Common Glass Fusing Issues and How to Remedy Them

Heating glass in order to change its composition, to fuse it or to manipulate it to create a project of your choosing can be fraught with difficulties. When heated, glass is prone to many potential faults, particularly when heated to high temperatures within a kiln. As a beginner, there is the potential for a number of things to go wrong if you do not know how to remedy them. Below are a few common defects of glass and how to remedy them:

Bubbles forming in a fused glass piece

During the intense heating stage when fusing glass in a kiln, occasionally fusing bubbles can form in the heated glass. Some people may choose to accept this as a natural occurrence but there are ways to mediate this issue. Bubbles are normally caused by air being trapped somewhere either between the glass layers themselves or within the kiln.

This issue can be remedied in a number of ways. Firstly air can be trapped due to an uneven kiln shelf. To remedy this, check that the shelf is level using a ruler or turn the shelf over to see if it is more even. This ensures that the glass sits more flush on the shelf and air cannot get between the two elements.

To prevent air getting between layers of the glass when fusing, try firing the glass at a lower temperature and holding the glass for longer. Ensure that your glass has a small space between its different elements before fusing to allow the air to escape.

An additional method to the firing schedule is to slow down the firing rate and add an additional hold during the fusing cycle. This will ensure that air will be more controlled and slowly released from between the glass layers.

Finally, selecting the correct firing schedules can be key to keeping bubble forming under control. Check the manufacture instructions of your glass kiln to determine optimum programs for your project. Soul Ceramics can guide you to the best glass kilns for your needs.

Thermal shock cracks

Thermal shock cracking is a common occurrence during glass fusion. It occurs when one part of the glass is heating or cooling faster than another part. This causes a difference in expansion rates and hence creates cracks and fusions.

Most people would check the kiln to see if there is a fault within the heating element as a first port of call. In general, this is the least likely cause, as modern electric kilns generally work to a program unless there is a failed element within the kiln, causing an uneven heat.

More likely is to check that you are using the right kiln. Most kilns offer heating from the top and sides in order to create an even heating throughout the piece and reducing the likelihood of cracking. The other consideration here is that it depends on the thickness and composition of your piece. On a wide flat piece of glass that is not very thick, it is best to heat the piece from the top of the kiln. This helps ensure that the heat is dispersed across the entire piece evenly.

Taller pieces benefit from being heated from both the top and the sides as do thicker pieces to ensure even heating throughout. It’s best to understand what your kiln is capable of before deciding which kiln you are going to use for your projects. 

The final answer is to slow down your firing rate. This helps to better control the heat, ensuring that heating is not too rapid and therefore creating cracks once it is cooled. A slow, even heat is likely to produce more consistent results.

Glass sticking within the kiln

Kiln wash is used to ensure that there is no residue left within the kiln and that future projects do not stick to the shelf and cause issues. If applied unevenly, the glass may stick as it moves during firing. This can mean that glass can break apart.

Kiln wash should be applied carefully and evenly to prevent the glass from sticking, but not too much as this may adhere it to the bottom of the finished piece.

Glass annealed too quickly

Annealing glass is a process which ensures that glass is not overly stressed and therefore is not prone to cracks or breaks. This process is achieved by soaking during the cooling cycle and then slowly cooling the glass down from 900ºF to 500ºF. If the process occurs too quickly, it is likely to cause cracking.

If your project is thick, your annealing process may need to include a stage called firing down. Firing down essentially adds a small amount of heat during the cooling process to ensure that the glass does not cool too quickly. The best way to achieve this is to use a kiln with a pre-program option, such as the glass kilns and annealing kilns available at Soul Ceramics, but care still must be taken to monitor the temperature change as even pre-programmed cycles may not achieve the required effect.

Glass compatibility

As previously mentioned, when fusing glass it needs to expand and contract at the same rate to avoid cracking. This means that it is important to use glass with the same or similar properties, otherwise fused glass will not be compatible together and ultimately break. 

Glasses are classified using their expansion rate and so when you source glass for your project, ensure that you select glass with a similar expansion rate to avoid the potentials of cracking and incompatibility.

Many issues can arise when heating glass for fusing within a kiln. The best advice is to ensure that the heating is controlled and extensive preparation is undertaken prior to your heating schedule. Some experimentation may be required to ensure that your heating and cooling schedules are set correctly to avoid some of these common pitfalls. Please contact us at Soul Ceramics if you need to know more about the most suited glass kilns for your glass fusing project.

Devitrification (Crystallization of Glass)

Devitrification occurs when the surface of the glass becomes crystallized during firing, resulting in a cloudy or rough finish.

To prevent devitrification, ensure that your glass is clean and free of contaminants. Use a devitrification spray or apply a thin layer of clear glass frit or powder to the surface of your glass before firing. Additionally, avoid over-firing your glass and anneal properly to reduce stress.

Slumping or Shape Distortion

Uneven heating or improper support during slumping can cause the glass to slump unevenly or distort its shape.

Ensure that your kiln is heating the glass evenly by using a proper firing schedule. Use kiln furniture, molds, or shelves to support the glass during slumping, and make sure the weight is distributed evenly.

Warping or Sagging

Over-firing or improper support can cause the glass to warp or sag excessively.

Follow a proper firing schedule to avoid over-firing the glass. Use molds or slumping forms designed to support the glass during shaping, and ensure the kiln shelves are level.

Additional resources on glass arts

• All the supplies and tools you need for glass fusing
• The best kilns for glass fusing
• Glass fusing techniques to make glass partterns
• How to start a glass fusing business