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The Basics of Firing Electric Kilns

The image that many immediately conjure when envisioning a ceramics artist is of an individual throwing clay or molding a figure, not standing idly in front of a kiln. While firing may not be the most visually exciting part of the ceramic-making process, it is the most critical part, as it ensures the creative energy put forth in all previous stages will be properly preserved in a durable, functional end-product.

Due to the fact that they are readily available and easy to install, electric kilns are the most common way to fire ceramic pieces, so knowing how to operate them effectively is an essential skill for ceramics artists. In order to help you achieve the best results with your electric kiln, we’ve created a guide composed of useful information, tips, and general advice to direct you through the process.

From the moment you acquire your kiln to the instant you reveal your completed pieces, we hope to provide you with the confidence, capability, and knowledge to create the best work you can.

Understanding the Process

Before considering the specifics of operating an electric kiln, it’s important to first understand a few basics about the firing process:

Firing Stages

Typical ceramics firing occurs in two stages: bisque firing and glaze firing. During the first firing - bisque - greenware transforms into a durable, semi-vitrified porous state where it can be handled safely while being glazed and decorated. Carbonaceous materials are also burned out in this phase.

The second firing occurs after the artist has applied glaze to the piece, hence the name, and is typically faster than bisque firings because most of the water has already been driven out of the clay.

How Heat is Transferred in a Kiln

All electric kilns have heating elements, which are essentially pieces of wire designed to resist the passage of electricity. Similar to a stovetop or other home heating appliance (except designed for far higher temperatures), coiled heating elements inside a kiln impede electrical movement and subsequently cause the wire to heat up and radiate heat throughout the kiln’s interior. This radiant heat rises and is absorbed by everything in the kiln.

Pyrometric Cones

Pyrometric cones are devices that measure how much heat is being absorbed inside a kiln. The higher a cone’s number, the higher temperature it responds to, so an artist chooses cones to match the internal temperature they plan to fire to. As the cone nears its peak range, it softens at the tip and begins to bend. This lets the artist know that their work has reached the intended temperature.

Low cone temperatures appear with a 0 in front of the number, such as 018, 019, and 020, while higher cone temperatures are two-digit numbers like 12, 13, and 14. Traditionally, a three-cone system is used with every firing, consisting of a guide cone (numbered one below the target temperature), a firing cone (set at the target temperature), and a guard cone (numbered one above the target temperature).

If you have an automatic controller for your kiln, pyrometric cones are not required. However, cones still provide the best feedback while firing, and their use is suggested at least every three firings in an automatic kiln to ensure your kiln temperature is calibrated correctly, to help you determine if there are hot or cold spots in the kiln, and to troubleshoot glaze color and inconsistencies. Especially as heating elements age and therefore generate less reliable temperatures, pyrometric cones are essential.

For more information, read our guide on cones, including a full pyrometric chart.

Before You Begin

As you prepare to fire, be sure to double check your needs and take the following into consideration:

Determine your Required Temperature

All clays and glazes are created to mature at specific temperatures. If you fire clay at too high a temperature, it will deform or even melt. If you fire it at too low a temperature, it will not solidify and be durable. As for glazes, firing too high can result in runoff, firing too low will make your pieces dry and rough, and a glaze’s color can be affected negatively by any temperature variance as well.

Be sure to consult the labels on your glazes for cone requirements and consider the type of clay you’re using before choosing a firing temperature. Specific temperatures for bisque and glaze firing will be discussed below.

Invest in Kiln Furniture

There are an assortment of accessories you will need to hold and support your work during firing. Kiln furniture is designed to withstand the repeated high temperatures of firing without deforming. Here are some of the most important accessories to consider buying:

  • Kiln shelves: The most common kiln shelves are made from Cordierite, a naturally occurring mineral made of silica and alumina. While Cordierite kiln shelves can be fired repeatedly at cone 10, they are more susceptible to warping at temperatures above cone 8. If you will be firing at higher temperatures, high-alumina kiln shelves can withstand cone 11. Silicon carbide shelves are thinner, more lightweight options that will not warp in high temperatures, but cost twice as much as Cordierite or high-alumina shelves.

  • Kiln stilts: These are used to hold and protect the edges of pieces in low to medium-low firings. Though some stilts may be able to stay in form at cone 10, most are better suited for temperatures at and below cone 6.

  • Kiln posts: Kiln posts support shelves while optimizing space within the kiln for firing. They are available to order in many different levels of thickness and height.

  • Furniture kits: Including a variety of posts, shelves, heat resistant gloves, and cleaning equipment, a furniture kit might be the easiest way to acquire all the accessories you need at once. There are dozens of different kinds, so you can chose one that best fits your kiln’s needs.

  • Bricks: Used as an insulator by lining the inside of a kiln, bricks come in a variety of shapes and two main materials. Hard bricks are strong and dense, and are great for structural support. Soft bricks are less capable of withstanding high temperatures, but retain heat more effectively.

Getting Ready to Fire your Kiln

In preparation for firing, be sure your kiln is clean and in top condition. Check heating elements for damage, kiln lid braces for security, and electrical cords and connections.

After making any repairs that are required, vacuum out your kiln if necessary and chisel off any glaze drips on the shelves. Especially for kilns insulated with firebrick, frequent and thorough cleanings are crucial so that no foreign matter will be exposed to the heating elements and short them out.

Finally, double check that all potentially combustible materials have been removed from the surrounding area and that there is at least a foot of clearance around your kiln.

The Bisque Load

When you’re all set to fire, here are some basic rules you should follow for a safe and successful bisque:

Your Work Must be Bone Dry

Do not fire your ceramic pieces until you are certain there is no water left in the clay. If your piece is even slightly damp and you fire too fast, the steam will cause it to explode. “Candling” is one technique you can use to dry your clay if it is still damp; in this process, the kiln is programmed to remain at around 180 degrees Fahrenheit for eight to ten hours. This method eliminates chances of breakage even with thicker pieces, and is particularly helpful when pieces of varied clay thickness are in the kiln together.

Fire a Full Load

In order to take advantage of conduction heating and save electricity, try your best to fire a full load every time you use your kiln.

Handle Work Carefully

Because all work is very fragile at this stage, it is crucial to handle your pieces with great care.

Low Fire or High Fire?

Bisque firing requires between cone 010-04, with cone 08-06 being the most common. There are two main approaches to bisque firing: low fire or high fire. Counterintuitively, in low fire, the cone number is higher - up to 04 - to ensure that all the carbon and other materials in the clay burn out during the first firing. This eliminates the chance of carbon burning out during the glaze firing and causing blisters in the glaze. It also makes your pieces stronger, lowering the chance of cracking during cooling. However, the risks of the high fire route include the chance that the clay will not absorb enough glaze, and it leaves pieces less porous and flexible, and therefore more sensitive to thermal shock.

The other option is low fire; bisquing closer to cone 010 leaves the ware very porous and able to absorb glazes well. However, if it absorbs too much glaze, the piece can be too thick and actually fall apart. Additionally, firing at cone 010 leads to a piece that is weaker. Therefore, you should chose a cone based on the type of glaze you’re using as well as the type of project you’re creating.

Fire Slower if You’re Uncertain

A good rule of thumb is that a slow fire is safer than a fast one. Unless a glaze looks better fired fast, slow down the process to ensure your pieces don’t become damaged by the quickly rising temperatures.

Properly Place Your Pieces

Use one-inch stilts to hold up the bottom shelf of your kiln to aid circulation, and keep all of your pieces one inch away from heating elements, walls, the thermocouple, and the KilnSitter (if you have one).

Make Sure Steam Can Escape

To keep your pieces from exploding, steam needs a way to exit your kiln. If you don’t have a venting system, prop your kiln lid open a few inches with a kiln brick or similar item during the first few hours of firing. If your kiln has an upper peep hole, leave this unplugged during this time as well.

Don’t Unload Until the Kiln is Completely Cooled

Even after your kiln has been turn off, it will remain hot for hours. Don’t open or touch it until it is fully cooled to avoid injury or thermal shock that could endanger your work and kiln elements. Unloading the kiln should only take place when your pieces can be easily touched by hand.

Glaze Firing

Though many of the same rules apply to glaze firing as bisque firing - such as the need for ventilation the first couple of hours and a suggested tendency to fire slower when you’re uncertain of how best to proceed - there are a few key differences to keep in mind during this second firing:

Glaze Firings Take Less Time

As mentioned in the beginning of the guide, glaze firing typically is a faster process than bisque firing because most of the moisture in the clay has been driven out. However, some glazes look better after a slow firing, so you’ll want to experiment.

Glaze Fire at a Lower or Equal Cone to Your Bisque Firing

Most commercial glazes recommend glaze firing two cones cooler than your chosen bisque firing cone. However, many artists find that firing low-fire pieces at the same cone as they glaze them works well. Cone 06 is a good choice a typical glaze firing, though of course what will work best depends on the project, glaze type, and kiln.

Mid- to High-Fire Glazes Often Look Better if they are Cooled Slowly

If using a manual kiln, turn the switch down to “medium” when you would normally just turn the kiln off, and if using an automatic controller, program the final cooling ahead of time.

Use a Soak / Hold Temperature at the End of Firing

To even the temperature throughout the kiln and assure the pieces have all achieved the desired temperature, a soak or holding temperature can be extremely useful. Particularly if the kiln is densely packed, this step is recommended. However, don’t soak for too long, as this can overfire your pieces.

Kiln Operation Safety

Due to a kiln’s extremely high temperature and the potentially dangerous gases it releases, firing poses a number of hazards all artists should do their best to avoid. Here are some of the most crucial safety rules to keep in mind:

Before Firing

  • Purchase personal protective gear, such as kiln mitts and dark glasses

  • Read the manual that comes with your kiln so you know how to safely install and operate it

  • Set up a ventilation system to ensure harmful fumes and gases have an escape route

  • Check and make sure your shelves aren’t cracked; firing with cracked shelves can break during firing and damage your work

  • Prior to loading your kiln, make sure it is unplugged and turned off

During Firing

  • Don’t touch your kiln’s heating elements with anything, as it can cause electrical shock

  • Don’t leave your kiln unattended while firing

  • Only look into a hot kiln while wearing dark safety glasses

  • If you smell burning plastic, turn the kiln off, then examine the wall outlet and cord for signs of burning

  • Don’t wear loose-fitting clothing around a hot kiln

  • Don’t add extra insulation around a kiln; this can cause its wiring and steel case to overheat

After Firing

  • Don’t open a kiln until it has cooled completely to room temperature

  • Turn off and unplug your kiln prior to unloading

  • Keep the kiln lid closed when not in use and never place anything on the kiln lid

  • Keep pets and children away from your kiln

Now that you know how to get the most out of your electric kiln, happy firing! If you have any questions, don’t hesitate to contact us, and we hope this guide has provided you with the proper know-how to operate your kiln with confidence and ease.