High-temperature ceramic Furnace Linings, or Refractories, are a crucial part of pottery production. While numerous compositions and shapes are available for kiln construction, potters can effectively build any kiln with just a few. To ensure the most cost-effective kiln lining, in terms of initial cost and ongoing operation, it’s essential to grasp the basics of refractory composition, shape types, and construction techniques.
Compositions:
Fireclay-based refractories are the most common and least expensive dense compositions used for floors, chimneys, and salt soda kiln construction. Fireclays are available in several grades. Starting from the lowest (low duty), then intermediate duty, high duty, and super duty, and finishing with high-fired super duty at the top end of the fireclay spectrum. Most dense refractory kilns used in firing pottery are constructed of high-duty or super-duty fire clay-based refractories and perform exceptionally well.
High alumina compositions start at 50% alumina and increase in alumina content to 98% for the highest purity and most expensive; a potter would rarely require an alumina content exceeding 70%.
Insulating compositions range from 1600F to 330oF and are used primarily in reduction kilns or as a backup layer(s) to dense refractory hot face linings to help reduce energy losses through the lining. A point to remember is that the higher the density of the refractory (excluding fiber-based products), the more energy it takes to heat it, and the greater the thermal losses can be if not appropriately insulated.
Each composition of brick can be made into various shaped and unshaped refractories.
Bricks:
Most bricks are pressed or extruded. Common shapes include straight, arches, wedges, keys, rotary kiln blocks (RKBs), and square edge tiles. Larger pieces are typically produced by air hammering the brick mix into wooden or steel molds sized for the desired shape dimensions.
The standard refractory brick size is 9″ x 4.5″ x 2.5″ series or one brick equivalent (eq); this size is the most commonly used in pottery kiln construction. However, an equally popular standard size used in industrial furnace construction is the 9″ x 4 1/2″ x 3″ series. 3″ series brick reduces the number of joints in the kiln. For sprung arch and barrel arch crowns, the use of 3″ series eliminates the use of straights except for very large kilns. Remember, straights do not have a taper and can slip and or fall out in a crown that does not stay in compression.
Monolithics:
Commonly referred to as unshaped or specialty refractories, Monolithics are available in several forms and bonding systems:
- Mortars – air set, heat set, and phosphate bonded
- Castable and gunning mixes (both dense and lightweight) – cement, chemically and no cement bonded
- Plastics – air set, heat set, phosphate bonded
- Ramming mixes (wet and dry) – air set, heat set, phosphate and ceramic bonded
It is highly recommended that mortar be used when bricking a kiln. Although many pottery kilns are constructed without mortar, using mortar serves several purposes. First, it helps the bricklayer keep walls square and level by easing the minor variations in brick sizing during manufacture. Second, mortar helps reduce heat loss in the lining, conserving energy and reducing the uneven temperature in the kiln. Mortars can be thinned with water to desired consistencies and further diluted so they can be sprayed or brushed as protective coatings.
Castables make precast shapes of various sizes, bases, or floors of kilns, cover the outside of a kiln (like a stucco), and sometimes pour the “key” in roof construction to avoid cutting brick.
Plastics can be used to construct an entire kiln, just as in large industrial furnace linings. They can also patch worn kiln linings and seal up large cracks.
Fiber-Based Products:
Mineral wool and ceramic fiber-based products are used to produce insulating blankets and vacuum-formed boards and shapes. Additionally, ceramic fiber materials are available in papers of thickness ranging from 1132 ta L/2 inch, ropes of various diameters and shapes (tadpole type), woven tapes for gaskets and cloth for heat-resisting curtains, and moldable and pumpable compositions. Service temperatures range from 1200″F to 3000’F.
A point to remember is that typically, the higher the density of the fiber-based products, the more insulating they become. This is the reverse of denser brick and monolithic materials.
Construction:
The following illustrations show recommended methods of wall and arch construction.
Walls—Figure 1 illustrates a recommended wall construction utilizing alternate header and stretcher course construction. Another acceptable method uses four or five stretcher courses plus one header course. This construction is typically used for dense hot faces and insulating backup layers. The header course is made of dense material. The 4 and 1 construction is more energy efficient than the alternating method.
The level and square are your best friends when properly constructing walls. Mortar joints should be fragile (no more than 1/8″ thick) and applied like putting peanut butter on a slice of bread.
Since bricks are available in sizes other than the standard series format, obtain to minimize brick cutting and to NINE THICK Alternate header and stretcher courses than the typical 9″ x 41/2″ x21/2″ or 3″, do not limit design the kiln for sizes and shapes you can obtain adequate bonding.
Arches:
Figure 2 illustrates typical sprung arch construction. Each row (or ring) is the same depth; for additional stability, a bonded arch-type construction utilizes bricks of different widths (or lengths), as shown in Figure 3.
The construction of the arch begins by fixing the steel supporting structure to the kiln walls and the skew retainer steel to the vertical supports connected by tie rods, Caution: Do not forget this essential first step as the arch will likely collapse if you wait until the end to install your steelwork.
Next, lay out one ring on a flat surface marked with a chalk line for squareness. This establishes the combination of arches or wedges you will use to obtain the desired rise of the arch and to assist you in cutting your skews (in place of preshaped skew backs).
Arch Form Construction:
Carefully transfer the measurements to a sheet of 3/4″ plywood and draw the shape of the inside of the ring on the plywood. Then, the plywood is cut so that the outer edge corresponds to the inside dimension of the walls and the curvature of the arch. Using the first piece as a pattern, cut a second piece of plywood to the exact shape.
Next, separate the two pieces with 2 x 4 short lengths. The size of the 2 x 4’s is determined by the length of the brick plus 3 inches; a minimum length should be 12 inches. Attach sufficient 2 x 4s around the radius to adequately support a thin cover of flexible Masonite or luan sheeting over the top of the form.
Now, you are ready to install the form in your kiln. Cut four vertical support boards of either 2 x 4’s or 4 x 4’s in the corners to length to hold the arch in the correct height position. It is easier to cut the vertical supports 1/z to 3/q” shorter and then use wooden wedges under the supports for the final leveling of the form. Wedges are more readily removed for repositioning the form as you work your way from one end to the other.
Installing the Arch: Install the skew line using preformed skews or built-up ones you cut from straight brick (Figures 4 & 5). Then, lay your arch or wedge brick according to the combination you determined by laying out your first course on a flat surface. Note: your brick supplier should be able to provide you with a close approximation of quantities if you tell them the desired span, rise, and length of the arch for your kiln.
Sometimes, the center brick or “key” brick may have to be cut to achieve a tight arch. Never cut a brick less than 7z of its original thickness for the “key” brick to maintain sufficient strength.
After completing each ring, reposition the form and continue as before until completion.
Figure 8 shows a half circle constructed of arch brick. Again, typically, two sizes are necessary for most arches.
A point to consider is that the thicker the arch, the more structurally stable it is. This assumes that good bricklaying techniques have been employed throughout the kiln construction.
Many people try to save a few dollars by choosing a four 4.5″ arch thickness, whereas a 6″ or 9″ thick arch will better serve you in the long term.
Final Thoughts:
- Use extreme care when considering refractories used for building kilns. In most cases/ one doesn’t know the conditions under which they were exposed. If they have been removed from a furnace, it is usually because the lining failed. Properties of refractories deteriorate with exposure to extreme heat, chemical vapors, mechanical stress, and thermal cycling,
- Ensure that the flu opening from the kiln to the chimney and the chimney’s size and height are sufficient for the volume of the kiln. Error on a larger size and control with dampers,
- Just as an automobile needs routine maintenance, so does a kiln. If gas or oil is fired, disassemble the burners periodically and clean them for maximum performance. Large cracks should be repaired with refractory mortar or plastic. Dipping pieces of ceramic fiber in a mortar before sealing cracks is also an effective patch.
- Increasing the amount of insulation in a kiln lining allows it to be fired more economically and reduces the tendency for cold spots.
- Before you purchase an electric or gas kiln, investigate the possibility of building your own. The refractory cost is modest, and the reward is priceless.
Acknowledgements: The author wishes to recognize and thank Harbison-Walker Refractories and W. J. Wunch for permission to utilize these illustrations.