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n ro UCin • Kwi Rak Choung has experimented with ancient and modern techniques. This article is the first in a series that details his research Vase. Natural ash glaze. 35 x 16 ern .

r-- ........ HE TECHNIQUE OF INTRODUCING WATER.

into woodfiring kilns is little known in the modern age. This is not because the technique is new but because of a disconnection from past experimentation. In many countries, woodfiring had aln10st completely ceased as more advanced and convenient kiln types were introduced during the late 19th - early 20th century. Consequently, woodfiring has been reintroduced by ceramic artists seeking new qualities in their work. Among the techniques being re-invented, the use of water is an intriguing one. Interesting investigations and explorations into the longwoodfiring process have been reported by ceramic artists. The possibilities suggested in these reports indicate that enhanced colour and mattness of the glaze can be achieved by using water during firing. My experience in 1999 at the Sturt Pottery in NSW was when firing an almost abandoned anagama which had been dampened by wet weather for three years I obtained exceptional colour on some pots. T hroughout four more firings, however, I could not achieve the same level of colour development. I did not appreciate the effect of dampened earth under and around the kiln Ul)til I had a conversation with Owen R ye at Monash University, Victoria, about the possible effects of water as it gradually penetrates into the kiln as the heat in the kiln increased. This experience, and curiosity about the possible effects of water in woodfiring, provided an impetus for further study. In searching for information on the role and effect of water, my initial interest was doubled by encountering some relevant literature. Experimentation from the brick making industry, ceramic engineering and laboratory tests in the glass industry provided an analytical base for my experiments.,

HI STORICAL AND MODERN PRACTICES

The use of water in firing ceramic wares is an old Chinese technique. YingKing1 o utlined the method of using water for firing bricks in a Chinese. pottery 300 years ago. The top of the kiln was made level then terraced w ith elevated edges and filled with water like a rice field (the kiln wall would be soaked by water seeping through gaps). When added at the appropriate stage of the firing, the water turns into steam and reacts with other kiln gases. Funk reported that water is introduced for the firing of porcelain ware at Jingdezhen, China: "The Chinese were most concerned about the phase of firing when water was introduced and worked itself through many channels, causing the heat to penetrate in all part of the kiln, thereby providing uniform ware whic h would stay free from cracks".2 The use of vvater in kilns has other ancient precedents. Yang Mo Jung reports on an excavation of a Korean celadon kiln site in use 400 years ago. 3 Holes (70 x 30 em deep) fo und on the floor of the firebox were filled w ith unburned charcoal. These holes were connected to each other and to a water inlet outside the kiln. The report suggests that charcoal was used for holding the introduced water to enhance the reduction atmosphere during firing. This is convincing when considering the firing method of ancient Korean celadon kilns. These anagama style kilns were fired with a fire-mouth door wide open, resulting in an oxidised effect on a large number of pots. This is evident on many excavated celadon shards from the earlier period kiln sites. The use of water would have been an alternative way to prevent the effect of oxidation. Karen Terpstra researched a woodfiri ng method of Qing dynasty (1644 1912) kiln injingdezhen, China. Translating a conversation with Zhu Gui H ong, she reports: "The front area of the kiln reached a higher temperature. Wet wood was stoked periodically and water was sprayed on the fro nt part of the frame of the kiln causing heavy reduction. Also, it was believed that this chemical reaction gave a greater degree of vitrification of the clay body from the hydrogen reduction. This exothermic reaction flowed to the back of the kiln and distributed the heat and raised the temperature in the back of the kiln." '1 Another discovery of the use of water in kiln firing in ancient times is reported by Richard Bresnahan in relation to an excavation of a group of 12th - 13th century anagama kiln sites in Japan.5 During the excavation, large water storage jars were found at the base of the kilns. This • led to further research involving experimental firings with water. The results showed that by pouring eight to 10 litres of water into the ash pit of the anagama before sealing it off, effects similar to the old Echizen ware were achieved.

Bowl. Natural ash glaz e. 15 x 2 8 em..

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Bottle form. Fe!spar glaze . 23 x 12 ern/h.

In Australia it was not until relatively recently that the role of water introduced during the firing process has been taken into consideration. Ivan McMeekin suggested that the significance of using wood as a fuel is that the reduction atmosphere in a wood-burning kiln can only be achieved by carbon monoxide and hydrogen.6 H e noted that completely dry wood still contains son1e 20 per cent H zO in the cell structure. What is significant here is that this is directly related to the question 'how much water' is necessary for best possible results, because woodfiring naturally contains a certain level ofwater in the atmosphere. This leads to another question: What other role does water have b esides as a red ucing agent? With limited answers for those questions a number of experiments and studies have contributed to the area. Owen R ye suggested various ways of using water as a way of achieving the b est possible results for colour and matt quality on the surface of glazes and clay bodies: " I have been experimenting for eight or nine years with the effects of introducing water into an anagama kiln. The aim has been to enhance the colour of glazes via extra reduction; enhance flashing colours and colour un der wads; and matt the glazes via increased reduction especially during cooling. " 7 Another ceramic artist, Gail Nich ols, has reported that water introduced during soda fir ing in a gas kiln enhanced the colour and mattness on the glazed surface. 8 Nichols agrees w ith Rye that water causes a reduction atmosphere, but she has since shown that even in oxidation firing the results are similar to the results of reduction firing when water is introduced at top temperatures. Susan' Scott9 ob tained similar res ults in an experiment with the effect of water on natural ash glazes using a gas kiln. Her findings show the reduction effect of water, especially when water is introduced during heating between 900°C -1 000°C (1652° -1832°F) even in oxidising atmospheres. Also ofinterest is Nisse H olmstrom's study. H e states that the ancient Chinese used water in the cooling stage of firing brick kilns to achieve a red brick colour. 10 Air inlets were sealed before the water application in this firing m ethod. Without water the bricks would remain grey because of the domination of the reduced iron oxide FeO. A difficulty here lies in explaining how water contributes to the change ofFeO to Fe20 3, w hich means, theoretically, the change in the iron state is caused by the oxidising effect of water. It is necessary then to examine the atmospheric conditions w hen water is introduced into the kiln during firing. The chemical reactions in a high temperature woodfiring kiln atmosphere, when water is present, can be represented (water-gas reacti on) as: C + H zO = CO + H 2 (reaction 1) C O+ H 20 = C02 + H 2 (reaction 2) The outcome of these reactions res ults in mostly reduction gases with the exception of C02, which is a oxidising agent, in reaction (2) . And, the simultaneous occurrence of reactions (1) and (2), especially in firings w ith a solid fuel, can be expected. Therefore, the dominating gases in high temperature kiln firings when water is introduced are th e reducing ones that, consequently, establish a reduction atmosphere. This explanation contradicts the suggestion of the 'oxidising effects of water' for the change ofFeO to Fe203. To understand the' oxidising effect ofwater', two related factors can be illustrated. Firstly, in the cooling stage th e water introduced into the kiln oxidises the lower oxides of iron (FeO, Fe30~) in the clay bodies, 'not' the atmosphere. This effect explained by the chemical changes in iron oxides when reacting with water: 2Fe0 + H 20 = Fe20 3 + H z and 2Fe30 4 + H zO = 3Fe203 + H z

This reaction shows the 'oxidising effect of water' on the lower oxides of iron. Therefore, it can be speculated that the water introduced into the kiln acts as an oxidising agent on the lower oxides ofiron while still contributing in the formation of reduction atmosphere by extra hydrogen gases (Hz) generated from the water-gas reaction. Secondly, in relation to the above speculation, to be able to oxidise the lower oxides of iron, the water (steam, H zO) in the firing atmosphere may have to be retained without chemical breakdown. During a high temperature firing the water introduced reacts strongly with other gases present, consequently the majority of water/steam breaks down chemically to form more reduction gases. For the 'oxidising effect of water' to be viable, it is necessary to introduce water during the cooling stage because less chemical reactions occur at lower temperatures in the cooling stage and there is less breakdown of water vapour (HzO). This may explain the colour change of the grey to red bricks when water is introduced during the cooling stage of the firing in Chinese practice. This speculation on the chemical role of water requires further discussion of related literature. In a technical article, Phillip Cornelius reported his experiments with water as a reducing agent in firing. 11 He reported a development of rose/ red colours and a skin-like surface effect on stoneware bodies. Charcoal was added before the water application at the temperature of cone 11 to achieve this effect. American ceramists, Joy Brown and Richard Bresnahan also introduce ·water in their anagama firings . 12 They both are inspired by the Tanegashima-style firing method. The name is derived from Tanegashima Island in Japan where the effects of water in a woodfiring kiln were found by accident when trying to stop the kiln firing by hosing it down with water. Later when the kiln was opened they discovered pots with exceptional colour development. After the experience with this firing method Bresnahan built a kiln with four chambers. One of the chambers contains a water inlet under the kiln. An open water pit (approx. 30 em from the kiln floor level) in which a 50 mm pipe was installed and connected to the outside for adding water, was constructed behind the exit flue of the previous chamber. Over the past 20 years, Bresnahan has been experimenting by varying the amount of water and he concludes that only a small amount of water is necessary for the colour development and surface effects. EFFECT OF WATER ON COLOUR DEVELOPMENT

Colout developinent in most clay bodies depends on the iron oxides present and the physical state of such oxides. Experiments with some iron bearing clays fired under various conditions and atmospheres have been tried. 13 One of the results showed a buff coloured clay fired under oxidising conditions. This colour was explained by the presence ofFez03 in the mullite structure of the clay body. Under oxidising conditions, the iron oxide exists in a ferric form (Fez03) and dissolves into the mullite structure, losing its colouring power. If the iron oxide is reduced, iron oxide (ferrous iron, FeO) is not taken into the mullite structure, and reoxidation re-forms as Fez03, which contributes to the red colour. This suggests that establishing a reducing atmosphere at an early stage, around 850°C (1560°F) recommended by some practitioners, is effective for colour development on the clay bodies.

Top: Vase. Natural ash glaze . 33 x 16 em. Below: Bottle form. Natural ash glaze. 20 x 21 em .

REFERENCES:

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1. Sung Ying-King, 'T'ien Kung K'ai Wu', Bull Am CeramSoc20(1)p.15-24, 1941. 2. W. Funk, 'The Technkal Production of Chinese Porcelain', Ber Deut Keram Ges, 22 (6) p.223, 1941. 3. Yang Mo Jung, Ceramics of Korea, Mun Yei Press, Seoul, p.289, 1994. 4. Karen Terpstra, 'Firing Method of a Woodfired Jingdezhen Qing Dynasty Kiln', Ceramics Technical12, 2001. 5. Richard Bresnahan, 'Water and Woodfiring', The Studio Potter, vol. 28. 6. Ivan McMeekin, 'The Bourry Box', Woodfire '89 Gulgong, 1989. 7. Owen Rye, 'Water in Woodfired Kilns', Ceran1-ics Technica/1 0, 2000. 8. Gail Nichols, 'Colour and Ice -Atmospheric Effectc; in Soda Glazing', Ceramics Technical10, 2000. 9. Susan Scott, 'The Change is the Weather', Ceramics J'v1onthly, May, 2001. 10. Nisse Holmstrom, 'Aquagama', Dffferent Stokes, Conference Proceedings, 1999. 11. Phillip Cornelius, 'A Chemical Look at Forn1S of Reduction', Ceramics Technical, 11, 2000. 12. Richard Bresnahan, p. 20- 22. 13. ]. E. Houseman and C. J. Koenig, 'Influence ofKiln Atmospheres in Firing Structural Clay Products: 1, Bull. Amer. Ceram . Soc, vol. 54, p. 88.

Considering the above discussion, the influe nce of H 20 on the state of iron oxides in various atmosp heres is interesting but the question here is: Which atmospheric condition favours each of the two different roles ofwater? Two roles ofHzO exist because the rates of chemical reaction b etween the heating and cooling stages differ co nsiderably. In the heating stage, where there is a large amount of free carbon and carbon monoxide, water introduced into the kiln provides the active water-gas reaction forming the extra reducing gases of hydrogen (H 2) . Therefore, when water is introduced in the h eating stages an enhanced reduction atmosphere will be expected. When water is introduced in the cooling stage, a diminished water-gas reaction, due to a limited amount of reducing gases present, allows H 20 vapour partly to remain as a free gas w ithout chemical breakdown. This H 20 vapour then may act as a re-oxidiser on lower oxides of iron in the clay body. Permeability of kiln gases into clay bodies is an important factor in colour development because kiln gases directly influence the state of iron oxides present in the clay body. This means that the atmosphere with more permeable gases is more effective in converting or saturating the iron oxides present. Generally, permeability of kiln gases is dependent on the porosity of the clay body, firing temperature and the molecular size and weight of gases. With a given temperature and clay body, therefore, it can be expected that the smaller and lighter kiln gases such as H 2 and H 20 have greater permeability than other gases. As the discussion indicates, water in the kiln enhances the reduction atmosphere in the heating stages, and may act as an oxidiser of reduced iron oxides in the cooling stages. The role of water in either case is different, but it can be said that the subsequent effect of water in both cases is enhanced colour on the surface of the clay body

Kwi Rak Cheung is a ceramic artist living in Sydney. This artide is the first in a series outlining his research.

Vase. Natural ash glaze. 45 x 48 em.