An electric kiln atmosphere rich in oxygen will make elements, kiln-sitters, and thermocouples last as long as possible. All the materials used in L&L kilns like to be in oxygen. Fumes are generated by carbonaceous materials in clay, china paints and glazes containing oils, glue from decals, and certain glazes and other miscellaneous products. Fumes include carbon monoxide, sulfur oxides, hydrogen fluoride and metal vapors. These fumes are unhealthy and can adversely affect your work. You MUST VENT YOUR KILN if you are doing ceramics.
General Room Ventilation
Your kiln room should be dry and well ventilated. Never operate in an enclosed space unless you have good ventilation. Aside from issues of ventilating the fumes from the firing, the heat build up in an enclosed room could present a significant fire hazard. We recommend room ventilation of at least 10-25 times the cubic feet of the kiln per hour. For example, if a kiln has 10 cubic feet then 250 cubic feet per hour (about 4 cubic feet per minute) should be adequate. Our suggestion is to get a variable speed fan for ambient room ventilation and keep a thermometer on the wall. That way you can vary the ventilation to suit the needs of ambient heat conditions in the room.
For many years people only vented their kilns by propping up the lids for the first part of the ceramic firing and taking out peepholes. You can still do this if you want. However, be sure to have proper room ventilation at least to get rid of the fumes that get vented to the room.
We recommend our VENT-SURE downdraft kiln vent system. This will do most of the venting of the fumes of the kiln, will help cool the kiln, will improve uniformity of firing in the kiln, and will help maintain the oxygen level in the kiln (which is important for certain glaze effects).
With a downdraft vent system air is pulled from tiny holes in the bottom of the kiln, which creates a slight negative pressure in the kiln. Just enough fresh air is drawn into the kiln to continuously replace the air being sucked out.
The heat in the kiln is then forced to move about. The slight downdraft effect of the vent system counteracts the tendency of heat to rise in the kiln (which would otherwise lead to uneven temperatures top to bottom in the kiln). The amount sucked out should not be enough to compromise the rate of temperature climb, but must be enough to suck out all impurities (i.e. carbon, fluorine, water vapor etc.). L&L's Vent-Sure system only requires between one 1/4” hole and four 5/16” holes, depending on the size of the kiln. Too many holes can cause slower firings and a lower maximum temperature. In addition, the vent system duct-work could get too hot, and potentially melt, if there are too many holes. The Bypass Collection box (included with the Vent-Sure vent system) allows to adjust the amount of air being sucked from the kiln. Basically you want it to just vent the fumes. You may need to turn the vent off near the end of the firing especially if you are having a hard time reaching final set point.
One thing to keep in mind about venting at high temperatures is that you are actually venting less air the higher in temperature the kiln goes. This is because the air in the kiln expands with temperature so less molecules of air (which hold the heat) are being removed from the kiln the hotter the kiln gets. Keep in mind that even the best vent systems cannot handle lots of smoke from newspaper or a lot of wax resist, and still let the kiln reach its highest temperature. To be sure you have not created an unsafe situation, you should check the temperature of the flexible duct-work while the kiln is at its maximum temperature. Most flexible aluminum duct-work is rated for at least 350°F, so if it is hotter than the rating you must plug up at least one hole. High Temperature Cement (available from L&L) works well for this. Kao-wool and other high temperature fiber products can work too. However, the fibers may get stuck in the fan motor, and potentially burn it out. Other residue, particularly wax resist, can build up on the fan motor and the inside of the duct-work. A periodic cleaning will help. CAUTION: Be careful if you are doing wax resist. The wax may condense on the inside of the aluminum duct-work and this could be dangerously flammable. Check this periodically if you are doing this.
Holes in the Lid
L&L does not feel it is necessary to put air-intake holes in the lid on sectional L&L kilns, although you can if you prefer. If the kiln is not sectional, or fits together extraordinarily well, you may want to drill air intake holes in the lid. The number and size of these holes should never exceed the number and size of the air exhaust holes.