PPP: An Ounce of Prevention

March 1, 2010
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Preventive kiln maintenance and care helps enable these valuable pieces of equipment to provide many years of reliable service.

Having a preventive maintenance and care plan for your electric kiln or kilns will help enable these valuable pieces of equipment to provide many years of reliable service and production. A kiln that is not set up or located in a climate-controlled environment (heated and cooled), or a kiln that is not well-vented, will require much more care than one that is in a climate-controlled location. This applies to both manual and computerized kilns.

Humidity, dampness, high heat and cold temperatures increase corrosion, oxidation and the rusting of electrical connections on switches, KilnSitterTM components, the electronic circuit board, relays, power blocks and hardware. In addition, not having a kiln fume vent or kiln room vent reduces heating element life and compounds corrosion to the kiln's components.

Heavy producers should plan to do preventive maintenance check-ups at the beginning and middle of each year. Others should do a check-up at least once a year. Most preventive kiln care takes approximately 1-11/2 hours per kiln. Repairs require more time, depending on the extent of the problem and what remedies are required.

Figure 1. Check the main power cord plug and receptacle, looking for signs of melting or burning.

Electrical

Disconnect the power before doing anything to the kiln. Start by checking the main power cord plug and receptacle (see Figure 1). Sometimes, the cord will touch and melt to the side of kiln, so look for signs of melting or burning. If the kiln is direct-wired, check the wire connection from the disconnect box to the kiln. A repair will be required if the connection is melted or burnt; consult an electrician, if necessary.

The next step is to open the switch boxes. You will need a good National Institute of Occupational Safety and Health (NIOSH)-rated dust mask while cleaning the kiln. Clean dust and dirt by vacuuming, or use a can of compressed air to blow out dust (use caution not to spray liquid from the can). Use a dry 1-in. paint brush to help loosen dust and dirt.

Inspect all wiring connections and power cord connections at the KilnSitter or connection block. Check wires to switches, element connections, relays and any other wire connections to make sure that everything is clean and tight-fitting. Clean any corroded connections with a small wire brush or emory cloth. Replace any suspicious wires and connections if they appear melted or burnt. Use caution when working with the connections to the elements; the element pigtail is brittle and can break off.

Figure 2. If the KilnSitter assembly appears to be damaged, it should be replaced.

Kiln Control

Next, check the KilnSitter tube assembly for damage and corrosion, or a bent or worn rod (see Figure 2). Rods can exhibit signs of pencil pointing or bending over time from exposure to high temperatures; replace the assembly if the rod appears diminished. If the cone supports appear bent or worn, they should be replaced. (A new assembly will come complete with cone supports and a sensing rod.)

Periodically (about every 20 firings) check the calibration alignment with a firing gauge on your tube assembly. (Consult your manual for the proper setting.) Add a small touch of kiln wash to the top of the cone supports and the underside of the sensing rod (where it rests on the firing cone) to prevent the cone from sticking.

If you have a computerized kiln, check the thermocouple(s). If a thermocouple is bent, cracked, has a split weld-on tip or shows signs of other damage, it should be replaced. Also, check and clean the thermocouple protection tubes for metal powdering. Some kilns may be equipped with both a KilnSitter and a computerized controller, so both should be checked and serviced.

Reattach the boxes on the kiln and replace any rusted or loose screws with new stainless sheet metal screws.

Figure 3. If an element is bulging out or sagging, it can be heated with a plumber's torch to red hot. While wearing kiln gloves, use a pair of needle-nose pliers to compress the wire back into the brick.

Kiln Chamber

The next step is to check inside the kiln chamber. If dirt is present, a good vacuuming with a HEPA fine-particle filter is needed. Vacuuming should be done periodically or when any sign of dirt is visible, since dirt in the element track can damage the elements and the brick. When vacuuming, use a crevice nozzle to clean the element tracks. Don't put too much pressure on the bricks or elements. If you are using a downdraft fume vent system, check the air holes and ensure that they're free of any blockages.

If glaze drips or melts exist, use a knife, dental tool or pick to remove them. It's best to remove drips right away because they continue to melt during each firing and will cause more damage to the brick. Don't worry about the small divots left from picking out the melts. Large spots can be filled with kiln cement, which is available at most suppliers and cures in less than 24 hours.

If the elements are bulging out of their tracks, don't try to bend or apply pressure because elements are brittle and will break (see Figure 3). If an element breaks, it needs to be replaced. To repair an element that is bulging out or sagging, heat it with a plumber's torch to red hot. While wearing kiln gloves, use a pair of needle-nose pliers to compress the wire back into the brick. (Do not attempt this repair if you are not experienced or confident.) Add element pins, if necessary, to help keep the element in its place.

Broken brick pieces can be cemented back in place with kiln cement; this is especially important for any brick pieces that support the elements. It's best to replace any damaged brick when an element change is necessary. Cracks in the kiln floor are normal and occur with expansion and contraction over time. Check the tightening band for tightness. If it's loose, hold it with pliers at the band clamps and tighten it with a screwdriver.

Figure 4. Kiln elements wear out and weaken over time.

Kiln elements wear out and weaken over time, and this occurs more rapidly when firing at higher temperatures, with more frequent use and in cases where a fume vent system is not used (see Figure 4). Signs of element wear include longer firing times; the kiln's inability to reach temperature; collapsed, bunched or stretched element shape; and dark blacking or burning. An approximate average for element life is about 120 combined firings at cone seven or less. If firing times increase to around 2 hours or more, you will be wasting energy and adding additional wear to the kiln's electrical components.

Test elements with an ohms meter and reference the manufacturer's specifications to make sure they fall in an acceptable range. Keep a firing log book and compare firing times. Count the total number of combined bisque and glaze firings.

When changing elements, it is best to replace a complete set for better balance of heat-work throughout the kiln, especially if the elements are a year old or more. Some kilns use element pins to hold elements in the track. Pay close attention to where they are positioned and remove all of them, taking care not to break out any brick. You should get new pins with your elements when purchased.

Clean and vacuum the element tracks and scrape out any dirt that may contaminate the new elements.

Finishing Touches

Check the kiln's hardware for tightness. Screws usually rust out from inside the case, and rusted, loose or missing screws should be replaced. Check the lid hinges for tightness, and examine the lid supports to make sure they are sound and working. An unsecured lid can be very dangerous. Also check the kiln lid for cracks on the underside and the case bands for tightness. If the bands are loose, tighten the hose clamps with a screwdriver and pliers. Inspect the kiln stand to make sure it is sound and not rusting out or weak.

The next step is to check the kiln fume vent system. The motors of under-the-kiln vents will most likely have collected dust from the floor, though wall-mounted types will need to be cleaned as well. Vacuum the dust and dirt that has built up in the motor air ports. Inspect vent ducts for corrosion and holes, and replace the ductwork, if necessary.

Next, inspect the kiln shelves for cracks and glaze melts. Never use your bare hand to brush or wipe a kiln shelf, as the glaze and shards are very sharp. Scrape and chisel any spots or loose kiln wash. After cleaning the shelves, apply fresh kiln wash to protect the shelves and help keep replacement costs in line. The kiln shelf support posts should also be inspected for glaze drips and replaced, if necessary. Inspect and replace broken or missing peep plugs.

Worth the Effort

Some people say, "If it ain't broke, don't fix it." For production potters who value and rely on their kilns, the opposite is true. Establishing and following a kiln maintenance and preventive care schedule will help keep you firing.

For more information regarding kiln maintenance and care, contact The Kiln Doctor, Inc., 202 East Main St., Front Royal, VA 22630; (877) 545-6362; e-mail mike@thekilndoctor.com; or visit www.thekilndoctor.com.

SIDEBAR: General Precautions

  • Do not use kiln shelf supports to prop a kiln lid. Their sharp edges will cut into and damage the lid, as well as the top row of brick. Instead, use a soft brick cut into a wedge or add a lid prop kit.
  • Use a full-size kiln shelf as a foundation shelf raised a minimum of 1/2 to 1 in. above the kiln floor to allow an air space.
  • Spacing around the kiln should be a minimum of 12 to 18 in., depending on the wall material. The floor where the kiln is located should be cement, woodstove board, or 2 x 12-in. or 2 x 18-in. square landscaping blocks. Do not place the kiln on wood, linoleum or tile floors.
  • Clutter and dust in and around the kiln can pose safety hazards. Do not store anything on or near your kiln. Some fire codes require any combustibles to be 36 in. away from the kiln.
  • If you are a heavy/frequent firer, consider keeping certain wear parts on hand for emergency repairs:spare elements; thermocouples (automated kilns); tube assembly (KilnSitter); kiln cement; relay (automated kilns); high-temperature wire and an assortment of high-temp connectors for your kiln model; an assortment of stainless sheet metal screws for hardware; element pins


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