My Brick Is Damaged So I Cannot Fire The Kiln

Insulated firebricks can withstand significant abuse and damage before they degrade sufficiently to cause kilns to become unusable. The kilns may not have much aesthetic appeal after many years, but they remain functional. This article will discuss particular examples of brick damage and degradation along with which ones require the attention of the glass artist and how to repair.

In general, insulated firebrick serves only two purposes in the glass kiln. They prevent the heat from escaping from the kiln (insulate), and they support or retain elements. If the brick serves both of these purposes, then the brick remains functional and does not need to be replaced but may require some repair.

However, glass kilns present two additional important nuances with brick: dusting and overfired glass on the bricks. Brick dust is a bane of the glass artist. Brick dust or small chunks can make a beautiful piece of glass art an exercise in frustration. Glass melting into firebrick is normally not a terminal event for a kiln, and it can be repaired.

Before proceeding to specific examples, some understanding of the brick material itself and its characteristics is helpful. Insulated firebrick is a combination of special clays mixed with other materials to give the porosity seen in the bricks. This porosity makes them light weight and have good insulating properties. The mixture has a cake batter appearance and is poured into molds to dry. The bricks are removed from the molds and fired to 2300-2400F in a tunnel furnace. They are manufactured in thicknesses of 2.5” and 3” and capable of withstanding temperatures of 2350F for extended time periods. This means most art glass application temperatures are far below any maximum rating of the firebrick material.

Firebricks can have voids or cracks or impurities like metal oxides in the finished product. Some voids or cracks are not visible and can result in certain portions of the brick being structurally weak. As bricks heat up, they will expand at their coefficient of expansion. The bricks also produce brick particles or dusting.

The dusting issue is normally mitigated by coating the brick surface of the lid with a mixture of brick cement, water, brick dust and a gumming agent. This brick coating is combined to a consistency similar to that of cream. It should go on the brick like a wood stain in that it soaks into the brick and hardens the surface. The grain of the brick should remain somewhat visible. If the coating is either mixed too thick or applied too thick like enamel paint, it may chip off the brick. This coating can be reapplied if it abrades away. The brick cement, the main ingredient, has a different COE than the brick itself.

The most common brick defect on glass kiln lids is chipped brick at the outer corners or edges.






As the bricks increase in temperature, they expand. The only place for the bricks to expand is toward the edges of the lid. The corners of the lid are the furthest point from the center of the lid, and the brick corners push against the metal band on the lid. If the brick has any structural weakness from voids or cracks, that is where the brick will chip or break. These chips should not concern the glass artists. There is negligible loss of insulating capacity with the loss of so little brick material, and this portion of the brick does not retain any elements. It is suggested that the chip be recoated if the chip is on the bottom of the kiln lid where there is potential dusting onto the artwork. Do not try to fill the void with kiln cement. The cement will eventually break out and take more brick with it.

The most common problem with the kiln floor and walls is surface cracking.


These cracks should be of no concern as they are not retaining the elements nor impacting the insulating capability of the firebrick. The cracks are generally on the surface and do not go through the brick. Moreover, when the kiln heats up, the brick expands to close the cracks. These should be left alone. Do not try to fill the cracks with cement as the cracks will get larger and the cement will break out. If a crack is large enough to warrant replacement, there will be damage to the outside metal of the kiln where the heat has been able to reach the metal. In this instance, the burn mark will be very apparent on the outside of the kiln metal. This is extremely rare.

The next brick issue is glass getting onto the floor of the kiln.



This type of damage is not reason for replacing the brick in the kiln. Normally, the glass has only gone into the surface of the brick less than one inch. Most kiln floors are at least 2.5” thick and many are 3” thick firebrick. The glass can be removed with a putty knife or a pocket knife and some patience.


This photo shows the floor of a large glass kiln that had damage from molten glass, and the glass was removed. The kiln remains operational. Some areas are not flat enough for placing kiln posts, but the posts can be placed around the damage to still support shelves. If the floor is damaged to such an extent that posts cannot be placed on it, shelves can be placed on the damaged floor to make a new floor that is sufficiently flat. Do remove all the molten glass from the kiln walls and floor. The glass can cause more damage later if not removed.

A brick issue that does require immediate attention and probably replacement is when the brick supporting the lid element breaks and the element is falling from the lid brick.






This condition may be repaired in the short run by using element pins or staples to hold the element in place, but the lid may need to be replaced for it to operate well over the long run.

Hopefully, this article explained how to address many of the common brick issues encountered by glass artists and how to decide which brick problems warrant attention. Just because the kiln is cosmetically challenged with worn paint and every brick is cracked, does not mean the kiln will not fire like a dream.

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This article appears in the July/August, 2009 issue of Glass Art magazine.

How to Choose an Electric Kiln

Over the years, the selection of kilns has grown steadily. To help you sort through the confusion of choosing from so many different types, we have narrowed the selection criteria to eight.

1) Temperature

The kiln you choose must be rated hot enough for the ware that you will fire:

2350°F: Porcelain and stoneware

2300° - 2000°F: Low-fire ceramics

1400°-1700°F: China painting, glass fusing, glass slumping, enameling, bead annealing

It is a good idea to buy a kiln that will fire hotter than you need it to. If you are firing glass to 1500°F, buy a kiln rated to 1700°F. If you fire ceramics to cone 6, buy a cone 10 kiln. As heating elements age, they draw less and less power. Generally, the higher the kiln’s maximum temperature rating, the longer the elements last. This is because even after the elements begin to wear, they still draw enough amperage to fire the ware.

Another advantage to higher temperature capacity is that during periods of low voltage, your kiln will still likely reach the temperature you need.

2) Size

In general, the larger the kiln, the lower the cost per cubic foot of interior space. Divide a kiln price by its cubic feet, and you’ll see what I mean. (This principle also applies to houses. I learned this when I built a house years ago.)

Will you want to fire many small loads or a few large ones? Some people prefer to fire frequent small loads to see how special effects turn out before spending time on other projects. Others prefer firing fewer large loads. This may be another factor in choosing kiln size.

Choose a kiln that will fire the largest ware that you produce, and decide how often you want to fire the kiln. Figure how long it will take you to make enough ware to fill a kiln of a given size. Do you think your needs will expand later? Kiln owners will typically tell you to buy more capacity than you currently need, because you’ll probably outgrow your kiln later.

Before purchasing a 10- or 12-sided top-loading kiln, visit a studio that has one. Reach down inside the kiln to be sure you are tall enough to load it. This is important. I know people who cannot touch the floor of their kiln, so they leave a shelf supported by posts in the bottom. If you have difficulty loading a studio kiln, consider the short and wide 12-sided, 22” deep kilns.

If you fire ware of a particular size such as tiles or bowls, plan the kiln load on paper. Draw diagrams of different sized kiln shelves and determine how many pieces of ware will fit onto each shelf. You may find that the ware fires most efficiently in a particular size kiln. For instance, the 10- and 12-sided kilns can both fire 10 in. bowls. But since both kilns fire four bowls per shelf, the bowls fire more efficiently in the 10-sided kiln than in the 12-sided.

The 10-sided kiln is also a good choice for those who need short firing cycles. Since 10-sided kilns are smaller than 12-sided, they can heat and cool faster. In addition, kiln shelves for 10-sided kilns are lighter than those for 12-sided kilns and are easier to lift.

3) Electrical

Will you need a new circuit installed for your kiln? This may affect your choice of kilns. Only a licensed electrician should install a new circuit. Use copper wiring, not aluminum.

Homes in the U.S. and Canada usually receive 120/240 volts. If your studio is in a business district, strip mall, or school, it is likely that your voltage is 208, not 240. It is important that you know your voltage before ordering the kiln. 208 volt and 240 volt circuits use the same wall outlets, so you can’t visually tell them apart.

Call your power company or electrician if you are not sure about your voltage or phase. If you fire a 240 volt kiln on a 208 volt circuit, the kiln will fire slowly and may never reach maximum temperature. This is an expensive mistake, because you will need to order new elements of the correct voltage and possibly have the switch box rewired.

Contrary to logic, 240 volt kilns do not necessarily fire hotter or faster than 120 volt kilns. Some 120 volt kilns can reach 1000° F. in five minutes!

4) Round or Square

On a per-cubic-foot basis, the “round” kilns (6-, 7-, 8-, 10- and 12-sided) are less expensive than the square because they are easier to build. Ceramists usually buy the round models while schools and potters sometimes buy the large square kilns, because they are especially durable and slow cooling.

5) Top- or Front-loading

Front-loading kilns are preferred for enameling, where pieces are removed from the kiln at 1450° F. This would be difficult with a top-loading kiln since the heat rises when you open the lid. Ceramists typically use the small front loaders for glaze testing and small pieces. Large front-loading studio kilns are easier to load than top-loading models because you don’t have to bend down into the kiln.

6) Firebrick or Ceramic Fiber

Though ceramic fiber heats and cools faster, insulated firebrick (used in most kilns) outlasts ceramic fiber. So each material has its advantages.

In addition, heating elements are easy to replace in a firebrick kiln, because they are exposed in firebrick grooves. Most ceramic fiber kilns use elements embedded into the ceramic fiber. Therefore, these elements cannot be replaced. Instead, the ceramic fiber firing chamber and elements are replaced as a single unit.

7) Insulating Firebrick Wall Thickness

Most ceramic kiln walls are either 2½” or 3” thick. Kilns with 3" walls and lid take slightly less energy to fire due to the extra insulation. However, their main advantage is that they reach a higher temperature than their 2½” counterparts. They also cool more slowly, which is important when firing heavy pieces prone to cracking and for special glaze effects. To fire stoneware or porcelain, buy a kiln with walls at least 3” thick.

8) Manual or Automatic

Most manual-fire kilns operate with infinite control switches, the type used on electric ranges. They contain a bi-metallic timer that cycles on and off. As you turn the switch clockwise, the heating elements stay on longer and longer. On High, the elements stay on continuously.

Manual-fire kilns are gradually being replaced by automatic models. If you are planning on a manual-fire because that is what you are accustomed to, at least consider an automatic kiln. Once you understand them, automatic kilns are easier to use than manual kilns. Before making your final decision, ask your dealer to demonstrate an automatic kiln for you.

Automatic kilns are of two general types: mechanical and digital. Mechanical automatics use timers to advance the switch settings and the Dawson Kiln Sitter to turn the kiln off. Digital kilns use an electronic controller.

Some people think mechanical kilns are more reliable than digital kilns. It is true that the wiring of a digital kiln is more complicated than that of most manual-fire kilns. Digital kilns use a transformer and relays, which are often not needed in a mechanical kiln. However, digital kilns are reliable if designed properly. Ceramists have been firing them successfully for over two decades.

Digital kilns are also easier to repair than some people think. The heart of the system is a small circuit board that, in a well-designed kiln, can be removed in minutes and repaired or replaced.

The biggest mistake kiln operators make is assuming that an automatic kiln will shut off as it should every time. Every automatic kiln needs human monitoring, especially near the shut-off time. So even though the kiln you are buying is automatic, plan to be near it at the end of firing. If the kiln takes longer than expected, look through the peephole at the pyrometric cones on the shelf. The cones will warn you if the kiln has fired to maturity and should be shut off manually.

Once you begin to understand the reasons for so many different types of kilns, choosing the best one for you will be easy.

Firing Glass Kilns Is Very Expensive

All kiln manufacturers encounter this thought with customers. However, the reality is much different even with the increased cost of electricity in the last few years. The firing cost for glass kilns is lower than most customers believe.Here is the basic information needed to calculate the firing costs of a kiln. After the formula are two examples applying the formula to typical kilns operated by artists.

To calculate the firing cost we need to understand the formula:

(cost per kilowatt hour charged by electric company) x (kilowatt rating on kiln) x (the number of hours the kiln takes to complete a firing) x (the duty cycle of the kiln)

The electric company charges for power in kilowatt hours. Electric costs range from $.10 to $.20 per kilowatt hour depending upon your location. The cost per kilowatt hour can be found on the bill from your power company.

The kilowatt rating of the kiln can be found on the electrical data plate located on the side of the control box on the kiln. The data plate has the volts, phase, amps and watts. Most of the small introductory kilns that operate on 120 volt standard household outlets have about 1500 to 1800 watts or 1.5 to 1.8 kilowatts. The medium sized kilns that are about 17-23 inches wide are rated around 5.0 to 8.0 kilowatts. Some large glass kilns can be rated around 11 kilowatts. To convert watts on the data plate to kilowatts, divide the watts by 1000.

The number of hours the kiln fires is displayed on the digital controller at the end of the firing. The firing time can be under an hour to 20 hours depending upon the project. If the kiln does not have a digital controller, just measure the time from start to when you turn off the kiln.

The duty cycle for the kiln is the amount of time the elements are actually having electricity going through them. Electricity is only going through the elements when the relays are ON. This is the clicking or humming sound heard when the kiln is operating. The kiln is using electricity only when the relays are ON. The general duty cycle for firing of glass kilns with HOLD times, controlled ramp rates, etc is 50-60%. For example, if the fusing program takes about six hours, the relays are only ON for about 3-4 hours of that six-hour firing.

Here are two examples applying the formula to the firing of two kilns.

EXAMPLE #1 Small glass kiln that operates on standard household 120 volts and is rated at 1700 watts or 1.7 kilowatts. Firing program is a fast fusing with a total firing time of 1 hour and 30 minutes. That is 1.5 hours. A duty cycle of 60% is assumed.

Putting this information together with a cost per kilowatt hour of $0.12, the formula looks like this:

$0.12/KwHr x 1.7 Kw x 1.5 hours x .6 = $0.18.

Therefore the cost of firing a small glass kiln with a 1.5 hour fusing program is about $0.18 when the cost per kilowatt hour is $0.12. Here is a table showing the firing costs at different costs per kilowatt hour.

FIRING COSTS FOR 1.7 KILOWATT KILN Example #1

Cost per Kw Hr: $0.10, Cost per firing: $0.15

Cost per Kw Hr: $0.12, Cost per firing: $0.18

Cost per Kw Hr: $0.14, Cost per firing: $0.21

Cost per Kw Hr: $0.16, Cost per firing: $0.24

Cost per Kw Hr: $0.18, Cost per firing: $0.28

Cost per Kw Hr: $0.20, Cost per firing: $0.31

EXAMPLE #2 Medium glass kiln that operates on 240 volt circuit and is rated at 7000 watts or 7.0 kilowatts. Firing program is a basic fusing program with a total firing time of 8 hours and fifteen minutes. That is 8.25 hours. The program includes some controlled ramps and hold times. A dusty cycle of 60% is assumed.

Putting this information together with a cost per kilowatt hour of $0.12, the formula looks like this:

$0.12/KwHr x 7.0 Kw x 8.25 hours x .6 = $4.16.

Therefore the cost of firing this program in this kiln is about $4.16 when the cost per kilowatt hour is $0.12. Here is a table showing the firing costs at different costs per kilowatt hour.

FIRING COSTS FOR 7.0 KILOWATT KILN Example # 2

Cost per Kw Hr: $0.10, Cost per firing: $3.47

Cost per Kw Hr: $0.12, Cost per firing: $4.16

Cost per Kw Hr: $0.14, Cost per firing: $4.85

Cost per Kw Hr: $0.16, Cost per firing: $5.54

Cost per Kw Hr: $0.18, Cost per firing: $6.24

Cost per Kw Hr: $0.20, Cost per firing: $6.93

The costs are generally less than most people expect. There is no change to the cost per firing if the kiln uses three-phase power, because the kilowatt usage remains the same. Firing with large amounts of glass has only a marginal impact on the firing cost.

Some customers do not need to perform these calculations as some of the controllers provided by the kiln manufacturers have the capability now to calculate the firing cost of each firing. The customer just needs to enter the cost of their electricity, and the controller does the rest. At the end of the firing, the customer presses a button and the firing cost is displayed.

Hopefully the explanation and the examples offer an understanding of how to calculate the firing costs.

(This article first appeared in Glass Art magazine.)

An Introduction to Relays

This summary will explain the different types of relays available along with their benefits and detractions. Hopefully this will clarify some of the myths about relays.

Mechanical relays are standard in most kiln manufacturers’ products. Mechanical relays are the small black plastic boxes in the control box. The advantages of mechanical relays include relatively low cost and small size so they can easily be placed in small control boxes. This makes them the preferred choice for most kiln manufacturers.

They produce the clicking sound as two metal contacts come together and then separate. When the contacts are touching, the electrical current flows through the contacts to the elements, which generate the heat. Some kilns will click hundreds of times in a firing. The life of a mechanical relay is influenced by several factors.

First, the relay has only a certain number of clicks in its life. The more clicks in a firing, the fewer firings it will produce. Glass programs utilizing slow ramp rates and long holds cause the relay to cycle many times. Annealing ovens that hold for many hours also cause the relay to cycle frequently.

Second, the closer the relay is operated to its rated amps, the shorter its operational life. This stands to reason. Most mechanical relays are rated at 30 amps. If the relay is operated at 28 amps, its lifespan will be shorter than one operated at 8 amps. Most kiln manufacturers do a good job designing products, so almost all relays operate below 20 amps, and the majority are below 15 amps. This design preference cannot always be achieved, and some models may have mechanical relays used in circuit above 20 amps.

Third, relays with prolonged exposure to high heat environments will have a shorter lifespan and be more prone to periodic locking in the Closed or On position. Many shops have kilns located in areas without environmental control and poor airflow, which causes the ambient temperature to be well above 110 F. This is hard not just on the relays but also on the digital controllers. To mitigate this high ambient temperature, use a small fan to blow air across the electric components. It does not need to be a large fan. Small four inch to eight inch fans on low speed are very effective at lowering the temperature of the electronic components and thereby improving their lifespan.

Most kiln manufacturers have mitigated these potential drawbacks in their designs. Most kilns that operate at 30-45 amps, for example, have two to four mechanical relays. Each relay operates a certain set of elements. If one relay fails in the ON position, the remaining relays are OFF so the kiln does not receive more than 30-60% of the total power. This normally leaves the kiln to reach a steady state temperature far below fusing or melting temperatures for glass so a long anneal may occur. For example, one relay may fail ON and the kiln remains at 1000 F for an extra six hours. This normally does not adversely impact the glass, much less the kiln.

Mercury displacement relays (MDRs) are standard in few kilns, but some manufacturers offer them as upgrades. These are basically two vertical cylinders with a floating plunger inside. When the relay coil is activated, the generated electromagnetic field pushes the plunger up and completes the connection between the bottom and the top of the cylinder. The power is then allowed to flow through the cylinder to the elements. They address some of the detractions of the mechanical relays.

First, their expected life is in the millions of activations, which is far greater than that of mechanical relays. They have no physical contacts to wear out. Their failure mode is almost exclusively OPEN, not closed. This means almost no overfires. Since the relay works with the plunger moving upward in the cylinder when the relay is activated, gravity causes the cylinder to fall to the bottom of the cylinder when not activated, thereby opening the circuit.

The main detractions of MDRs are the use of mercury, cost, and large size. There is a small amount of mercury in these products, and we always dispose of these by returning them to the relay manufacturer. They recapture the mercury for later use and address all disposal issues. We encourage customers to return them to us for disposal. They cost about 4-5 times more than mechanical relays so they are not worthwhile in some applications, primarily small introductory models. However, the expense may be justified where the time and effort invested in the work outweighs the cost of a misfire from relay malfunction. Lastly, their large size precludes their installation in many kilns that are in the field, as many of the control boxes are too small for the MDRs. For many models, the metal control boxes are larger when MDRs are installed at the factory.

We are asked many times if the extra cost is worth it. Our normal analysis of the question follows these lines. Adding MDRs is not worth the expense to small intro kilns that operate on 120 volts and use a standard house outlet. The MDRs are very expensive relative to the price of the kilns. Moreover, the single mechanical relay in the small kiln operates at no more than 15 amps.

Medium size kilns that will be operated 5-7 days per week and are used in a studio environment for classes may benefit from MDRs. The studio owner may prefer to pay a little more rather than accept replacing mechanical relays and the possible delays in firing. Large studio kilns that fire projects with extensive preparation time are the main candidate for MDRs. The added cost of the MDRs is far outweighed by the potential loss of high value projects from relay failure. Many of the larger studio kilns come with MDRs standard and almost all have them available as upgrades.

Lastly, here are a few additional technical points on the use of MDRs. If retrofitting MDRs into an existing model, the MDRs must be mounted vertically so the plunger stays at the bottom of the cylinder when the coil is not activated. Also, most MDRs require the use of a mechanical relay to drive the coil, as most digital controllers do not have sufficient output power to operate the coil of an MDR. Therefore, the use of MDR does not eliminate the clicking from the mechanical relays, but the voltage through the mechanical relay is reduced to less than one amp so its life is very long.

Solid state relays are not used extensively in heating devices for two reasons: First, their predominant failure mode is the ON position, and they generate their own heat, which must be dissipated by use of a heat sync or fans on the control box. Most kilns used by hobbyists and studios have their control boxes mounted to the kilns. Kilns generate heat, which makes it more difficult for these relays to dissipate the heat in the control boxes. Excessive heat causes solid state relays to fail. Most ovens that use solid state relays have them mounted in control boxes that are remote to the kilns. These kilns are used in industrial application where the added costs of remote boxes is not much of an issue.

Hopefully this explanation of the relay types eliminates some of the kilns myths surrounding their use in glass kilns.

(This article first appeared in Glass Art magazine.)

An Introduction to Heating Elements

Every owner/operator of a kiln knows one thing about elements: They are the things that are very difficult to change and fail at the most inopportune time. Their design and theory of operation are however not generally understood. This article is intended to present the basics of element design and answer some of the basic questions of elements such as, “Why do I have to buy a different element for 208 volt kilns than 240 volt kilns?” “Why is the size of that element different from the one in my other kiln?” and “What makes elements last longer?”

We will begin with an overview of heat generation and kiln design. Some basic math equations are required to sufficiently explain the basic concepts of physics that apply to kiln design. Then we will turn to the specifics of element design including mandrel size and wire gauge. Lastly, the question of why elements wear out and melt will be discussed.

An element’s place in kiln design

A kiln is box that gets hot in order to make other items hot. Heat is required to make the kiln hot. Heat is generated by turning electric energy into thermal energy. Elements are the means to that end. The size of the kiln, the type of insulation, and the desired operating temperature determine the amount of thermal energy required to obtain the temperature. Therefore a larger kiln (10 cubic feet) requires more heat to obtain the same temperature than a smaller kiln (6 cubic feet), and more heat is required to go to cone 6 than cone 04. Heat generated by the elements is measured in watts. Kiln wattage is calculated by multiplying the voltage by the amperage:

Wattage = Voltage x Amperage

(All kiln manufacturers have this data located somewhere on the switch box.) For example, a kiln rated at 240 volts and 45 amps would have 10,800 watts (45 x 240=10,800). If the elements actually cycled on for five hours during a firing, the kiln would use 54,000 watt hours (10,800 watts x 5 hours) or 54 kilowatt hours. Power companies sell in kilowatt-hours. The amperage is calculated by dividing the voltage by the ohms in the element.

Amps =Voltage / Ohms.

For example, an element of 10 ohms with 240 volts running through it would have 24 amps (240 / 10 = 24). If you combine the two equations you get:

Wattage = Voltage / Ohms x Voltage

The meaning of this equation in relation to elements is that as ohms of the elements go down, the amperage increases. Subsequently, as the amperage increases, the wattage increases. Therefore, the lower the ohms, the “hotter” the element at a set voltage. The reverse of this is also true; as ohms increase, the amperage decreases and along with lower amperage comes lower wattage. Ohms are determined by the elements.

Element design

Once the size of the kiln is determined, the required watts are calculated, and the voltage is specified, the elements can be designed since the required ohms can now be calculated using the above formulas. The next step is to design an element with the determined ohms that will have a long operating life. The first decision is the type of element material used.

Elements are made of one of three types of material: Iron Chrome (FeCrAl), Nickel Chrome (NiCr), and powdered metal (APM). Iron Chrome is the most popular because of its high temperature rating, about 2500 degrees F, its Alumina Oxide coating adheres well to the element, and its higher surface load (discussed below) enhances element life. Nickel Chrome elements are used mostly for temperatures below 2000 degrees F. APM elements use powdered metal technology to make a very durable element. The drawback to APM wire is its cost, which is about five to seven times that of Iron Chrome. For the remainder of the article I will discuss the design for coiled iron chrome elements in recessed grooves of insulated firebrick, as it is the most common design in ceramic and pottery applications.

Everyone wants elements that last a long time. The element design by the kiln manufacturer is an important first step toward long element life. There are two basic concepts of element design. First, the element must be able to dissipate heat into the firing chamber fast enough to prevent the element itself from becoming too hot and melting. Second, as the element becomes hot, it becomes very soft and loses its shape and must not be permitted to fall or lean on another element coil because of the first limitation.

Therefore in designing an element, two important parameters are monitored: the element pitch and the watt density. The element pitch is the number of wire diameters between coils. The pitch should be above 2 wire diameters i.e. there should be at least two wire diameters of distance between the coils. This permits enough distance between the coils to prevent them from sagging and touching one another and enough space for air to move around them to transfer the heat energy from the element to the air in the firing chamber.

The other design parameter is surface load or watt density measured in watts per square inch of element surface area. This should be below 20 watts per square inch for kilns that fire to 2000-2350°F. The closer the individual operates the kiln to 2350 F, the lower the watt density should be. This is because the closer the firing chamber temperature is to the temperature at the element, the less thermal energy is moved from the element. If the element becomes too hot, it melts and breaks.

Wire gauge and mandrel size

Watt density and pitch are controlled in element design by the wire gauge and the mandrel size used to wind the element. Each wire gauge has a set number of ohms per linear foot of element wire. Thicker wire has less resistance than thinner wire and hence, thicker wire has less ohms per linear foot. Therefore, 12-gauge wire has less resistance than 15-gauge wire. The logic behind this is the narrower the “pipe,” the more resistance there is to move something (electrons) through it. Therefore a 10-ohm element of 12-gauge wire is longer than a 10-ohm element of 15-gauge wire. This logic would suggest smaller gauge wire because less would be used and therefore the cost would be less. However, thicker element wire has a property that makes it a better design choice, longer element life.

Thicker wire has longer element life because it has more resistive alloy (Iron Chrome) in relation to its volume to form a new oxide coating thereby delaying element breakdown. Thicker elements are better. As in life, there is always a but. Thicker wire causes problems with pitch. Thicker wire requires more space between elements because the pitch is measured in diameters of wire.

Mandrel size is the way to control pitch. Here is how this works. Remember we already have defined the ohms the element must be. Therefore we need to use a certain length of element wire to obtain the desired number of ohms. For example, a 15-foot piece of element wire with 1 ohm per foot will have 15 ohms in total. Now we must wind this wire around a metal rod. The number of times a 15-foot wire goes around a ¼ diameter rod is more times than it will go around a ½ inch rod. More coils will be on the ¼ inch rod than on the ½ inch rod, and when the element is stretched to the same length, the coils will be closer together and have a lower pitch. Therefore, one way to mitigate the drawback to thick element wire is to increase the mandrel. But the mandrel cannot be increased beyond the size of the groove in the insulated firebrick.

Why elements wear out

If one took a cross section of an element when it was hot, one would see that the outside is an oxide coating, and the middle section is molten and soft. This is why elements sag when hot. The oxide coating is the protective layer for the element. A good oxide coating is required for long element life. Hence, firing with peepholes open is very good for the elements as oxygen is introduced into the firing chamber and replenishes the oxide coating. On the other hand, reduction firing is not good for element life as the atmosphere is deprived of oxygen. As the element fires, the oxide coating breaks or flakes off and more alloy is used to create the oxide coating. As the alloy is transformed into an oxide, the amount of material is decreased making the element diameter smaller. This is the same affect as lowering wire gauge. As the ohms/resistance increase, the wattage output of the kiln decreases. In order to produce the same heat out of an element with higher ohms, the element must have power running through it for more time. As the element runs more, it causes further breakdown of the element, and the cycle continues. Eventually the element cannot produce enough thermal energy, and the kiln will take too long to reach temperature or it will not reach temperature at all. The elements will require replacement.

Element life is related to wire gauge, watt density, pitch, and as seen above, the number of hours the elements are actually on. The number of hours elements are actually on can be controlled to an extent by overpowering a kiln. For example, a 3 cubic foot kiln which is fired to cone 06 regularly is well powered with 7,200 watts. In general, the elements will have a long life assuming the other design parameters were met. But if the kiln was made with 8,000 watts, the elements would be running for less time to reach the same cone 06 firing and therefore the element life would increase slightly. However, as the elements become overpowered, they tend to creep out of the grooves more. Also, as elements become overpowered their watt density increases as more wattage is produced by less wire, thereby increasing the watts per square inch of surface area of the wire. So movement in this direction is also limited by the characteristics of the element wire.

As can be seen, optimal element design requires the balancing of many factors. However, even the best element design cannot prevent an element from eventually wearing out.

When Heat No Longer Rises

We regularly encounter a myth in kiln firing: heat rises so the top of the kiln is always hotter than the bottom. This is something we all learned years ago and see in everyday life. However, in the kiln world, this is both true and false. The answer depends upon which temperature the statement regards. At temperatures below 1100°F this is generally true, but above 1100°F it is generally false. Let me explain.

Convection, the normal action of hotter molecules rising and cooler ones falling, occurs at temperatures below 1100°F. This normal action of molecules allows hotter molecules to physically collide with cooler molecules and transfer the heat energy. As temperatures approach 1100°F, the amount of convection diminishes quickly as the primary method of heat transference, because there are so few molecules remaining in the fire chamber. The density of air diminishes dramatically with temperature. As the density of air diminishes, the number of air molecules decreases, and the ability to transfer heat by collision decreases accordingly.

The primary method of heat transfer at temperatures above 1100°F is radiation. This is transferring heat from the elements to the ware by line of sight high energy, high frequency "light." Think of sunlight on a cool, clear day. You can feel the radiant heat from the sun on your face, but when the cloud passes over, it feels much cooler. This is line of sight radiant heat transfer. The molecules between the earth and the sun are not transferring the heat by bouncing into each other and heating the earth. Space is cold. Except for annealing, the temperature at which most of the critical heat work occurs on art glass is above 1100°F. Therefore, radiant heat is the primary method by which heat is transferred to glass, and convection has little to no impact on the glass.

Now that we understand how heat energy is transferred, here are a few points of how this principle is applied in the design of a kiln. First, most glass kilns, other than the smallest models, have elements in the top. Smaller models with 8” x 8” or smaller interior chamber sizes do not require top elements as their uniformity of radiant heat distribution will be sufficient with side elements only. The glass is no more than about 3 or 4 inches from an element. Most kilns with an interior diameter above 10 inches have elements in the top. The top elements are normally laid out in a pattern in the lid to radiate heat evenly onto a flat piece of glass. This reduces the temperature variances on the glass and prevents cracking. Moreover, this is why stacking shelves is discouraged. They act as a barrier and prevent the radiant heat generated by the elements in the lid from reaching the glass on the lower shelf. If shelves are used with top element kilns, the bottom shelf will severely under-fire almost every time.

Second, the kiln’s shelf or art glass should not be placed in the firing chamber in such a way that they act as a barrier between the elements and the thermocouple. The thermocouple should reflect the temperature that is experienced by the glass.

Third, if firing projects with drape molds, drop molds, etc., where a large percentage of the glass faces the side wall of the kiln and not the top elements, the use of kilns with side elements is encouraged for this application. Most kilns that are above 7 to 9” deep either come with top and side elements as standard in the design or have side elements as an option at the time of purchase. The idea is the same. As parts of the glass bend and no longer face the top elements, the side elements provide the radiant heat to the glass that now faces the inside walls. There are no drawbacks to firing flat glass in a kiln with top and side elements.

Many times people ask if they can fire glass in old ceramic kilns, which have only side elements. The answer is yes, it can be fired, but it usually takes much testing to adjust the programs to have successful firing. Better firing results are obtained by slowing down the firing profiles and doing smaller sized projects. The smaller glass pieces have diminished capacity for temperature variances across the piece. Large flat pieces are very difficult to fire successfully in side element only kilns.

Remember these points when firing to improve your results and end another kiln myth.

(This article first appeared in Glass Art magazine.)

Light Around the Kiln Lid

I received an interesting note from a customer: "I know of someone who complained that there was a gap between her lid and body of the kiln and that she could see the inside of her kiln through that gap. She had been told by Paragon's customer service that it's supposed to be that way and she isn't having any heat loss from the gap. What?? That's the most ridiculous statement I've ever heard!"

The complaint of the lid is a normal misunderstanding. The kilns are manufactured using a special insulated firebrick that expands at high temperatures. The lid is installed so the lid by the hinge does not touch the top of the firebrick sidewall. This is to allow the sidewall brick to expand vertically and not put pressure on the lid. Furthermore, the slot in the metal through which the hinge rod goes is oval in shape not round. This is again to allow the lid to move and "float" on top of the kiln as the kiln grows vertically during the firing. All kiln manufacturers understand this issue and design kilns with "float" in the lid. There is a colored glow visible between the top of the sidewalls and the lid. The kiln lid of any of our ceramic, pottery, or top loading glass kilns uses the same design concept.

A significant portion of the lid touches the top of the kiln and it is normally the front of the kiln. Furthermore, the lid at temperature bows with the edges bowing up and the middle bowing down toward the firing chamber. This will be exaggerated in larger diameter kilns and almost negligent in smaller diameter kilns. This is how the brick material behaves at temperature. The amount of heat "escaping" is very small and has almost no impact on the kiln's performance. Allow me to explain.

People do have a perception that if they see any light around the lid, or the lid is not touching the top of the kiln sides all around, massive amounts of heat are escaping or there is a hazard. This is not correct.

First, almost every kiln sold by Paragon and the other kiln manufacturers are certified to Underwriter Laboratories Section 499 that regulates kilns. If the design itself were hazardous they would not allow it to be certified.

Second, the assumption that heat "escapes" assumes the impact of convection on the firing process. Above 1100 F the heat transference in kilns is through radiant heat, not convection. Physics dictates that the particle density of air diminishes rapidly with temperature. Convection relies on particles physically colliding to transfer heat energy. At higher temperatures, the density of the molecules diminishes to the point that convection is not an effective method of heat transference. Radiant energy transfers heat at these temperatures.

This is the same as the sun on your face on a cloudy day. When the sun is out, you feel warm on your face, and when clouds appear, the temperature you feel drops. This is why the glass kilns use top elements to uniformly heat flat glass and why you should not use shelves stacked in glass kilns. The glass that does not receive direct radiant heat will not heat properly. If convection were the only method of heat transference, this would not be the case.