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This the first in a series of tips on firing your kiln. Whether you are new to firing, or experienced, you are sure to find something that will improve your firing results.
Most people do a bisque firing, then a separate glaze firing. We will cover both.
First some general thoughts about firing:
Kiln vents assist greatly in simplifying firing and reducing firing defects. In addition, they improve element life by removing damaging fumes quickly. Of course, if you are firing in a structure where people are working or living, a vent is required to remove the harmful fumes from the area. This is true for bisque firing as well as glaze firing.
You should regularly vacuum out the kiln, including the element grooves.
New kilns, or kilns with new elements, should be fired once to Cone 6 without any ware in the kiln. This correctly oxidizes the elements and ensures a longer life. They may smoke a little due to oil left on the element wire during manufacturing. (Note: I found conflicting information regarding the temperature of this first firing, everything from Cone 04 to Cone 6. But general consensus now is that you should fire to Cone 6 to fully oxidize the elements.)
It is helpful to understand the changes that clay goes through in firing. You can read about this a previous tip: Clay, the Drying and Firing Process
Kilns should be placed at least 12-18" from all walls during firing. Remove all combustible materials from the area around the kiln.
All furniture and ware in the kiln should be placed at least 1” from any element.
The bottom layer of shelving should be on 1/2" - 1" posts. Your ware is placed on this first shelf, posts are set around the perimeter, and another layer of shelving placed. Make sure you leave at least 1/2" between the top of your tallest piece and the next shelf, to allow for expansion during firing. Usually for whole round shelves you will use 3 or 4 posts. For half rounds, you will use 3 posts per side. Posts should be placed over each other as you rise up through the kiln.
Some people believe that if you have half shelves, it is best to stagger their heights to get more even heating. But after talking with many potters I have concluded this is not very important in an electric kiln. It is very important in a gas kiln, and that may be where the practice came from.
Be careful placing and moving your pieces around. I have often broken pieces while moving them slightly around. Instead of grabbing them at their tops, lift them up from their bottoms.
Make sure your pieces are dry. Pieces that are still wet will feel cool when placed on your cheek. Wet ware can blow up in the kiln, as the water inside the clay expands.
Items may be stacked on and inside one another for bisque firings. They won't stick together. However, you can cause problems by doing this.
The carbons may not burn out completely from an area that is covered by another piece, and this may cause defects during the glaze firing.
Some items may break if they are not allowed to expand and contract freely. So if stacking two bowls for example, make sure there is plenty of room between them. Remember that items will shrink during firing.
Stacking may cause more uneven temperatures throughout the kiln.
Some people think the tighter they stuff the bisque load, the better. And you may have success with this method. But other people find that they do better when pieces are given space. My advice is that you can pack a bisque load tighter than a glaze load, but don't overdo it.
Items such as plates and tiles often warp less if bisqued on their side. You can lean them against the side of the kiln.
Fire items with lids in place, to prevent distortion.
Leave at least 1/2" between all pieces. Remember that the piece will expand during the firing cycle before it contracts and shrinks.
If you have a vent, you don't have to worry about colors bleeding from one piece onto another. If you don't fire with a vent this can be a problem, so you might want to segregate colors while loading.
Make sure you have a good layer of kiln wash on the shelf. This is to facilitate the removal of glaze drips. Do not apply kiln wash to the sides of the kiln, or underside of the kiln shelves. The wash is likely to flake off land on your glazed pieces.
Of course, glazed areas cannot be put directly on kiln shelves, or they will fuse with the shelf. If doing low fire, place your pieces on firing stilts. If doing mid to high fire, only glaze within 1/2" of the bottom of your piece (sometimes more if you have a runny glaze.) Wax the bottom of the pieces before glazing to make it easier to remove the excess glaze.
If firing a new glaze you are unsure of, put the whole piece on a piece of bisque you have pre-made. Then if glaze runs, it will run onto this piece instead of ruining your kiln shelves.
Don't forget to add your witness cones while loading. You will want to do this even if you have a kiln sitter or electronic controller, to see what Cone was actually achieved inside the kiln. If you plan to use the cones to determine when to turn off the kiln, make sure they are visible through the peephole. Read Tip 42 for more information on Cones and Cone packs.
Items such as large plates benefit from being in the center of the shelf for even heating. This helps reduce warpage. For more information read Tip 26 on Plate and Platters, or Tip 30 on Making Tiles.
If using an electronic kiln, make sure you place the shelves at heights that avoid the thermocouples. The thermocouple should be at least 1-2 inches from a kiln shelf or ware.
If you have half shelves, it is perfectly acceptable to have them staggered if it fits the size of your pieces. You can even use pieces of broken shelves to add additional mini-shelves.
It is usually easiest to put the tallest pieces on top, so you don't have to use very tall kiln posts.
It is very useful to keep a firing log. Start your log entry by describing how the kiln is loaded (types and sizes of items, density of packing.) You will find that the more densely the kiln is loaded, the longer it will take the kiln to reach the appropriate temperature. You will learn how to adjust for this with your particular kiln. Typically you will add a longer soak at the top temperature when the kiln is more densely packed.
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