Troubleshooting Glass Defects – Thermal Shock Cracks

There are few things more disappointing than opening your kiln and finding your glass project destroyed by a cracks, bubbles, divitrification, foreign objects, or other reasons.
We do not consider ourselves glass experts (although some of us love fusing glass), but as kiln experts we have chased down a lot of solutions to imperfections through research. Usually the best people to ask about glass problems are the glass manufacturers themselves. All of them have great information on their website and are more than happy to help. That being said, we would like to share some of the successful solutions we have come across. In this blog we will start with cracks caused by thermal shock.

What are Thermal Shock Cracks

Thermal shock cracks occur when one part of the glass is expanding (heating) or contracting (cooling) faster than another part of the glass. This difference in expansion rates creates cracks and causes the glass to crack.

Glass will not crack after it reaches 1000 °F because above this temperature it behaves more like a liquid than a solid. Glass is considered an Amorphous Solid which technically is neither a liquid or a solid but that is a whole different conversation.

How Can You Avoid Them

Potential Kiln Issues

Many people’s first instinct is to blame the kiln. Sometimes it is the kiln, which is great news for us because we know how to fix kilns. More often than not thou there is something else going on. Kilns are pretty simple creatures. They basically just do what they are told. The most common kiln problems associated with thermal shock would be a failed element or relay that causes the kiln to cool too quickly or unevenly.

You can tell if a crack occurred during the heating or cooling process by looking at the edge of the crack. You know that they occurred before the kiln reached temperature when the edges had time to soften after the crack occurred. If it occurred on the cooling phase of the firing it will have sharp edges. If this is the case you may want to test fire your kiln to see if all the elements are working correctly

Use the Right Kiln

To avoid these cracks you want to make sure the piece expands and contracts evenly throughout the piece. This is easy to do with small thin pieces because heat can transfer through the whole piece fairly quickly and thus stay uniform. Really wide, tall or thick pieces are more difficult to manage the expansion uniformity.
The keys to success lie in the direction of heat radiation for a given project and the rate at which you increase or decrease temperature.

Most glass kilns have elements, in the lid (FireBox14), the side walls (GM1018) or both (GM1014). Most larger kilns will have at least one element in the bottom to provide a heat source under the shelf. Kilns that are designed to fire on multiple layers will not have lid elements because the top shelf would fire too hot. Kilns with elements in the bottom or rare due to the fact that debris tends to collect in the the bottom of the kiln and can contaminate the elements. Conductive heat from the shelf helps to compensate for this.


On a wide project that is not very thick, like a flat fused panel or a shallow slump or drape mold, it is best to heat the piece from the top. This helps ensure that the heat is dispersed across the entire project evenly.

Tall pieces are kind of hybrids. Good examples would be tall drape molds or very deep slump molds. In both cases, the project starts out flat and then becomes tall as it deforms. Ideally you would want to heat these pieces from both the top and the side. Thick pieces like deep cast molds would also benefit from top and side heating.

Slow it Down

Okay so not everyone has the benefit of owning multiple kilns designed to fire a variety of projects so what do you do if you want to fire a flat fused panel in a side fired kiln or a tall drape mold in a top fired kiln. The answer is slow down your firing rate.

If you are using Skutt’s Glass Fire Mode, be sure to consult the GlassFire Mode sizing chart in your manual before you choose a firing speed.

If you are writing your own programs using Ramp/Hold, you may want to do a little research on the proper firing speed for the size of the piece you are firing. Sometimes building steps into your program can help. An example of this would be adding a 15 minute hold at every 250 degree increment to allow the heat to balance out before you move to the next segment in the program.


Glass Speed Chart



  • Keep projects at least 2 inches from the elements. If you still have problems, shield your project from direct exposure form the elements with shelf posts or fiber.
  • Post up your shelf at least 1 inch to allow heat to flow under the shelf.
  • When using GlassFire Mode use the slow speed and make sure your piece falls within the guidelines of the Glass Fire Mode Chart.
  • When designing your own programs in Ramp/Hold Mode, slow down the firing rate between start up and 1000F and again from 1000F to room temperature and add holding steps to the program.
  • Wait until the kiln is at room temperature before you open the lid.


Annealing Cracks look very similar to Thermal Shock Cracks. Happy Firing!

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Top 5 Ways to Extend the Life of your Kiln Elements

elements Let’s face it, elements are expensive and there are plenty of things you would rather be doing on a Saturday than spending an afternoon replacing them. Unfortunately, just like the brakes on your car, you are eventually going to have to replace them. The good news is that, also just like brakes, there are a number of things you can do to extend their life. In order to better understand why we recommend certain practices it is helpful to know a little something about how elements age.

The most widely used element wire is Kanthal A-1. It is composed primarily of Iron, Cromium and Aluminum. Element life is determined by the rate at which the Aluminum oxidizes. The hotter the element temperature, the faster it oxidizes. The amount of time you spend at high temperatures (ex: slow cooling Cone 6 glazes) compounds the problem.

Whether it is a new kiln, or you have just installed new replacement elements, the first firing is critical. You will notice new elements are bright and shiny. The outside surface of the element begins to oxidize when it reaches 1832 F. The temperature of the kiln could be significantly less. The degree of temperature difference will depend on the element design, the rate at which you are firing and a number of other factors. As the element oxidizes it forms an aluminum-oxide layer around the core. The goal is to form a uniform, dense oxide layer that is well bonded to the base metal. We have found a Medium Speed, Cone 04 ConeFire program does a very good job of achieving this.


This oxide layer prevents the further oxidation of the alloy base metals by providing a barrier that resists the diffusion of oxygen into to the alloy and also prevents the diffusion of metal ions to the alloy surface. The first time you fire the element it is basically naked with nothing to protect it. If you fire the kiln empty with nothing to release damaging gases, it helps the element form a nice uniform oxide coating with minimal initial damage to the metal.

As you continue to heat and cool your kiln the element continues to expand and contract. Every time it expands and contracts, tension is formed between the the oxide coating and the base metal because they expand at slightly different rates. This forms little fissures in the oxide layer which expose the core. The exposed core then oxidizes and fill the fissures with new aluminum oxide. This both elongates the element and reduces it’s diameter.

As the core gets smaller and smaller, the resistance of the wire increases causing the element to run hotter thus compounding the problem.The net effect is a reduction in the power potential of the element.

As the element gets longer and thinner, the distance between coils becomes less. This is what is called element creep. When the distance between coil turns becomes less, the element gets hotter because the coil turns are radiating more heat on to each other. Remember that hotter is bad so this also compounds the problem.

A well designed element will ideally have the largest diameter (more core material to fill the cracks) and the largest distance between the coils (runs cooler). The heavier gauge elements also have more dimensional stability. When elements get above 1700 F they start turning into the consistency of cooked spaghetti. As they soften, the coils want to start laying down, causing the distance between the coil turns to lessen. Therefore, the thicker they are, the better they can support themselves at higher temperatures. Because of this you can also increase the distance between the coils (increase the pitch).

Okay, now that we know how elements degrade, lets look at what we can do to slow the process.

1. Choose the Right Kiln for the Job

Every kiln has a maximum temperature rating listed on the serial plate. It is the maximum temperature a kiln can achieve under “normal” conditions. This is a lot like the maximum load capacity for a pick-up truck. Just because a 1/2 ton truck is capable of hauling a bed full of gravel, it does not mean it is the best choice if you are a contractor who hauls gravel everyday, up hill, low on transmission fluid, in the Mojave Desert… you get the point. Things always last longer when you do not push them to their limits. A good example of this are 10 cu/ft kilns.

Kilns like the KM1227-3 are designed to use a common sized 50 amp plug. Manufacturers need to limit the amperage of this size model to accomodate the plug, therefore they cannot put the power into the design that they would prefer. At Skutt we offer the “PK” version of these size kilns for those people who are not restricted by this plug requirement. This would be a much better choice for artists that are consistently wanting to fire their kilns at high temperatures. One of the biggest reasons why elements last longer in more powerful kilns is because they can reach temperature faster and are not spending unnecessary time at high temperatures. If you are consistently firing to cone 6, you will be much happier with a kiln rated to cone 10

Ideally everyone would use low fire ceramics. In reality, there are a lot of characteristics (i.e crystal glazes) and properties (i.e. freeze resistant tile) that are impossible to achieve at low fire temperatures.

Cup Head System Skutt Potters Wheels

2. Keep Your Elements Clean

Road or rail vibration during shipping can cause brick dust to become airborne and settle on the elements. When you fire your kiln for he first time this dust can interfere with the creation of a good oxidation coating on the elements. It can also cause the element to run hotter as it restricts heat from escaping and burns off any combustibles. This also happens over time with just normal use. As the elements expand and contract they can loosen brick material that settle on the elements. In addition the normal dust associated with a clay studio can settle on the elements when the kiln is being loaded and unloaded. Occasionally bits of clay and glaze can fall into the element groove. These are extremely important to remove since they have the potential to completely burn through the element.

A good rule of thumb is to vacuum out your element grooves at least one a month and always vacuum out the kiln before it is fired with new elements.

3. Fire New Elements With the Kiln Empty

Just as dust is bad for the creation of a good oxide coating, so are the fumes that can emit from clay and glazes. Therefore, make sure your first firing with new elements is with the kiln chamber empty. As mentioned earlier, run a Cone 04, Medium Speed ConeFire program for best results.

4. Vent! Vent! Vent!

Even if you have the best oxide coating ever, the fumes that emit from clay and glazes can still attack the element when it expands through heating and forms cracks in the coating. Downdraft vents are your best defense against potentially harmful fumes. Downdraft vents pull the fumes from the kiln chamber before they have a chance to damage the elements. If you do not have a downdraft vent your next best option is to prop the lid a couple of inches until the kiln reaches 1000 F to allow the fumes a path to flow out of the chamber. You should also leave the top peephole out during the entire firing to handle those fumes that escape above 1000 F. Trying to reduce in your kiln will clobber their useful life and is definitely not recommended.

5. Keep the Elements Supported

We mentioned earlier that elements soften when the are heated above 1700 F (again this is the temperature of the element not the kiln chamber) and can actually droop out of a broken element groove and eventually break from elongation. Eventually this brick should be replaced but if you do not have the time you can help things by creating a little fence to hold the element in place using element pins. Usually a good time to replace brick is when it is time to replace your elements.

By following these guide lines you can easily double the life of your elements. In future blogs we will discuss the benefits of APM elements. Until then, be safe, creative and happy!

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