One of my most successful soda firings, Pottery Northwest, Seattle, WA

The three types of firing methods presented to the viewer were conducted in a wood-fired kiln (including sagger containers), a pit-type kiln, and an electric kiln. The wood kiln and the pit-type kiln both possess a revealing nature (what I shall call "painterly" quality) of a recorded history of fluctuating yet enduring internal atmospheric activities superimposed onto the surface of the vessels. The immediacy of the moment captured, which reflects the intrinsic firing characteristics of that particular kiln has been the compelling aspect of these firing methods. The electric kiln can also produce exciting glaze effects such as "oil spot" and crystalline effects within the glaze.

The Sagger consists of a semi-closed container that allows for pieces to be contained within a separate environment from that of the kiln chamber. Pieces are placed into the sagger container along with combustibles (sawdust, vegetable matter, salts, oxides, etc.), which is then placed into the kiln chamber. The kiln simply provides the maturing temperature necessary for the desired effects. Sagger-firings can be accomplished at almost any temperature range; however, I prefer the results achieved at pyrometric cone 07 (1783°F) and occassionally pyrometric cone 9 (2295°F). I commonly apply terra sigillata (literally, "sealed earth" - originating from the Etruscans or the Hellenistic Greeks to produce red and black ware), which consists of very fine clay particles and added oxides to give color (usually iron oxide - producing reds and oranges). I then burnish the terra sigillata with a polished stone at a certain stage of drying. This process flattens the very fine clay particles onto the surface of the vessel. The burnished surface displays a shine or lustrous sheen.
As the sagger reaches maturing temperature in the kiln chamber, the combustibles fume and explode onto, and into the clay body of the vessel. The variations of color are indelible as they penetrate the clay walls. Due to the low-fire nature of the sagger-firing process, the clay body used to make the vessels has not completely vitrified and is somewhat porous and therefore is never to be exposed to water.

The immediacy of the moment captured, which reflects the intrinsic firing characteristics of the wood firing process has been the compelling aspect of these firing methods. In my quest for achieving a greater color range from the wood-firing process, I decided to incorporate the reduction-cooling process. Through consultation with Seo Eo of East Carolina University and John Neely of Utah State University, I discovered a vast array of colors with the use of this procedure and the proper clay body. This clay body is an iron-rich clay. The iron content within the clay plays a crucial role in the reduction-cooling process in that it is the reduction of the iron that gives the blue, gray and black coloration. The process involves maturing the clay body to pyrometric cone 8 or 9 (2277°F and 2295°F, respectively), and bringing down the kiln in temperature, keeping it in a reduced state. The idea is to convert the oxidized state of iron (red ferric iron oxide - Fe2O3) into a reduced iron (black ferrous iron - FeO) by continuing to introduce wood into the kiln's chamber. The continued stoking of wood is just enough to introduce excess carbon into the kiln without raising the temperature. The excess carbon searches for oxygen to allow for combustion. Due to the anoxic environment created within the kiln, the only oxygen molecules available are tied up in the ferric iron within the clay body. The carbon matches with one or two oxygen molecules from the iron compound and is exhausted out of the chimney (or stack) of the kiln as CO or CO2 (carbon monoxide and carbon dioxide, respectively). This leaves behind free metallic iron (Fe) and ferrous iron (FeO).
To achieve this anoxic environment, the kiln must be tightly sealed to prevent oxygen from entering into the kiln's chamber. To seal the kiln, I use newspaper coated in clay slip, which is applied to the entire surface area of the kiln. This process is very similar to applying wallpaper.
The cooling process is slow (5-10 hours of light stoking in addition to the 24-30 hours of heavy stoking that is required to reach the maturing temperature). A pyrometric temperature gage (inserted into the kiln at the beginning of the firing) is closely observed. The iron in the clay body remains active (can convert from either oxidation or reduction) until a temperature of 1450°F is registered. At this temperature (and lower) the iron "freezes" within an oxidized or reduced state. If all is successful, the state will be that of the reduced, grayish-blue ferrous iron. The reduction-cooling process can provide a whole spectrum of the iron's potential colorations. The most successful and treasured vessels for me are the ones that exhibit all of these colors.

With this process, the wood (or gas) kiln is fired up to cone 9 to 10 (2295°F and 2340°F). Beginning at around cone 7 (2259°F), soda ash (NaCO3) is introduced into the kiln through ports in the front and sides of the kiln. The soda ash is dissolved in water and sprayed into the kiln. This compound of sodium carbonate separates at such high temperatures liberating the sodium to become attracted to the silica composition of the clay bodies used to form the vessels within the kiln. Thereby creating a self-glazing firing process. Differing from salt (NaCl) firings, soda firings have a tendancy to create more of a dry surface on the pottery with the occasional glossy and pitted "orange peel" texture commonly associated with salt firings having been treated with large quantities of salt. The appealing difference is that with the soda-fired surface one may identify a darker grayish-blue "orange peel" texture on one side and a dry flash of orange/red on the other. This phenomenon could be due to the residual carbonate molecule (CO3) splitting further into carbon (C), carbon monoxide (CO), carbon dioxide ((CO2), and oxygen (monoxide-O) and oxygen (dioxide O2). If more water is used to dissolve the soda ash, there can also be a better chance of heavy water reduction (hydrogen reduction), which can also lead to some very nice coloration. A combination of all isolated atmospheric conditions present within the kiln's chamber during each spraying event would explain the variegations of both oxidation and reduction atmospheres commonly occurring on the surface of the individual vessels.

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