How to fire your digital kiln

How To Fire

By Steven Wilburn, ex-fireman for NMclay.com.

Firing your manual kiln

Before loading your kiln be sure that it's not near anything combustible as the exterior becomes very hot. Most kiln companies recommend a 12" space between the kiln and the wall.

Loading your kiln

It is important to have a shelf in the very bottom resting on three 1" posts in a triangular arrangement. This allows air to flow under the ware and give them even heating. If additional shelves are used be sure that the posts supporting them are placed directly above the original 1" posts located under the first shelf in the bottom. This prevents warping or cracking.

Determine what type of firing you will be doing. Clay or greenware being fired for the first time will require what is called "bisque" firing. This will remove carbon and other impurities that can cause problems in glazes. It also hardens the ware so that it can be handled and glazed more safely. Place your pieces in the kiln according to like sizes in order to increase efficiency of space. It is a good idea to put shorter pieces at the bottom and taller pieces at the top because tall posts tend to fall over easily, sometimes breaking things. Arrange your artwork evenly in the kiln to allow airflow to pass between everything, be especially careful not to have glazed pieces touching each other or they will stick together. Glazed ware needs to be at least 1/4" apart as when they are heated they can expand and touch. It is a good idea when firing glazes to have a thin layer of kiln wash on the shelves so that if glaze drips on them it can be easily removed. Also remember to put stilts under pieces that are glazed on the bottom to avoid sticking; this can only be done in low fire. Stoneware or porcelain pieces need to be dry footed (no glaze on bottom). Always be sure to use the proper cone for the type of firing you are doing and avoid the confusion commonly associated with cone 06 (low fire) and cone 6 (high fire). Always read labels on products. Manuel kilns require a bar cone in the sitter so that the kiln will shut off at a specified temperature, this temperature is determined by a cone number marked on the side of the cone (cone 06 approx.1830F,cone 04 approx.1940F, cone 6 approx.2232F). Digital kilns don't require a bar cone but it is important to use witness cones to determine if the kiln fired correctly. Slip cast ware or relatively thin walled hand built items can be fired on a medium to fast speed schedule, thicker pieces should be fired slowly. Digital controllers have pre-programmed schedules for different speeds (Fast-Fst, Medium-Med, Slow-SLO)

A good schedule for firing pieces that are thin:

(1/8") to medium (1/4") in a manual kiln is- 1. turn switch(s) to low for one hour, 2.go to medium for another hour, 3.then to high where it will stay until firing is done. After the firing is complete let the kiln cool till it is nearly room temperature, lifting the lid too early can cause cracking or damage to the kiln. If an explosion occurs during the firing, (usually due to air pockets or moisture), it is important to remove any fragments that may have fallen into the element grooves. Clay or glaze touching the elements will cause them to burn out. Sometimes an explosion can cause the sitter to get jammed, if this occurs the kiln will not turn off and the contents will melt down (total disaster). Don't be discouraged, many kilns come with a built-in safety timer that you can set for the estimated firing time, this will vary depending on the size of kiln and the density of the load. When the timer reaches 0 the power to the kiln will shut off. It is important to set the timer for more time than the firing takes, if the timer turns the kiln off before the sitter does, than the firing has not been completed. If this happens you can simply re-fire the load being sure the timer is set for more time than before. If your kiln doesn't have a timer you should check it when it should be turning off. If it goes a whole hour past the it's usual time then turn it off by hand and examine the ware and the sitter when it cools off. This is another good reason for using witness cones. Make records of your firings so you can get to know your kiln better, if you notice the results changing than it may need an adjustment or new elements.

Firing thick walled pieces or sculptures.

Thicker, heavier things need to be fired slow, especially in the earlier stages of the firing. As heat is applied to the ware moisture begins to escape, no matter how long a piece has been drying there still is water locked into the molecules of the clay. If the water is not gone when the clay reaches 212F (the boiling point of water) it will turn to steam and the clay will blow up. On a manual kiln the schedule will be similar to the one above except you'll want to keep it on low for two to three hours and medium for about two hours. You may even prop the lid up with a brick or post about 3" high till its ready to go to high, and then close the lid. If you are near the kiln during the firing and you hear something like the faint sounds of shoveled earth, most likely that was your stuff being blown to smithereens. Don't let this discourage you, its all just part of the learning process. Digital kilns have a slow program but sometimes it's not slow enough for some items. You may program a custom firing of your own that will be stored in the memory to be used when ever needed. Most digital controllers have similar features, What you'll need to look for is the Ramp/Hold button. Press Ramp/Hold and it will display USEr 1(most controllers can hold up to 6 custom programs) If this is your first program press Enter. Now it asks how many segments of the program you will need SEGS (most controllers can store as many as 8 segments) the schedule we recommend only uses 4.

The following program has been successful for us:

Heavy sculptural firing program:

1.USEr 1, Enter, SEGS-4, Enter, rA 1-180, Enter,F1-195,Enter,HLd1-4:00,Enter rA 2-150, Enter, F2-450, Enter,HLd 2-2:00, Enter,rA3-300, Enter, F3-1000, Enter, HLd3-0, Enter, rA4-780, Enter, F4-Ending temperature(see cone chart) HLd4-0. This firing could take 12 to 14 hours to complete. When completed the display should read CPLt and flash the amount of time the firing took. Press enter or stop and the display should read the current temperature inside the kiln. If you want to put in another custom program press Ramp/Hold and USEr2. (When the display says USEr its asking what program you want to use). Keep a record of all your custom programs.(USEr1,USEr2,USEr3 etc.) When the display says rA it is asking what ramp or rate of temperature climb you want and when it says HLd it's asking how long you want to hold at that temperature. Each segment (SEGS) has a ramp and a hold.