Firing your kiln is really simple, but there are so many variables and factors that come into play that one should not take anything for granted.
There are many types of kilns and several types of firing processes. Here we will primarily address the basics of firing and address specific types and situations in separate articles.
Newer kilns have computerized digital controls which is pretty much just a matter of setting and forgetting. These will be discussed another time.
Older kiln styles are either manual or come equipped with kiln sitters and/or timers. Some people with manual kilns will use Guide Cones and/or pyrometers to help control temperatures. A timer is used as a back up to help prevent over firing and meltdowns.
Pyrometers are also used in the place of kiln sitters and timers.
For the sake of this discussion, a kiln equipped with both a sitter and timer will be point of reference
Before loading and starting the kiln, the operator needs to know what the purpose of the firing is. The following are most common firing purposes. Greenware, Glaze, and Overglazes. Within each of these are additional considerations, for example: Greenware - Will the items be used for utility purposes, decorative only, will they be glazed or will they be stained. Glaze – do the instructions tell you what cone to fire and what cone was used to fire the greenware; Overglazes – are you firing gold, decals, lusters, china paints? Each of these things requires different temperatures or cones. (see article about cones)
So let’s begin. The first thing is to prepare your kiln. You should make sure the kiln chamber is free of dust and particles. A shop vac is a worthwhile investment. Next, make sure you have applied one or two coats of Kiln wash to the surface of all shelves, the bottom of the kiln chamber and I recommend the ends of your posts. I also encourage brushing some on the top of the two bottom 'sitter' prongs. (Not the movable rod) Some companies say do this, others say do not. In any case, clean the prongs with a file periodically so there is no buildup on them from the cones. It is important they be kept clean. DO NOT put kiln wash on the sides of your kiln or the lid. You just need to let the application dry, it will cure during its first fire. You only have to do this once, unless for some reason you have to sandblast off glaze drips etc. at some later time.
When not using your shelves stack them so the washed sides are facing each other.
The kiln wash helps to prevent any glazes from fusing to your kiln furniture. If drops of glaze melt on anything, they can be removed a lot more easily than if you don’t use kiln wash. Kiln furniture is expensive and it is well worth the time to make this preparation.
Now your kiln is ready to fire. It is a good idea to test fire your kiln with a fast fire to calibrate that the sitter will shut off at the appropriate time. To do this, all you need to do is put a low temp cone, like 018 in the sitter, turn the empty kiln on high, set the timer for about 1 ½ hour, shut the lid and let the kiln fire until it shuts off. This should take about an hour (depending upon the size of the kiln). When cool, remove the cone and make sure it melted properly. If it is shaped somewhat like an ‘L’, your kiln is calibrated appropriately. If the angle is sharper, more like a “V” or a “U” your kiln is over-firing. If the angle did not reach an “L” shape, then the kiln is under firing. If your kiln is under-firing or over-firing, you will need to calibrate it before attempting to fire any of your ware. (How to calibrate your kiln is another article)
How to set your cone
Modern day cones are small rods about one inch in length. These rods have either three or four sides. In the case of the four sided or square rod, it does not matter how you place it in your sitter. In the case of the three sided or triangle rod, it does make a difference. If you examine the sides you will see one of them is ever so slightly wider than the other two. This is the base and it goes on the bottom. (Note: there may be some brands who have equalized all sides, so it won’t matter)
Place the cone between the prongs of the sitter, with the movable prong/rod on top, the two stationary prongs on the bottom. This can be manipulated internally or externally.
Steps to Firing a Kiln (basic process)
Place cone in sitter
Load Kiln
Lower lid, propping open 3-4 inches
Set timer for ½ more than estimated firing time
Turn controls to low for an hour to ninety minutes
Turn controls to medium for an hour to ninety minutes
Close lid completely
Plug one or more peep holes (Leaving one open)
Verify timer has ½ hour more time than estimated
Allow kiln to cool completely before opening
Examine cone after unloading the kiln
Examine time remaining on timer
Charting is a great idea
Loading your kiln
It is not a good idea to load your items on the bottom of your kiln, you should use some short stilts and place one shelf over the bottom and then build up your shelves to heights needed.
Try to load your kiln evenly, arranging items of the same height on each shelf.
Shelves should be placed so that at least one kiln element is in between them in order to make sure heat is evenly distributed.
You should leave about one inch of head room between your tallest item and the shelf above it.
It is a good idea to put smaller items on the bottom shelf and the very top shelf with the larger/medium sized items in the middle. Of course, it is not a good idea to stack space bars too high as they become unstable, so if you have very large pieces, they should be on the top.
Do not over crowd pieces, allow space so that the heat/air may flow freely around them. Unpainted greeware can touch if needed and you can loosely pile small items like ornaments or magnets. Glazes of course, should not touch and you should leave about 1 inch of space between them so they do not grab each other during the firing. Your glazes will bubble up and if the touch each other, they can pull each other off their stilts.
Firing Greenware (NOT PORCELAIN OR STONEWARE)
Knowing ahead of time what your greenware will be used for, is important so you will know what cone you will fire the kiln. Utility pieces such as cups, dishes, bakeware, etc. should be fired at a hotter temperature to give them maximum durability. If a cup is fired too low and then eventually put in a microwave, it can break, same with pieces you may use to bake with. It is most commonly recommended that utility pieces as mentioned be fired to cone 02. This will make them less porous and stronger. Items that will be glazed will need to be fired at least one cone, preferably two cones hotter than the glaze firing unless the glaze instructions direct other wise. This is necessary so that the ware will shrink enough during its initial greenware firing that it won’t cause stress during the glaze firing. Items that will be stained can be fired at cooler temperatures as low as 05-07 and they will produce a softer, more porous surface.
As a rule of thumb, I recommend that greenware for all utility pieces be fired at cone 02 and all other pieces be greenware fired to cone 04. This allows the operator to load the kiln full and not have to break out every item into many smaller loads. However, the artist/ceramist may have specific reasons and desire to fire at a specific temperature and their wishes should be accommodated if at all possible. At cone 04 the ceramist can choose to use glazes or stains and know that their item will be compatible to either.
In most cases, greenware that has underglazes applied, can be fired with any other greenware as long as they do not touch each other. Greenware that has no underglazes can be touching each other and even layered lightly. If you use any underglazes, be sure to read the instructions before firing. Do not over crowd your ware, pack it loosely so that the heat and air can circulate and gases vent out.
Items with lids should be fired with lids in place. The mouth of the container should be big enough that the lid has some movement when in place or they may not come off after the firing. Greenware shrinks when fired, anywhere from 8 – 25 percent depending upon what kind of slip is being used. If you fire items with lids separately, they may not fit afterwards because of shrinkage or warpage from the firing process.
Where possible, items should be fired in the same position they will be used. Example figures, boxes, jars, etc. should be set in the kiln upright and not lying on their sides. There are exceptions to this such as large flat items, platters, trivets, etc. – these may require special attention. Items such as ornaments, round items, etc. can just lie on the shelves loosely.
Firing Glazes
The first thing I want you to do is pay attention to the labels and instructions that come on the jars of glaze. READ THEM
The labels will tell you the recommended firing temperature/cone range. It will tell you if the glazes are compatible with each other. It will tell you if the greenware needs to be fired hotter (and how much hotter) than the glaze.
Some glazes cannot be fired in the same kiln load. Most of the more modern glazes are compatible, but some are not. At one time you could not fire red and green glazes in the same load or the colors would jump. So it is better to be safe than sorry. KNOW YOUR PRODUCT.
Most glazes have a firing range. Some fire at 04-05, some at 04-06, some at 05-06. All of these can be fired in the same kiln load. To do this, you find the lowest common cone. From the examples given, all can be fired at cone 05.
Typically, glazes should be fired between one and two cones cooler than the bisque (fired greenware). Some special effects require they be fired at the same temperatures as the greenware.
It is not a good idea to fire glazes and greenware in the same load, though it can be done. It will only take one disaster to stop you from doing it again.
You can fire glaze ON greenware as long as it does not cover more than about 10 percent of the total surface. For example: areas where you plan on applying fired gold to accent a piece that you are planning on staining. Please do not glaze the interior of items in greenware, this can only result in crazing. You may not see it right away, but over time, your glaze surface will develop hairline cracks.
Once you know what temperature an item needs to be fired at, the rest is simple. Just follow the same basic steps to load and fire your kiln.
With glazes, you will need to use stilts under your pieces. Stilts are ceramic bases that have teeth or prongs on them that are used to support glazed items during the firing process so that they will not fuse to the shelving. The ceramic base goes on the shelf with the pins up. The glazed item sits on top of the pins.
It is more important with glazes than with greenware, that the kiln is not open until fully cooled. Premature opening can cause great stress on the ware and can create crazes/cracks in the glazes and can even cause the item to break from thermal shock.
WARNING: Stilt marks can leave glaze shards that are VERY sharp. Handle carefully and sand them off to prevent future cuts or scratches.
Firing Overglazes
Overglazes are fired at the coolest temperatures - ranging from 015-022. If you are adding multiple overglazes, you will fire the hotter ones first.
DO NOT fire gold and mother of pearl at the same time. These two overglazes create gases that are not compatible. If you want both on your piece, fire one first and then the other.
DO NOT put gold or decals or mother of pearl on the bottoms of your items
Overglazed items do not need to be stilted.
Make sure kiln is cooled completely before opening. The more often you fire an item, the greater risk that the item will suffer thermal shock, so the more care should be given to the cooling process in order to lower the risk.
Do not plug peepholes during overglaze firings. Make sure the firing room or kiln is well ventilated. It is inadvisable to be in the room during the firing process.
