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Loading & Firing Guidelines

Kiln Installation and Firing Tips

The furnace should not be placed closer than 18" to any wall or vertical surface or under shelving or other projections. The room in which the furnace is operated should be well ventilated. Before plugging the furnace into your outlet or connecting the furnace to a power source, be sure that the toggle switch or electrical timer is in the "off" position. Be certain that your electrical wiring, receptacle, circuit breaker, and fuses are in good condition and adequate for the furnace before connecting. If you are not sure, consult a qualified, licensed electrician. The metal case of the furnace which comes equipped with a cord is grounded through a ground terminal on the plug. Furnaces not equipped with cords must be grounded by a separate grounding wire of a suitable wire gauge to handle the entire amperage possible on the incoming power line. Power should come from a fused disconnect switch in the immediate vicinity of the furnace.

The temperature control indicates the temperature at the tip of the thermocouple. That is the point the two different alloy wires are welded. Therefore, this thermocouple must extend into the furnace 1-1/2" to 2" to obtain a correct temperature reading. You should keep in mind that contamination from products fired may alter its calibration. Change the thermocouple should you doubt the temperature control readings. Pyrometric cones are another inexpensive way to check the control reading.

Care should be taken to keep the furnace away from flammable surfaces. A good rule is to keep the furnace at least 18" or more from all vertical surfaces. Do not use the furnace under a shelf or other obstruction to the flow of air. Keep all flammable liquids out of the room with the furnace. Be sure no curtains or other material that could change position with wind or opening of a door or window can come within an unsafe distance of the furnace.

If the furnace is to be used for a lost wax process, wax burnout or assaying, drill a 1" hole through the center top of the furnace chamber.


LOADING: Unit may be loaded "cold" or "hot". Even though the door safety switch will turn off power to the elements when door is opened, turn off furnace power manually before opening the door. Also avoid the possibility of shock by not touching the heating elements with your hand or tongs.

The load should be placed as near the center of the chamber as possible to give you a uniform temperature. For those operations requiring very accurate temperatures the hearth plate should be raised on ceramic blocks to provide air circulation below the hearth plate.

Also, your load must not touch the thermocouple (2 wires covered by ceramic beads at the rear of the chamber) as this will tend to give you the temperature of the load rather than the air and cause over-heating. Thermocouples should not be bent against the wall, but remain at least 1-1/2" into the chamber.

Loads should not be placed against the walls or elements. Space your load as much as possible for air circulating between parts to give you even heating throughout the chamber.

FIRST TIME USE: The elements are made of a very high temperature alloy wire. If your chamber is rated for 2250oF, you should fire the chamber without a load to 2000oF and allow to cool slowly with the door closed. This does not apply to draw chambers or chambers with a temperature rating of below 2000oF. This allows the elements to achieve a good oxide coating that protects the elements on subsequent firings. The high temperature also stabilizes the wire element to seat into the element plate or groove securely. High temperature wire elements tend to grow when only fired to low temperatures (below 1600oF) and may creep out of the element plate or groove. If after low temperature firings, you notice any tendency of the elements to creep out of the plate or groove, you should again fire the chamber empty to 2000oF. This will also help heal any thin areas of protective oxide coating on the element. This process should also be repeated when new elements are installed.

Turn on the toggle switch or electrical timer. The control should light. If the control does not light, check that the furnace is connected to a live power source. After the control starts indicating temperature, set the control to the desired (set point) temperature. See specific instructions for the control used on your particular model. Check the temperature reading on the control after the furnace has been on for about fifteen minutes. If the temperature has moved up scale go onto the next paragraph. If the temperature reading has not moved up scale, turn the toggle switch or timer "off". Open the door carefully and slowly and carefully feel the interior air temperature (do not touch elements). If the interior is warm, the thermocouple or the control is defective. Contact your dealer or Cress Manufacturing Company. Do not attempt to use the furnace in that condition.

When the furnace approaches the "set point" temperature, the control will start turning the current "on" and "off" to gradually slow the heating rate down to zero as the temperature set point is reached. Except for a 460 volt furnace, the furnace uses efficient solid state silent zero voltage switching relays for this purpose.

Do not heat the furnace to temperatures higher than necessary for your process and not for longer periods than required. The higher the temperature and the longer the time in use the shorter the element life and the thermocouple life.

If the furnace is used at temperatures over 1600oF, the thermocouple and the elements will eventually burn out. The thermocouple should be replaced about once a year in heavy use. The calibration of the thermocouple can be affected by contaminants fired in the furnace. We do not recommend changing heating elements until they burn out or are so worn that they slow the heating time materially.

All controls, switches, and electrical components are subject to failure; therefore, you should check the furnace periodically to be sure it is heating properly.

UNLOADING: Furnace may be unloaded hot or cold. Avoid the possibility of shock by not touching the heating elements with your hand or tongs. The furnace brick may develop cracks due to the heat shock of the cold air if loaded or unloaded when hot.

When opening the door "hot" you should wear protective gloves to prevent burns on hands and arms. If very hot, you should wear a face shield also. Wear welder's goggles to prevent infrared heat from damaging eyes. Do not wear loose clothing that could catch on fire should it come in contact with very hot air or heated furnace parts or heated furnace loads.

Keep your face and eyes as far away from the hot opening as possible. Be sure your tongs or lifting tool has a secure grip on the parts being removed.

Have a temperature resistant surface on which to place parts removed from the furnace.

GENERAL: Parts placed in furnace must be degreased and dry. Oil, paint, wax, or other matter that could give off fumes may coat element plates or bricks with enough conductive material to cause arcing between element coils. Zinc or tin plated articles should not be placed in furnaces exceeding 800o F. The chamber should be cleaned whenever a deposit or oxide or other material collects on floor or walls. Acid or other corrosive particles in room atmosphere will react with metal elements causing them to fail.

Reasonable care should be taken in opening and closing furnace door. If furnace is operating at high temperatures, gloves or other protective material should be used by the operator. The insulating brick will last longer if not subjected to excessive thermal shock (opening the door at high temperature) or allowing door to "bang" open or closed.

Combustible materials must not be placed on or close to the furnace as the heat will build up over a period of time if the air circulation around the furnace is impaired and combustion will result.

Because of the large number of models and modifications required for industrial use, most of our furnace models are not UL listed; however, we use the same materials and safety restrictions as are incorporated in our listed models. You should obtain local listing if required in your area. This is normally a reasonably inexpensive and easily obtainable service.

NOTICE: Please read all instructions and observe the following safety warnings before operating your furnace.

1. Install furnace 18" or more from any wall or combustibles.

2. No flammable liquids, sprays, or gases can be used or stored in the same room as the furnace.

3. Never fire hotter than the furnace rating specified on the furnace or the instruction sheets.

4. Do not fire hotter than the manufacturer's recommendation for any material put into the furnace or permanent damage may result to your furnace.

5. Do not open door until furnace has cooled.

6. When opening a vertically rising door, carefully open the door vertically while holding firmly onto the handle. Carefully test that the door is safely in place before allowing the door to stand open by itself. Do not let the door stay in an open position while the furnace is unattended. Turn off power to the furnace before opening its door. Do not touch heating elements with anything.

7. Do not leave furnace unattended while operating.

8. Never use an extension cord.

9. Operate furnace only in a well ventilated room.

10. Unplug or disconnect power from the furnace before servicing or cleaning.

11. Dangerous Voltage - Do not touch heating elements with anything.

12. Do not touch hot sides of furnace or hot door - Burns may result.

13. Keep children away from furnace at all times.

14. Never store anything under furnace; never lean objects against furnace.

15. Do not store or use flammable liquids or sprays in the same room with your furnace. Do not operate while wearing flammable loose fitting clothes.

16. Do not store or use your furnace outside - keep rain and moisture away from furnace.

17. Do not use furnace if cord or any other part is not in perfect working condition. Replace anything that is not in perfect working condition immediately. Keep furnace unplugged or power source disconnected when the furnace is not in perfect operating condition.

18. Wall receptacle must not be corroded. It must be in perfect operating condition in all respects. Have the wall receptacle changed by a licensed and qualified electrician if any doubt exists that the wall receptacle is in perfect condition. Check for heating of the plug and cord during each furnace or operation even if you believe the wall receptacle and cord are fine. Do not unplug cord by pulling on the cord. Unplug by carefully pulling straight out using only the plug cap itself.

19. Use furnace only with adequate electrical supply - with the correct voltage, amperage and correct fuse size (not too large or small). Be sure the wire size is large enough (avoid aluminum wiring). Do not use a 208 volt furnace on 220 volts.

20. Furnace must be properly grounded.

21. Wear welder goggles when viewing inside furnace to protect your eyes from infrared heat.

22. Avoid breathing fumes from material fired. Do not fire any materials that are toxic or that are not fully explained by manufacturer on all products considered for possible firing. Do not fire any product that is not fully known and properly labeled and known to be safe to be fired.

-Courtesy of Cress Kilns-


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