Pottery and Ceramics Tools
Choosing The Perfect Kiln
Your pottery and ceramics kiln will be with you for a long time, that is why it is so important to select the model that will meet your needs now and in the future.
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What are the dimensions of the largest piece you will want to fire?
OUTSIDE DIMENSIONS16 SIDED 62" - 36"W
12 SIDED 62" - 34"W
10 SIDED 62" - 29"W
8 SIDED 62" - 23"W
7 SIDED 62" - 20"W
6 SIDED 62" - 17"W
Calculating Kiln VolumeTo determine the amount of heat BTUs required for your kiln to reach a desired temperature in a certain time frame, you need to know your kiln's volume.
To calculate the volume, measure and record its height, width and depth.
When dealing with a flat top kiln with no arches or wall curves, simply multiply these measurements together to find out the volume.
Kilns with arches or round kilns are a bit more complicated, yet each volume is still the product of a simple mathematical formula:
Volume of a sprung arch kilnVolume = Width x Depth x (height of the side wall + 2/3 of the rise)
Volume of a catenary kilnVolume = Depth x Arch Area (4/3 height x 1/2 base width)
Volume of a barrel kilnVolume = pi x r2 x Height (r2 [radius] is the diameter divided by 2 then squared or multiplied by itself; pi = 3.14
Volume of a flat top kilnWidth x Height x Depth
For example: We have a sprung arch kiln with a width of 30", a depth of 30", and a 25" tall wall with a 5" arch rise.
The rise is from the peak to the top of the wall where the arch starts.
Multiply the rise 5" by .66. This equals 3.3.
Add the 3.3 to the 25".
This now gives us our revised height measurement 28.3 and the three dimensions we use for multiplication: 30 x 30 x 28.3 = 25,470 cubic inches.
For cubic feet, divide cubic inches by 1728. Our cu. in. number 25,470 divided by 1728 = 14.74 cu. ft.
Once you've determined your kiln volume, you can figure out how many BTUs are necessary to fire your kiln to a certain cone or temperature over a specified amount of time.
Three variables dictate the input for your kiln: the desired end temperature, the material the kiln is made of, and the time it will take to reach your end temperature.
There are other ways to determine BTU input, such as weighing the entire load, but I feel this is impractical for a studio setting. It is much easier to multiply your interior volume by a certain amount of BTU input, in other words, BTUs per cubic feet.
To fire to cone 06, you'll obviously need fewer BTUs per cubic feet than for firing to cone 10, and there's the time factor.
For a raku firing to cone 06 in 15 minutes, you'll need perhaps 2 to 3 times the BTU per cubic foot input of a stoneware kiln.
Below is a chart showing BTU per cubic foot necessary to reach temperature in a kiln used for other than raku.
Multiply your kiln's cubic feet by these figures to determine the total BTU input your kiln will need.
Below is a range of BTU per cubic feet and this range represents time.
The higher number will give you a firing of between 6 to 8 hours, while the lower BTU per cubic foot figure will give you a 14 to 16 hour firing.
Somewhere in between means a 10 to 12 hour firing.
BTUsBTU - British Thermal Units is a measure of heat.
1 BTU is the amount of heat required to raise one pound of water one degree. Burners are rated in BTU to specify the amount of heat that they release per hour, kilns are rated in BTU to show the total amount of heat per hour that they are capable of.
A 1,000,000 BTU kiln will have twice the heating power of a 500,000 BTU kiln and would be capable of heating the same load much faster.
1 BTU = 252.0 calories or = 1055 Joules.
Joule - is a measure of heat similar to BTU. 4186 Joules = 1,000 calories
BTUs per Cubic FootMaterials Cone 06 Cone 6 Cone 10
9" soft brick 6,000-10,000 8,000-13,000 10,000-16,000
9" hard brick 12,000-17,000 14,000-18,500 16,000-20,000
6" ceramic fiber 4,000- 6,000 6,000- 9,000 7,000-11,000
BTUs per Cubic Foot for Raku KilnsMaterials BTUs/cu. ft.
4 1/2" hard brick 70,000
2 1/2" soft brick 35,000
4 1/2" soft brick 30,000
1" ceramic fiber 25,000
2" ceramic fiber 20,000
Let's say, you have a 30 cubic foot soft brick kiln that you want to fire to cone 10 in 16 hours.
Multiply 30 by 10,000 for your answer of 300,000 BTUs.
As I said before, because of its rapid firing schedules, a raku kiln needs a lot higher BTU/CF figures than regular studio kilns.
The figures below will give you cycle times of 20 to 30 minutes.
These are simple numbers that assume the kiln is well built and well designed.
There are plenty of other variables that can affect time/temperature performance. These numbers are a guide, not a guarantee!
Once you've multiplied your cubic feet by the numbers above, you'll have total BTU input.
Divide this number by the number of burners you want to use and you'll have the necessary BTU output per burner.
YESany kiln that is located in a room where people are, should definitely be vented to the outside for safety reasons. It is also a good idea in rooms that are attached to living spaces, such as an attached garage.
But beyond that, a vent makes it much easier to fire the kiln and will give you better results.
Safety and ComfortRemoving fumes from your work environment is important, since these are unhealthy to breathe on a regular basis.
All products fired in electric kilns contain organic materials that make carbon monoxide and other fumes when they are burned.
These include clays, glazes, decorating products, decals and the like.
Fumes released may contain sulfur oxides, hydrogen fluoride and metal vapors, depending on the products fired.
Carbon monoxide can cause headaches and nausea.
Other fumes can also potentially cause health problems.
Glazes may contain chemicals such as lead and manganese which are not safe, at least in large quantities or on a regular basis.
Sometimes you may actually notice a smell when you fire your kiln.
Usually this comes from organics burning out of the clay.
And if you use newspaper or wood or other structural supports which burn out in the kiln, the smells will be very pronounced.
Ease of FiringIf you don't use a vent, you have to prop the lid of the kiln at the early stages of bisque firing to allow the moisture to escape.
Since most kilns now are sold with electronic controllers, having to manually do this step kind of defeats the purpose of having an automatic kiln.
It is much easier to start the vent when you start the kiln and not have to remember to close the lid after a few hours.
Improving your Fired ProductGlazes work best when the organics have been burned out of your clay at the bisque stage.
The best way to ensure that is to do a slow firing with a lot of air flow.
The vent helps remove the organics as they are burned out.
You will usually find you have fewer glaze defects when you have fired your bisque slowly, with a vent.
Glaze colors can also be affected by these fumes.
Carbon monoxide and other fumes can affect the color and properties of the fired product if they are not removed from the kiln earlier in the firing.
Red, green, gold and similar products are particularly affected by carbon monoxide at temperatures where the color develops.
Sometimes colors that are fired close to each other will affect each other by cross contamination.
This is significantly reduced with a vent.
Many bright glazes, like reds, need a lot of oxygen to develop their color.
One customer used a lot of reds and was often having bad results where the glazes would came out pink.
He added a vent to his kiln and his reds became very bright and predictable.
Extending the Life of Your KilnThe same fumes that are harmful to you can also cause corrosion on your elements, kiln sitter parts and thermocouples.
Pulling those fumes out of the kiln will reduce the wear and tear to those parts.
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