You may have noticed that we have several different kiln types and sizes in our firing room. That’s because it takes a mix of kilns to handle studio workflow, and to cover the different ways clays are fired – eg: oxidation, reduction, bisque, sculpture, earthenware, stoneware and porcelain. These are discussed within the kiln descriptions below. ….. (Click on photos to enlarge.)
……………………………… The Alpine Kiln
The first kiln you see as you walk into the firing room, directly ahead of you, is our forced air, natural gas, updraft kiln. It was built by the Alpine Manufacturing Company, so it’s usually just called “The Alpine”. Heat & flame are blown into the kiln from two floor level burners, one on the left and one on the right – they’re pointed out in Figure 1. Heat travels upward through the kiln and exits at the top at an opening called the flue, the size of which is controlled by closing a Damper (hence the term Updraft Kiln). Upper and lower peep holes in the door allow us to peer into the kiln during the firing. Figure 2 gives a view of the inside; it’s completely lined with firebrick to hold in the intense heat. This kiln can be fired to any temperature up to and including cone 10 (2340 F) – meaning that we can fire cone 6 glazes (2230 F) as well as cone 06 bisque (1832 F), and we can fire in either oxidation or reduction.
The ware is placed on two side-by-side 11 x 28 inch corderite kiln shelves, creating an overall 22″ deep x 28″ wide stacking space. Because of the arched ceiling, the stack height varies from 41 inches in the middle to 38 inches at either shelf edge – which yields an overall 17 cu ft ware capacity. One of the nice features of the Alpine is that it has a hinged door, making access to the interior really quick and easy.
Figure 2 illustrates how heat flows from the left burner. Thicker arrows indicate more flow. Flow from the right burner would be a mirror image of the flow arrows shown. Flames don’t actually make contact with the ware because the burners blow into a low, narrow trench located behind the left and right Bag Walls, which are actually made of loosely stacked firebricks. Then, the heated air and hot gasses push through gaps between the bricks, and travel up through the ware to the flue. For a given burner fan speed, closing the damper (Figures 1 & 3) slows down the flow and increases air pressure in the kiln. Increased pressure helps force the heated gasses into and around all the pots and “nooks and crannys” in the kiln – just as air spreads throughout the entire inside of a balloon as its blown up.
At the start of the firing, the damper opening (see Figure 3) is set to about 1.5 inches. This is wide enough to allow plenty of airflow into and through the kiln so that the burners operate in the combustion state known as Oxidation. When burning in oxidation, natural gas (which is primarily methane, or CH4 ) and air produce heat, carbon dioxide, water, & nitrogen according to the relationship:
CH4 + 2 (O2 + 4 N2) ===> Heat + CO2 + 2 H2O + 8 N2
When the gas/air ratio is perfect – that is: not to much air and not too much gas – the temperature of the flame reaches up to about 3450 F . With more air, there’s an excess of oxygen and it chemically enters the ware; with more gas, there’s a shortage of oxygen and it is chemically pulled from the ware. Since we can control the amount of both air and gas, the kiln can be fired in either oxidation or reduction.
A typical reduction glaze firing is lit in the evening, and the kiln is allowed to heat slowly in oxidation overnight. In the morning, the temperature has usually climbed to about 1500 F. When it reaches cone 010 (pronounced ” oh-ten “) 1650 F, the gas/air ratio is increased by either turning up the gas pressure, reducing blower air, or slightly closing the damper. This starves the hot methane of enough free oxygen to burn directly. BUT, with the methane as hot as it is, it actually pulls oxygen out of the metal oxides in the ware and glazes in order to burn. Doing so causes the colors in the clays and glazes to change. This is the process called Reduction because the oxygen content in the ware is reduced by the fuel trying to burn. Take an example, like the familiar glaze colorant Red Iron Oxide (Fe2O3) . It is red in its oxidized state, but when reduced it becomes Black Iron Oxide (FeO) – a very noticeable change !
The cone 010 reduction, which is often called the Body Reduction, continues for about 45 minutes while the kiln heats from cone 010 to cone 08. During this period, a six inch yellow-orange flame can be seen bursting forth from both peep holes if opened. This confirms that a substantial reduction has been established, and that the internal pressure is high enough for it to reach every part of the kiln.
Figure 4 shows what a cone pack looks like when viewed through one of the peep holes. The cone packs are placed on a shelf directly in front of each hole so that the firing can be monitored. At the point when cones 010 and 08 have collapsed downward, the gas/air ratio is lowered – using a combination of gas, blower, and damper controls – to set up just a slight reduction ( ie: just a whisper of flame exiting both peep holes) , and to make the kiln heat at a rate of about 120OF/hour (2OF/min). This firing approach is recommended by Val Cushing (Professor Emeritus; Alfred University) in the firing section of his text: Cushing’s Handbook .
Burner flame continues to heat the interior, circulating air by the ware, heating it as it moves. After the ware is hot enough to glow red, (1100 F) a degree of radiant heat transfer between the ware also occurs. The next two cones in the front row of the pack, ie: cones 1 and 5, help monitor the heating to make sure top and bottom temperatures are going up at the same rate.
Eventually all the front row cones melt and the view through the peep hole looks similiar to Figure 5. Later, when cone 8 starts to drop, kiln controls are adjusted for a second 45 minute heavy reduction, called the glaze reduction. This is followed by raising the temperature to the final level of cone 10, whereupon the firing is finished and the kiln is shut down. Cone 10 is approximately the same temperature as volcanic lava; or in other words, the temperature of the parent rock that weathered over geologic time into clay in the first place. Notice that the cone pack also includes a cone 11, known as the Guard Cone, which ideally does not show any bending – thus indicating the kiln was not overfired.
CAW Downdraft Kilns
The two other large kilns you see in the firing room, to the right of the Alpine, are sprung-arch downdraft kilns, custom made for CAW. They also use forced air, natural gas burners. Click here for more details about our burners. In a downdraft kiln, burner flames enter at the floor and the heated air circulates to the ceiling just as in an updraft kiln. See the flow arrows in Figure 6. But in this case, there is no opening in the ceiling, so heat and pressure build to effectively push air back down toward an exit trench located at floor level. The trench leads to a tall chimney that sucks air up and out of the kiln.
The longer, slower flow path for the hot gasses allows more time for heat to transfer to the ware. Consequently downdraft kilns are generally more fuel efficient than updraft kilns, and historically have reached higher temperatures. The concept was invented by the Chinese around 800 CE to help them reach the temperatures needed for porcelain. Both the kiln technology and porcelain body composition were once closely guarded secrets.
CAW’s largest kiln is pictured in Figure 6. It’s known as Ethel and is located in the center of the firing room. Ethel’s interior dimensions (45 w x 45 h x 36 d) give it a 45 cu ft capacity. It’s overall shape is close to a cube with an 8 inch arch capping the top. The bag wall is three bricks high with one inch gaps separating adjacent bricks. Our shelving plan allows for two side by side 14 x 28 shelves that together form a 28 x 28 inch ware space. Since the posts are 2.5 inch square, the largest circular dimension ( eg: platter or bowl size) that can be fit onto the shelf is 23 inches diameter.
The kiln on the right side of the room is known as Phoenix. It has the same general architecture as Ethel, but is smaller by 10 cu ft. This is primarily due to its shallower depth, the overall dimensions being 48 h x 44 w x 31 d . Phoenix’s shelving plan calls for two side by side 12 by 24 shelves, thus creating a 24 inch square space and allowing a maximum circular dimension of 19 inches.
Both kilns have a stand-alone steel frame door that is rolled into place in front of the kiln and clamped to the framework. The door is insulated with ceramic fiber blanket and hardboard to seal in the heat.
Figure 8 shows the components of a typical “sprung arch” kiln design. Tapered ceiling bricks form a circular arc between two “skewback” bricks located on either end. By alternating the tapered brick with straight brick, a range of radii can be achieved. The walls are held together with angle iron compressed and held vertical by heavy threaded rods. If you look closely at the arches of Ethel and Phoenix, you’ll notice No 2 arch brick, which taper from 2.5 to 1.75 inches over their face, intermingled with straight brick, some of which are trimmed.
The same firing overview discussed above holds for the downdraft kilns too. The difference in the firings is in the way controls are set to achieve the same time/temperature profile.
CAW electric kilns are generally used to bisque-fire stoneware and porcelain, and to glaze-fire terra cotta and Young People’s ware. We have three Skutt kilns available, two of which are located immediately to the right as you enter the firing room. These are pictured in Figure 8. Both are three segment, top loaded, automatically fired kilns with a 10-sided octagonal cross-section.
The kiln closest to the door, called the Door Skutt, is a model KM-1027 configured to fire as high as cone 10. It has a 7 cu ft capacity (23 in dia x 26 in deep) . The slightly larger kiln located between it and the clay mixer is called the Mixer Skutt.; it’s internal dimensions are 28 in dia x 26.5 deep. Either controller can be externally programmed for fast, slow, or custom firings. We generally use a slow, custom firing because our ware is a mix of wheel-thrown, hand-built, sculpture, and children’s ware, some of which can be pretty thick. This firing profile (shown below) includes a long pause at 200F (2 hours) in order to bake out deeply buried moisture.
All CAW bisque is ultimately fired to cone 06 (1000 C, 1832 F). This brings the ware up to the point where all organic material and chemically-combined water has been driven off, but where porosity remains suitably high for glazing. Lawrence and West have provided a chart of how stoneware porosity changes with firing temperature in their book Ceramic Science for the Potter, shown below for reference.
Notice Figure 10 also shows the effect of firing clay up to cone 10, our glaze firing temperature. At this level, porosity reduces to a minimum level, causing the ware to become vitrified. Clay vitrifies when its feldspar softens to a viscous liquid, filling voids, lowering porosity, and binding other minerals together into a single dense mass.