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Why soda glaze?

Maryke Henderson August 2006

Maryke Henderson graduated from the Australian National University School of Art in 2005. This is an excerpt from her Bachelor of Arts (Honours) research report.

Image of work by Maryke Henderson
Maryke Henderson Dialogue 2, 2005.
Stoneware, soda fired

With so many variables possible, every soda firing has the potential for experiment and can offer a wealth of new ideas and techniques. With the formation of glaze dependent upon the movement of vapour through the kiln, heavier glaze deposits form on the more exposed surfaces and only a blush of colour forms on a protected areas. (With conventional glazing techniques the glaze distribution does not rely upon this movement of vapours through the kiln chamber.)

This process makes soda glazing captivating and unpredictable. The vapours leave a mark showing where they have moved through the kiln and passed over the clay. Chance plays an important part in the process and the final outcome of the surface. These occurrences are an open field for exploitation rather than dictators of preconceived effects.

The unpredictable painting with fire on the surface over the controlled mark making and construction develops a dimension of tension between the organic and contrived. The works provide a formal canvas to explore the surface and to set up conditions for effects that can never be precisely repeated.

I have embraced the challenge to use the traditional techniques of throwing and vapour glazing to develop functional forms into a contemporary mode. I have done this through the development of surface and exploration of form. This has resulted in the integration of form, decoration and fired surface to create a lively personal statement. Skip to Maryke's work


Why soda glaze?

It captivates me with its unpredictability.

It offers the excitement of continual discovery with a continual feedback in the form of the unexpected results, which lead to possibilities for further development. Soda glazing reveals the material of the making, the soft plastic clay showing its responsiveness to the glaze surface. The organic surfaces produced create a tension with the formal. These surfaces have the impression of ageing, of layering and natural structures, and these formulate this tension with the structured forms. The forms still are functional, which is important for me as it allows an intimate relationship- maker to pot, pot to user while maintaining a visual dimension.


Image of work by Jane Hamlyn
Background - Artist - Jane Hamlyn. Jugs and teapot, salt-glaze with distinct orange peel surface

Vapour glazing has a long tradition in the history of world ceramics. The process depends upon materials volatilising at differing temperatures and interacting with suitable compounds in the clay body to produce a surface. Wood firers use the ash and alkaline vapours created from their fuel to form a glazed surface. Clay objects can be placed in an enclosed space (saggar) with combustibles, metals and salts which volatilise with heat and react with the clay to produce a decorative surface to the object.

Common salt (sodium chloride) is used to create a salt glazed surface. At temperatures of around 800°C salt will begin to vaporise leaving a surface blush of colour on the clay body. At this temperature about 7 to 8 percent salt is utilized per hour, however the addition of borax will assist the process of salt glazing. As the temperature increases the salt becomes an active vapour, forming a typical orange-peel surface texture depending upon the composition of the clay body.

Historical context

Salt glazing is thought to have originated in Germany's Rhineland from between the 12th century to the 15th century. The lower fired lead and tin glazes were fragile and chipped readily. There was a need for a more robust method of making everyday functional ware, however to fire the clay to a higher temperature, making it more durable meant that the lead and tin glazes would run and be unacceptable. Another glaze had to be found. There has been much speculation as to how the throwing of salt into a kiln to form a glaze came about. It has been suggested that barrels used to store food in brine or that driftwood from the sea may have been used in the latter part of the firing, thus forming a glazed surface. Ruthanne Tudball speculates that vapour glazing may have been brought into Germany around the 13th century by the return of the Crusaders from Egypt with information on the making of Egyptian faience, (which was made around 4500 BC)- faience being dependent on alkaline soda for fluxing.

Hamer writes of salt being placed on a tile to judge the kiln temperature during firing, melting at 800°C, other sources suggest 900°C which leads me to consider the possibility that an over-fired kiln may have produced blushes of colour or some glaze formation from the volatilised salt.

The colour and nature of traditional salt glazed ware in Germany depended upon the clay body or the application of clay slips to the clay object. Colours varied from browns, greys to creamy whites and yellows depending on the source of clay and firing method. Cobalt brushwork was also used for decoration. The glaze was hard and durable and did not require a first (or biscuit) firing making this an economical and useful method of production.

In the late 17th century salt glazing spread to London from Germany where Royal Doulton used the technique industrially for the production of utilitarian items, pitchers and plain figured bottles. John Doulton contributed greatly to the control of the cholera epidemics of 1832 and 1864 by the production of salt glazed sewer pipes which removed contaminated sewerage.

Industry benefited considerably from the development of salt glazing. The glazed surface is very hard and chemically resistant making it extremely useful in the production of acid containers, sewer pipes, and roof tiles. As mentioned previously, the work did not require two firings and therefore became a more economical method of production. This lead to the development of a manufactured commodity that was replaceable. It's spread was rapid and in Europe, wherever there were stoneware clays to make pots along with coal or wood as a heat source, salt-glaze could be found.

In the USA the same need for durability and inert surfaces lead to the introduction of the salt glazing technique. In 1785, public attention was drawn to the possible dangers of lead, which made salt glazed stoneware an attractive safe alternative. It was used mainly for plain domestic ware and was not associated with expressive or gestural surfaces. With the introduction and development of stoneware glazes from Japan, salt glaze became a forgotten technique for the production of domestic ware as stated by Glenn Nelson in his 1960 edition of Ceramics.

Image of work by Jane Hamlyn
Contemporary context - Artist - Don Rietz. Shirted vase, salt-glazed stoneware, 33cm

As soon as clays were discovered in Australia and there were pioneers with the skills to make and fire ware, salt glazing was used. Immigrants from Staffordshire, England, played an important role in the development of this salt glazed stoneware. It is interesting to note that the early German immigrants to South Australia known as Töpfer or potmakers did not produce salt glazed ware as did their compatriots in Germany. They made lead glazed earthenware in the tradition inherited from when the Romans occupied the Rhine Valley. Potteries were established at Lithgow, Sydney, Brisbane, Bendigo and South Australia where coal and clay were plentiful. Chimney pots, pipes and tiles along with practical functional ware were produced.

Relative to the thousands of years of ceramic history in Japan, salt glazing was a more recent development. Early pieces from the 19th century have been found, most probably inspired by those made in Germany. Hamada Shoji's interest in salt glaze during the 1950's encouraged other potters to take up the practice. In industry the use of salt glaze waned, not only because of pollution issues associated with the firing, but because of the development of alternative and cheaper products such as plastics in the production of drainage pipes.

Contemporary context

In the USA Paul Soldner began experimenting with salt vapours around 1965 and Don Reitz lead a movement for the reintroduction of more conventional high temperature salt firing. He was drawn to vapour glazing, as it does not obscure the marks of the surface, as do opaque glazes. In his desire for colour, Reitz would paint cobalt on a brick to flash colour onto his pots. His work transformed from the domestic. He took a traditional technique and reinvented it by the unorthodox application of coloured slips and oxides on contemporary forms and using a variety of light and heavy glazed surfaces.

Contemporary salt glaze practitioners

Walter Keeler (UK)

Walter Keeler's forms show a strong industrial influence with geometric forms comprising hard edges, articulation and assemblage. The glaze surface has an even coverage of strong orange-peel texture without any blemishes or flashings. He uses a china clay slip to give a smooth surface (with less orange peel texture) and where he applies oxides over the clay body he will obtain a heavily textured surface resulting in strong contrasts. There is a strong element of control over both form and surface unlike that of Patrick Sargent.

Patrick Sargent (UK/ Switzerland)

The creation of looseness and freedom in his forms and surfaces are important to Patrick Sargent. He used salt more as an enhancer, adding it to the later stages of his wood-firing. This left surfaces with blemishes from shell wads, wood ash and uneven salt glaze in the places where the pots are protected from the flame path. His work stands in marked contract to the more formalised forms and controlled surfaces of Walter Keeler.

Janet Mansfield (Australia)

The salt-glazed pots made by this Australian artist are strong, robust in form and covered by a generous, rich glazed surface. She wants to see 'the evidence of her ideas and the full revelation of the process of salt glaze,' and the pots to 'depict all the turbulent forces of the firing and look as though they have survived the heat and salt vapours and been enriched by them' Mansfield considers the position of her forms in relation to the flame path and the placement of wadding as an integral element of decoration. Heavily incised lines and the shapes and patterns of her handles catch the salt vapours, becoming prominent with the formation of heavy salt glaze deposits. Mansfield and Sargent both wood fire yet create totally different surfaces by the timing and quantity of salt introduction to the kiln.

David Miller (UK)

Unlike the heavy salt glazed surfaces of Janet Mansfield's pots, David Miller uses coloured slips and oxides to decorate his teapots and low fires with a minimum of salt to just 'kiss' and seal the surface leaving the original decoration unblemished. He uses traditional teapot forms to inspire his contemporary interpretations.

Contemporary soda glaze practitioners

More recently, experimentation with alternatives to salt has resulted in a new school of vapour glaze.

Image of work by Ruthanne Tudball
Ruthanne Tudball

Tudball played an important pioneer role in the development of soda glaze. Unable to use salt glaze because of pollution restrictions while she was attending university, she researched alternatives to introducing sodium to the kiln. She carried out further research and wrote of her findings in Ceramic Review in 1991 and later in her book, Soda Glazing, which increased the interest and popularity in this method of glaze application.

Tudball's work has an orange peel surface not unlike that of salt, yet her colours are softer with more peaches and oranges. The choice of clay bodies, application of slips and firing techniques would play an important part in achieving these surfaces. Tudball throws her forms, manipulates joins and textures the clay while in a plastic state resulting in forms that are soft and alive with a dancing quality. There is a rhythm about her work, the movement accentuated by the soda and the bleaching of colour by the soda on the raised, textured edges.

Gail Nichols (Australia)

Image of work by Gail Nichols
Gail Nichols

Nichols has developed a unique soda glazed surface of her own through thorough research and development of clay bodies and alterative methods of soda introduction. The alumia:silica ratio of her clay body has a high alumina content which reacts with the soda producing a thick dimpled opaque glaze with a matt finish unlike any produced by other soda firers. The glaze is still made of the same components, alumina, silica and sodium, however the higher the alumina, the more matt the surface and the higher the silica, the shinier the finish. This has lead to substantial differences in effects.

In describing her work she writes, (They are)...'evoking wintry images of snowdrifts and flowing ice. Rich matt soda glazes are formed over soft forms predominated by frosty blues, greys and whites. Yet the surfaces are not devoid of warmth. Strong red and orange flashings are evidence of the high-fire process that produced the works, representing a new development in sodium vapour firing.'

The most common method introducing the soda into the kiln is in a solution with water. Nichols mixes sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), calcium carbonate (CaCO3)and water into a slurry which sets like concrete and is broken into pieces before being dropped into the red hot firebox at appropriate intervals. This method, along with the addition of water and controlled atmospheric conditions at appropriate times, assist with the creation of her unique surfaces and colour response.

Other artists using soda glaze obtain entirely different surface qualities with the introduction of less soda at lower temperatures.

Jeff Oestreich (USA)

Oestreich does not depend upon an overall covering of soda glaze to decorate his functional yet sculptural forms. He combines form, decoration and firing to produce an overall effect. He creates variation in colour through the application of glazes that respond to the fluxing action of the soda allowing the soda to brighten the applied glaze. Oestreich introduces less soda at a lower temperature (1200 C) to provide added interest to the surface.

John Chalke (Canada)

Chalke soda glazes in a wood-fired kiln to1050C (cone 05) He adds the soda with the wood requiring no orange peel glaze formation, but just a sheen to the surface of his terracotta clay pots. With the addition of wood to the fire and the constantly changing atmospheres from reduction to oxidation, he obtains some carbon trapping within the glaze.

Differences between salt and soda glaze

Because salt (sodium chloride) glazing has a long history and was used in industry it benefited extensively from technical research, Soda glazing has not had the advantage of such a long period of exploration and investigation. When salt is introduced into the kiln, thick white clouds containing hydrogen chloride gas acid are emitted from the kiln. This gas forms hydrochloric acid when combined with water vapour.

With this growing environmental awareness and the development of strict anti-pollution laws other methods of producing a similar surface were pursued. The use of sodium bicarbonate as an alternative to salt was explored by Jeff Zamek at Alfred University, New York in 1974. His aim was to develop a non-polluting vapour glaze method that would give repeatable and acceptable orange peel surfaces not unlike those of salt. When Warren Mather and Bernice Hillman, also of the USA did further research and wrote an article Salt in the City: The Sodium Carbonate Solution for The Studio Potter, soda became a legitimate alternative to salt.


While it is possible to create a surface with marked similarities to salt by the quantity and method used of introducing the soda to the kiln, it is also possible to create wood-fired effects with a heavy glaze deposit on one side of the form and flashing on the other by giving consideration to the placement of the work within the kiln and by the rapid draft of soda vapours through the chamber.

Yet there can be delicate differences in the way the soda reacts to the clay and slipped surfaces. As John Chalke says, 'the characteristic soda blush makes it poetically superior to salt.' The colours tend to be brighter, yet softer which would appear to be due to the fact that when salt (sodium chloride) is added to the kiln the chlorides combine with the iron in the clay body resulting in more browns. Soda is gentler to the clay surface because of the slow rate of volatilising compared to the fast rate and 'aggressive nature of the insistent and demanding salt'. As with salt-glaze there are innumerable possibilities in variation through the use of different clay bodies, slips, firing temperatures and atmosphere, the time span of introduction and quantity of soda used and the positioning of the pots within the kiln.

Much of the North American ware is soda flashed rather than soda glazed. The work is frequently decorated with slips and underglaze stains, fired to a lower temperature before the addition of smaller amounts of soda, leaving the surface with a light blush of glaze and the decorated surface intact and undistorted. With higher temperatures there is a greater volatilisation of the soda resulting in heavier glaze coverage on exposed surfaces.


Environmental consideration was the main reason given for the use of soda in preference to salt as a glazing agent. Salt (sodium chloride) when subjected to heat usually in conjunction with water to disperse the salt vapours, splits the sodium from the chlorine, the sodium combining with the alumina and silica in the clay to form the glaze and the chloride combining with the water vapour to hydrochloric acid (2HCl). There is a lot of conjecture about the effects of both salt and soda glazing on the environment. Industry no longer has an interest in these methods of glazing and therefore little or no money is available for ongoing research. The artists who do research do so in an ad hoc manner due to lack of resources and experience in collecting hard data.

Kiln damage

John Chalke asserts that 'The clear advantages of sodium carbonate are less wear and tear on the kiln and kiln furniture', however while salt and soda appear to react differently, controlled research is required to prove this. Silicon carbide shelves (know for their resistance to corrosive vapours) have been used in the ANU soda kiln for 17 firings with surprising results. The shelves appears to froth in an attempt to repel the soda resulting in glaze drips onto the ware. Unexposed areas of the shelves are indicative of heavy corrosion. However, some props, being exposed to 75 soda firings, show no soda build up of glaze or corrosion due to good care. With conscientious care of bricks and furniture there will be less of this wear and tear whatever form of vapour glaze used.

One disadvantage salt has over soda is that the diluted hydrochloric acid resulting from salt glazing is corrosive on metal structures and reinforcement around the kiln. Soda does not have this corrosive element.

Technical aspects of soda glaze

How Soda Glaze works

In a stoneware firing, when the clay is just beginning to vitrify, at about 1260C (cone 8) soda, in the form of sodium carbonate (soda ash or Na2CO3), and sodium hydrogen carbonate (sodium bicarbonate or NaHCO3), is introduced to a kiln The sodium, a flux, combines with the silica (glass former) and alumina(stiffener or refractory element) within the clay body to produce a sodium-alumino-silicate glaze.

The composition of the clay body is critical in the formation of the glaze surface and it is recommended that a high silica body will encourage good glaze development, the ratio of Alumina (Al2O3) and silica (SiO2) being 1:5 in the clay body. The ratio of alumina to silica will determine the glaze surface, the higher the silica, the more sodium will be drawn to the clay with the outcome of a glassier thicker glaze and should the body be of a higher alumina ratio, the more the glaze will resist, resulting in a drier, matt or flashed surface. This can also be achieved by developing slips with variations of this ratio and by the addition of oxides or stains to create a palette of surface textures and colours.

Methods of soda introduction to the kiln

Soda can be introduced into the kiln in several different forms, borax, (sodium borate; Na2B4O7.10H2O) soda ash, (sodium carbonate;Na2CO3 ) sodium bicarbonate, or washing and bath crystals,(sodium carbonate decahydrate; NaCO3.10H2O ) When soda was first used as a substitute for salt it was often thrown into the firebox in large quantities as if it were salt. Salt volatilises rapidly whereas the soda will lie in melted pools in the firebox vaporising slowly. There was often a residue of a hard glass mass within the firebox after the firing. Water with salt helped dispersal and so experimentation began with the addition of water to the various forms of soda. But there are other means of adding soda compounds to the fire.

This can be achieved by the placement of crucibles containing soda throughout the kiln. Packages containing soda can also be placed amongst the waremwith additional packaging introduced into the fire at a later stage. Wood shavings or sawdust can be added to the soda package to increase the temperature of the fire, assisting to volatilise the soda. Jeff Zamek would drop soda into the kiln from a high position, allowing it to vaporize as it fell down through the kiln. Soda can also be placed upon pieces of wood and then fed into the hot firebox.

This is the most common practice of soda introduction. Soda in a super saturated solution is sprayed into the chamber at varying intervals through numerous ports. Martin Goerg injects a soda solution directly into the fire through a hole in a specially designed burner. Ruthanne Tudball used this method to introduce the soda but has recently commenced adding a paste of sodium bicarbonate to the kiln.

Introducing soda by this method is relatively new and is generally carried out by wood-firers who mix the soda into a paste and apply it directly onto the wood before placing it into the firebox.

Solid state
This method of soda introduction has been developed by Gail Nichols. A mixture of sodium bicarbonate, soda ash and calcium carbonate is mixed with water. When using light soda ash this mixture sets like plaster, is broken into pieces and added to the fire. The water assists dispersal and the calcium carbonate keeps the soda particles apart, allowing for a more efficient melt. In a presentation at the first Australian Soda Conference, Gail Nichols stated, "tests on the resulting glaze show that the calcium carbonate does not affect the glaze"

The amount of soda, the methods of introduction and the time over which the soda is introduced all play a part in the surface obtained and will depend upon the aesthetics required by the maker. The additions of water and combustible materials will create a reducing atmosphere within the kiln, again influencing the outcome of the firing.

The Work

The form

Image of work by Maryke Henderson
Maryke Henderson - Solitude 1, 2005.
Soda-fired, stoneware

The main inspiration for the form came from my fathers oil can. The form while industrial, maintains an elegance created by the fine elongated spout thrusting away from the body. I see this as a metaphor for the human form, perhaps in the stance of a dancer. The surface of his oil can had the appearance of aging as well as layered surface elements. Soda glaze fired under differing conditions and with varying clay surfaces underneath the glaze can also produced aged, layered and corroded surfaces. This is what I aspire to.

I have found collections of oil cans in museums and with antique dealers. They come in many different shapes and sizes, the length and width of the spout relevant to the use and speed of delivery of oil required. I have found that throwing is a good way of developing an idea, the process, feelings and ideas combining to develop the creative progression.

I start by throwing a number of simple, uniform forms that are remodelled by cutting, manipulation and joining. When I add the spouts and stoppers the pieces begin to develop as individuals. From each form new ideas spring or old ideas are discarded. I consider the joints. Sometimes they are obvious with a hard edge, other times softened and smoothed over. These hard (metallic) and soft (organic) edges create the tension that I seek within the work. Ultimately it is the fire that takes control, often changing the original intention by softening the surfaces, joints, curves and indentations. I investigated numerous different forms and processes of making and found that the forms that were thrown and cut and manipulated while wet better showed the plasticity of the clay. As the forms evolved the spouts became longer and more animated.

The forms originally stood on their own and as I thought more about my father as I was making, I could not disregard my mother and the close relationship they had. This is when the forms commenced to nestle together and began speaking of duality.

The pyrometric cones discarded after the firing display a charming quality, the cones soft, silken and white, the fireclay in which they stood, strong, robust and with a rough surface. This lead to an exploration with porcelain, using it as a contrast against clay bodies which contained iron. These explorations are put on 'hold' as I feel that this investigation can lead to a whole new body of work.

The bowls in which the ewers originally sat were inverted to become plinths on which the ewers now sat, first by themselves and later in groups. I looked at the work of Tom Risley, a Queensland sculptor who works with found objects. Risley makes objects with tomato stakes joining them with a corking gun, referencing the joins of silicone sealer in boats and expressing the act of joining. By pressing porcelain clay through cuts in the soda clay body I created joins that showed a gestural act of making by the impression of something soft oozing from a hard surface. As the ewers became more animated by their strong dynamics, the groups became larger and therefore the plinths grew also. I see this as an expression of my family, a large energetic group of people that continues to grow.


Image of work by Maryke Henderson
Maryke Henderson - Solitude 2, 2005.
Soda-fired, stoneware

Patterns surround us, but in art practice they are often associated with the decorative arts. They are described as "a design whose quality is marked by repetition" often with a metaphorical, cultural or decorative message. Linear and geometric patterns are strongly represented in architectural designs, however I find that these have a strong controlled hard edge to them compared to those found in nature, the two, used together create a tension, one against the other.

David Sequeira creates images using leaves that he then sews together in an overall geometric pattern. While there is a strong controlled uniform sewn line, there is also the underlining organic surface of the leaves that accentuates the natural against the contrived, the new against the old, the layering of human agency and natural structure.

Julie Ryder also uses the organic to create digital images by the use of scanning electron microscopy and digital software. These digital images of fungi have spectacular sculptural forms and textures that reveal a colourful and complex structure and natural patterning.

The patterns that I have chosen are drawn from architectural images and the constructed environment. The hard contrived line provides a contrast against the natural organic surfaces of the soda glaze. The white lines are carved through a layer of porcelain slip to produce intentional patterns.


With so many variables possible, every firing has the potential for experiment and can offer a wealth of new ideas and techniques. With the formation of glaze dependent upon the movement of vapour through the kiln, heavier glaze deposits form on the more exposed surfaces and only a blush of colour forms on a protected areas. (With conventional glazing techniques the glaze distribution does not rely upon this movement of vapours through the kiln chamber.)

This process makes soda glazing captivating and unpredictable. The vapours leave a mark showing where they have moved through the kiln and passed over the clay. Chance plays an important part in the process and the final outcome of the surface. These occurrences are an open field for exploitation rather than dictators of preconceived effects.

The unpredictable painting with fire on the surface over the controlled mark making and construction develops a dimension of tension between the organic and contrived. The works provide a formal canvas to explore the surface and to set up conditions for effects that can never be precisely repeated.

I have embraced the challenge to use the traditional techniques of throwing and vapour glazing to develop functional forms into a contemporary mode. I have done this through the development of surface and exploration of form. This has resulted in the integration of form, decoration and fired surface to create a lively personal statement.