where the vapor path directly impinges on the pot surface. Colors can range dramatically, from bright oranges and yellows to subtle grays, greens, and browns, often with a softer, more satin sheen compared to the sometimes glassier salt glaze.
Verified Fact: Both salt (NaCl) and soda (Na2CO3/NaHCO3) firing rely on sodium vapor interacting with silica and alumina in the clay at high temperatures. However, the source chemical dictates the byproducts and influences the vapor’s behaviour. Salt produces HCl gas, while soda primarily releases CO2 and water vapor, making it generally less corrosive to kiln refractories and less problematic environmentally, though proper ventilation is still essential.
Vapor Glazing: Unifying Concepts
Salt and soda firing fall under the umbrella term vapor glazing. This encompasses any ceramic firing process where glaze-forming materials are introduced into the kiln atmosphere as vapor, rather than being applied directly to the pot surface beforehand. The defining characteristic is this atmospheric interaction. While salt and soda are the most deliberate and common forms, the effects of wood firing can also be considered a type of vapor glazing. Fly ash from the burning wood, rich in silica, alumina, and fluxing oxides like potassium and calcium, circulates in the kiln. This ash settles on the wares, melts at high temperatures, and reacts with the clay body to form natural ash glazes, often producing beautifully varied and textured surfaces. The principles are similar – materials becoming airborne at temperature and reacting with the clay.
Surface, Texture, and the Artist’s Hand
The allure of salt and soda firing for many ceramic artists lies in the unique surface qualities achieved. The “orange peel” of salt, the directional flashing of soda, the subtle variations in color and sheen – these are effects intimately tied to the firing process itself. They speak of the journey through fire.
Comparing Salt and Soda Effects
While both use sodium, the results differ:
- Texture: Salt often yields a more pronounced, uniformly distributed pebbly texture. Soda can range from smooth satin surfaces to heavily textured areas where the spray or paste was concentrated.
- Flashing: Soda is renowned for its dramatic flashing – intense coloration on one side fading to a different quality on the other. Salt flashing occurs but can sometimes be more subtle or integrated into the overall texture.
- Color Palette: Both depend heavily on clay and slips, but soda is perhaps known for potentially brighter oranges and yellows under the right conditions, while salt often yields classic warm grays, browns, and tans.
The choice of clay body is paramount. Clays rich in silica are essential for good vapor glaze development. Artists often apply slips (liquid clay coatings, sometimes with added colorants like iron oxide or rutile) to enhance or modify the effects. A porcelain slip under a salt or soda atmosphere might yield cool grays or blues, while an iron-bearing slip can produce rich browns, oranges, or reds depending on the atmosphere and sodium exposure.
Loading the kiln becomes a critical part of the creative process. Artists carefully consider the flame path and where the vapor will be introduced, placing pieces to deliberately catch or shield them from the sodium vapor. The small marks left by the wadding used to separate pots are accepted, even celebrated, as evidence of the process – the ‘kiln’s kiss’. There’s an inherent element of unpredictability, a collaboration between the artist’s intent and the kiln’s personality, that many find exciting.
Contemporary Vapor Glazing
Today, salt and soda firing remain vibrant practices within contemporary ceramics. Artists working in functional pottery value the durable, unique surfaces for everyday objects, bringing a tactile connection to the process into the user’s hands. Sculptural ceramicists utilize the techniques to create complex, evocative surfaces that enhance form and convey meaning. Workshops and university programs continue to teach these methods, ensuring their survival despite the technical challenges and labor-intensive nature.
Modern practitioners often combine vapor glazing with other techniques, experiment with different soda introduction methods, diverse clay bodies, and sophisticated slip formulations to expand the aesthetic possibilities. They might build specialized kilns designed to optimize vapor circulation or use specific wood types in conjunction with salt or soda to layer effects. The dialogue between historical precedent and contemporary innovation continues to enrich the field.
Ultimately, salt glazing and soda firing represent a profound connection between material, process, and result. They demand skill, patience, and an acceptance of the unpredictable from the artist. The resulting ceramic pieces bear the direct imprint of the fire and vapor, offering surfaces imbued with a depth, texture, and liveliness that pre-applied glazes rarely achieve. It is this tangible record of transformation, this marriage of chemistry and artistry, that ensures the enduring appeal of vapor glazing in the world of ceramics.
