BASIC PRINCIPLES OF KILN FIRING CERAMICS
This is not an easy topic to cover briefly, so it will be necessary to break it down into several components.
Types of kilns
There are a wide range of kilns on the market today as well as many of the older ones that are still popular.
With technology, many new ones have controls that have become computerized and you need to have an instruction manual handy to operate them. For the sake of this discussion, we will primarily address the how-to’s of firing rather than how to operate a kiln.
A kiln is an oven, used to bake ceramic and clay items to a temperature high enough to reach their desired maturity and hardness. Clay bodies and paints have been formulated to mature at different temperatures. Most common clays are categorized as low-fire (cone 022 – 01) or high-fire (cone 1-10). Kilns are also used for other firing purposes such as working with glass or metals.
As to types of kilns, you have wood fueled, gas or propane, and electric. There are kilns that are round, square or oval-large and small. Kilns can open with lids on the top or front or can lift entirely leaving the bottom in place. Each design is a matter of preference and sometimes purpose. There are kilns best designed for glass, clay, raku, metals, dolls, special effects, high fire, low fire and combinations. Most popular is the hobby kiln, which can fire the entire range electrically. Older kilns are operated manually with cones and witness cones; or equipped with a pyrometer. Not-so-old kilns come equipped with a kiln sitter with or without a back up timer. Many newer kilns have computerized controls.
With the exception of the computer controls, a kiln is a very simple piece of equipment and very little can go wrong. They can last for years with minimal maintenance. Primarily, the only thing to go wrong with the older, manual kilns is burning out elements or replacing broken fire brick. Kilns with sitters and timers may need to have the sitter replaced or adjusted periodically. Timers rarely need to be replaced, but it can happen,
Firing Temperatures and Cones
Firing temperatures are defined by cones with numbers progressing on a scale from 022 to 10 – 022 being the coolest and 10 being the hottest. As you can see from the following chart, each cone has a small temperature difference; however each cone can make a big difference in the final result. Temperatures in the following chart are Fahrenheit
Cone 022 1085 Cone 021 1116 Cone 020 1157 Cone 019 1234 Cone 018 1285 Cone 017 1341 Cone 016 1407 Cone 015 1454 Cone 014 1533 Cone 013 1596 Cone 012 1591 Cone 011 1627 Cone 010 1629 Cone 09 1679 Cone 08 1733 Cone 07 1783 Cone 06 1816 Cone 05 1888 Cone 04 1922 Cone 03 1987 Cone 02 2014 Cone 01 2043 Cone 1 2077 Cone 2 2088 Cone 3 2106 Cone 4 2134 Cone 5 2151 Cone 6 2194 Cone 7 2219 Cone 8 2257 Cone 9 2300
Cone 10 2345
It is critical when referring to cones, that you clearly specify the zero in the number when firing low-fire material. As you can see in the chart, the difference between cone 010 and 10 is over 700 degrees. It can be a costly mistake to use the wrong cone in firing.
Cones are slender pyramid shaped rods that have been used to control firing temperature since late 1800’s. Prior to then, it was a matter of educated guessing and timing. Cones are designed to measure the heat absorbed and are formulated to melt at specific degrees. They are also affected by time as well as temperature.
Manual kilns use self-supporting shelf cones and witness cones. This requires that three cones be placed within view of the peep hole. If you are wanting to fire an item to cone 04, you will place three large cones on the shelf (a cone 05 – the witness cone, cone 04 and a cone 03-the guard cone). You will need to keep a close eye on the cones. As the witness cone (05) starts to bend you will be within a few minutes of finishing the firing process. The witness cone will bend over about 90 degrees (drawn by gravity) and the cone 04 will start to bend. Further firing you will see the witness cone bend completely over and the 04 will bend to about 90 degrees. The kiln should be shut off before the third cone, 03, barely starts to bend. The amount of time between each cone bending can be as few as 10-15 minutes. Though it has not bent, the third, guard cone should not be reused.
Kilns equipped with a pyrometer only need to use self-supporting cones on occasion to test the accuracy of the pyrometer. A pyrometer is a thermometer that is inserted in the kiln to measure the heat.
Kilns equipped with a cone sitter or kiln sitter use the smaller cones or rods. These cones are placed between prongs which hold the cone in place and one drops when the cone bends and thereby releases a lever to automatically shut off the kiln. It is not necessary to use more than the one cone. Even with the kiln sitter, you should keep track of your time as a back up to prevent over firing or misfiring the kiln because of a faulty sitter.
When the firing chamber cools, it is a good practice to examine the spent cone and see if it has bent properly. It is desired that the rate of bend be at a 90 degree angle. If the cone has not bent that far, it is under-fired. If it is bent more than 90 degrees, it is over-fired. Sometimes the kiln sitter can be adjusted. Other times, the sitter will need to be replaced. It is a good idea to put kiln wash on the two immovable prongs, NOT the one that drops. These two stationary prongs may also need to be replaced occasionally.
If a kiln over fires, it is most usually the kiln sitter at fault. If a kiln under fires, it is most usually a bad or broken element, but can also be the sitter. In both cases, however, it is a good idea to adjust the sitter and test it before continuing use of the kiln.
Prepping your kiln to fire
It is always a good idea to test fire your kiln and make sure your sitter is operating properly. To do this, fire an empty kiln at a low temperature – cone 018. There is no need to go through the warm-up process. Put your kiln on high, close the lid and after the kiln has finished firing and cooled, look at the cone and the angle of the bend. Adjust your sitter as needed.
There is a small plate on the drop flange of the sitter that shuts off the kiln. It is a good idea to make a scratch or mark as to where the plate is set. Loosen the screw holding the plate and move it up or down just a bare fraction. Extending it will adjust the time to be longer, shortening it will adjust the time to less. Test fire again and make any further adjustments as needed until the cone bends to the proper angle.
After you are sure your kiln is calibrated, prep your shelves, and posts by painting on one to two coats of kiln wash on one side of the shelves and both ends of the posts (also put wash on the two stationary prongs of the sitter). Also paint two coats of wash on the bottom of the kiln. Do NOT put wash on the sides or lid of your kiln.
It is recommended that you place a shelf with ½ inch or 1 inch posts under it, on the bottom of your kiln. This wash will help you in case there are glaze drips, you forget to stilt a glazed item or otherwise have something stick to your shelves. Never say never – it can happen.
Firing Greenware
Depending upon your clay body, low-fire or high-fire clay, you will make determination as to what cone or temperature you will need to use to fire your items.
Earthen ware (low-fire clay)
In most cases, you will fire greenware between 03 and 05. The cooler the firing, the softer and more porous the bisque. It is a good idea to know what you are going to do with finishing the item before you fire the greenware. Softer firing accepts stains (non-fired paints) much more easily than harder firing. However, if you are going to be firing items for utility purposes, you will most likely want to fire them to be the hardest.
Greenware should be fired at least one cone hotter than any glazes you will use on them.
To be more specific, there are a couple of classifications for greenware which will help you determine the ideal firing temperature.
Utility – includes kitchenware, vases, canisters, bakeware, boxes, etc. – functional items which are most often glazed or partially glazed.
Decorative – includes figurines, plaques, ornaments, magnets, jewelry, candle holders, shelving, frames, etc. – items which can be done with stains or glazes.
Utility items that are going to be used for food such as kitchenware (dishes, cups, bowls) and bakeware (casserole dishes, cake pans) need to be fired hottest – cone 02. This matures and hardens the bisque to withstand temperatures in microwaves, ovens, etc. It is also recommended for clay bodies that may not be food safe in their own right.
Utility items that are to be glazed such as vases, trinket boxes, canisters, etc., can be fired lower to cone 03 (even 04 in some cases) Be sure any lids are in place.
Decorative items that are to be glazed can be fired at cone 04. Decorative items that are not to be glazed can be fired at cone 04-05.
DO NOT FIRE GLAZE ON GREENWARE.
DO NOT FIRE GLAZES AND GREENWARE IN THE SAME LOAD.
There are exceptions to every rule, however it is better to be safe than sorry.
Put an appropriate cone in your sitter. (If using shelf-cones place them on the shelf level with top peephole) When loading your kiln, it is alright if your items touch; however you do want to allow for adequate circulation of air and ventilation. Make sure your greenware is thoroughly dry before firing. Place your items as they will be when finished, lids on boxes, cups and vases upright, etc. The one exception will be plates, platters and ceramic tiles (things that are flat). They need to be fired on their sides, supported fully so they don’t warp.
Build your load evenly. I usually put smaller items on the bottom and the largest near the top so my shelves are not stacked too high. Make sure there is at least one inch clearance between the ware and the shelf above it or the lid.
When your kiln is loaded, close the lid; however prop it open a couple of inches. Start the kiln on low setting. Let the kiln heat for about one hour (sometimes longer depending upon technique and other special situations). After initial hour, turn up the heat to medium and continue heating for another hour. When the kiln has been on medium for an hour, raise the control to high; close the lid, set your timer (about 15-30 minutes longer than you expect you will need for the load to reach cone). If you have more than one peephole, place a plug in all but one. I would recommend leaving the middle one or the bottom one open.
Until you have enough experience to judge, I would recommend you keep a log showing how full the kiln vs. the cone setting and the time it takes to reach maturity. From this you will be better able to gauge how much back up time you need to put on your timer. If the timer shuts the kiln off, you need to examine your cone and see if it bent fully or if it over fired (bent more than 90 degrees) or under-fired (bent less than 90 degrees). If it under-fired, you will need to re-fire, giving more time allowance on the sitter. If the cone never reaches temperature, you may need to check the elements. If it over-fired, you will want to reduce the time allowance in future and may also need to re-calibrate the sitter plate.
Allow your kiln to cool completely before opening to reduce the possibility of stress cracks (thermal-shock).
High-fire clays, porcelain, stoneware, etc.
Refer to your packaging as to the firing range you will need to use to reach maturity. Most porcelains and stoneware will mature at cone 6. Some pottery clays and stoneware will go as high as cone 8-10.
Firing glazes
When you fire your items from this point on, it is important that you read labels of your paints and glazes. You do not want to fire things together that require different cones. Colors and special effects are controlled by heat and firing at wrong temperatures will change the results. You will fire at the lowest common temperature. Example: a glaze with a firing range 04-06 and one that has a firing range 05-06 can only be fired at cone 05-06. If you add to the mix a glaze that requires to be fired at cone 05 then all of the examples can safely be fired to cone 05.
In most cases, your glazes should be fired at least one cone cooler than the bisque firing with the exception of some special glazes and to create certain effects.
Glazed pieces that are not dry-footed, need to be stilted so they will not fuse to the shelf during the firing process. Use a large enough stilt to balance your item without risk of it tipping. If you are firing items that are to be dry-footed, please examine them and make sure the glaze is far enough away from the bottom so any flow will not seep over to the shelf. This will depend upon the type of glaze and the thickness of application.
You should allow for about one inch spacing on all sides of the items to allow for air circulation and ventilation. If they are too close, there is a possibility they can reach out and grab each other during the firing and fuse together. When heated, your items expand and the glazes bubble.
Some glazes are not compatible to be fired together. This information should be on the bottles/containers. An example is that some reds and greens should not be fired in the same kiln/load as the fumes will contaminate each other.
Follow the same firing process as with greenware. Prop lid, heat one hour on low, one hour on medium, turn to high, close lid, close all but one peep hole, set timer.
Again, DO NOT open the kiln until it is completely cool.
Firing Overglazes
The primary difference with firing overglazes is that they fire at a very cool temperature comparatively. Check your labels for firing range of the products. Most often gold is cone 018, lusters 017-018, decals 015-018, china paint 015-022
Because of the fumes, you need to make sure both the kiln and the firing room is well ventilated.
Items do not need to be stilted as they don’t reach hot enough temperature for the glazes to melt. But, to be safe, stilt items at cone 015, or if the gold or luster is on the bottom edge or bottom.
Avoid firing Mother of Pearl and other lusters in the same load as gold. They can be fired together if the gold has been fired first and then add the Mother of
Pearl for a subsequent firing.
Decals will not fire successfully in layers. Fire one layer, add second layer and fire again in order to get the overlapping look.
China paints need multiple firings to deepen the color. Reds need lower temp as they fire out more easily.
Overglazes are NOT food safe and should not be used on food surfaces.
Again use the same heating process and do not open the kiln until it has completely cooled.
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