Power Supply
Be sure to ask what the voltage, amperage, and phase are. A 208v kiln (as used in many schools) is not compatible with residential (240v or 220v) wiring. If your wiring system is not rated high enough for the amperage the kiln will drawn, you could potentially burn down your building. Double-check what you are told by checking on the kiln itself. There should be a panel on the kiln's controller, switch panel, or outer casing giving all the specifications of that kiln.
For more on power and wiring specifics, see Electric Kiln Basics.
Temperature Rating and Reality
The same panel that gives the power supply specifications should also state what temperature the kiln is rated for. For example, the kiln may be rated for cone 10 (2,300 degrees F - 1,260 degrees C). This rating means that the kiln is designed and built to be able to reach that temperature.
Once you know what the kiln is supposed to be able to do, ask the seller what temperature the kiln actually can reach. Older kilns may not be able to reach their rated temperature any longer. If this is the case, the kiln either needs the elements replaced, the outer wiring needs to be refurbished, or there is damage done to the insulating firebricks.
Insulation Depth
Another consideration is the depth of insulation. Many older kilns were made with only 2 inches of firebrick. These kilns are inefficient due to heat leakage and therefore not very economical to fire. And although many kilns are currently made with 2.5 inches of firebrick, if you find one with 3 or 3.5 inches of insulation, it will save you money in the long term.
How Many Times Has the Kiln Been Moved?
Ask if and how many times the kiln has been moved. Every single time a kiln is moved from place to place, there is always danger of damage to the walls, floor, ceiling, lid (or door), elements, and all electrical controllers and monitors.
This is especially important for sectional electric kilns. Besides the above, the electrical joints between sections can also get bent and damaged.
All that potential wear and tear can seriously reduce a kiln's firing range, efficiency, and useful lifetime. Make certain you know how much traveling this kiln has done, so you can get a better idea of wear that may not be evident through a surface inspection.
Check the Metal Skin
Always inspect the metal skin of the kiln. The metal casing is there to protect and keep the firebrick in place. Any dents are cause for concern; they may indicate that the firebrick under the dented area is damaged.
Controllers, Switches, and Kiln Sitters
While examining the outer area of the kiln, also take note of what kind of controls this kiln has. Newer kilns often have a kiln controller, an electronic regulator that controls and automatically will modify the power going into the kiln elements, according to how it is programmed by the user. Older kilns may only have manual switches that you have to turn up yourself according to your firing schedule. All kilns should come with a kiln sitter. Kiln sitters are either the primary or backup method of shutting down the power once the kiln has reached the desired temperature.
Inspect the Firebrick
Thoroughly inspect the firebrick for damage. The areas that are most likely to become damaged with use are around the rim of the kiln's lid or door, and the areas surrounding the grooves where the elements are. Damaged firebrick significantly decreases the kiln's ability to hold heat and can also adversely effect element efficiency.
Firebrick that is crumbling around elements or unable to hold them in place can cause your elements to wear out within a few firings. Very small damaged areas can be repaired with insulating fiber and kiln cement, but if the wear is significant, you may need to replace firebrick. If there is excessive firebrick wear or damage, I would recommend against buying the kiln.
Examine the Elements
Elements have a limited lifespan, and they are expensive to replace. Make sure the elements in the kiln you are considering are in good condition, or else be aware that they may need to be replaced. (If the elements do need to be replaced, be sure the asking price for the kiln reflects that. Replacement sets of elements can cost up to $500.) Old, worn elements tend to sag and often begin to look weathered. The real telltale, however, is the length of time it takes for the kiln to make it to temperature (if it does so).
Interior Cubic Feet
How big is the kiln, in terms of its interior space? Is this kiln the right size to handle your pottery needs? Consider both the size of your individual pieces and also the quantity of your pottery output.
Kiln Furniture
Is the kiln furniture (shelves and posts) included in the sale? If so, check on the furniture included for condition and also for their number and sizes.
User's Manual
Many people loose track of the documentation that comes with equipment, including kilns. It will be of great benefit to you, however, if the seller does have the user's manual for the kiln, kiln controller, or switches. Manual switches are not difficult to manage if you have established a firing schedule, but controllers vary. It is much easier to have the directions on how to program or work with that particular controller.