Glass Fusing Problems

" Fusing problems…solutions for your glass fusing troubles."

To avoid glass fusing problems, keep accurate records of your firing schedules and results.

These records will help you diagnose problems and avoid them in future firings.

You can use the firing log provided here.

Keep notes of each project for future reference on what happened during your firing procedure; this will help you avoid future problems.

If you want to keep all of your notes in one place, and I highly recommend that you do!

Then check out the page on Note Taking. This is a list of some of the glass fusing problems you might encounter during your glass fusing or slumping procedures.

Bowing glass - Bowing glass can be caused by too much stress in the glass. This stress can be caused by a drafty kiln. Try plugging up the draft source and re-fire. Be sure to fire slowly to heat up the glass evenly, and anneal properly.

Bubbles – Bubbles can be caused by many different things. Here are a few:

  • Uneven stacking - Uneven stacking of glass can cause air to get trapped. Check the placement of all the glass pieces to see if there is a chance of trapping bubbles during the fusing process. The edges of the glass start fusing before the center of your glass has began to fuse and air can be trapped between the pieces. Holding the project at around 1000 degrees Fahrenheit for about 25-30 minutes can allow the piece to evenly distribute the temperature and allow the center to heat up as much as the edges of your piece.
  • Slumping - Bubbles can be formed during the slumping process. When slumping into a mold, be sure the hole in the bottom of the mold is open and not plugged with kiln wash. Trapped air can create a bubble between the glass and the mold if the hole is not open.
  • Bubbles in the glass - Check the glass that is being used for the project to see if there are any bubbles already trapped inside the glass. Some glass already has a few bubbles when purchased.
  • Rough pieces – If your piece is not smooth, then you did not fire it long enough or to a high enough temperature. Unless you want a tack fuse effect, and not a polished look, then you need to fire your pieces to a higher temperature.

    Sharp Spiky Edges - If your piece is over fired, it can have spiky edges from the glass grabbing as it is trying to shrink.

    Flattened out piece – If your piece is misshaped or has flattened out too much, you have over fired the piece. This is caused by firing to a high temperature for a long period of time, causing the glass to flatten out and become distorted.

    Shrunken/expanded piece – When heated, glass naturally wants to be ¼ inch thick. Your glass will shrink or expand to obtain this depth. A good tip to remember is that if your piece is less than ¼ inch when you start, it will shrink up to reach this depth. If your piece is larger than ¼ inch when you start, it will flatten out to reach this depth.

    Glass repelling – This will occur with dichroic and iridized coatings. The coatings can’t be placed together for fusing purposes, because they repel each other. The only way to avoid this is to place other pieces on top so that they will encase the pieces as they are fused and force them to be sealed.

    Foamy surface - This dull white crystalline substance on your glass is known as devitrification. This is one of the most talked about glass fusing problems. It can occur when your glass remains in a temperature range just before it begins to liquefy. You need to minimize the time spent in the temperature range of about 1400 degrees Fahrenheit. You can also use a devit spray to also help avoid this problem.

    Kiln wash stuck – This can be caused by either the kiln wash being damp before firing, or if you over fire or soak your glass for too long. This can be avoided by being sure your kiln wash is dry before firing, using slower and lower fusing temperatures, or using kiln paper. Before firing your piece, be sure that there is not moisture in your kiln wash. Either fire your kiln shelf or mold to remove the moisture or apply the wash and allowing it to thoroughly air dry before firing. Kiln wash can be removed by soaking your piece in a bathroom scum remover product. You can also use a scrubbing pad or steel wool to assist in removing the kiln wash.

    Gray or Scummy Edges - Gray or scummy edges can occur on pieces that have been fired once and then cold worked before re-firing. Cold working involves using either a grinder or glass saw on a piece of glass. These can be avoided by thoroughly cleaning the glass before re-firing the piece. Keep a bowl of clean water near your work area and soak the glass right after doing the cold work procedure. This will keep the edges damp and allow the piece to be cleaned easier. Scrub completely before proceeding with the re-firing process.

    Cracks – These glass fusing problems can be caused by a couple of things:

  • Thermal shock - You either took the piece out of the kiln too soon, or you opened the kiln and exposed the hot piece to cool air. This can transpire anywhere from room temperature to 1000 degrees. You might need to lower your ramp speed.

  • Heating up your piece too fast - If your piece cracked in the kiln and it has an “S” shaped crack, the piece has heated too quickly. Slow down!

  • Kiln Wash application - If it has cracked in different directions from a single spot, your kiln wash was not applied to the center of that cracked point.

  • Opening the kiln - When cooling your piece, do not open the kiln until it is at least below the 100 degree Fahrenheit temperature.

  • Compatibility – If the crack occurs along the line where your pieces of glass meet, then the two touching pieces are not compatible .

    I hope this list will help explain some of the glass fusing problems you might come across and how to avoid them in your future firings.

    Most glass fusing problems can be avoided by keeping track of how your glass reacts and keeping good notes on your procedures.

    Review this page when you encounter any glass fusing problems during your fusing experience.

    If you have any suggestions on items to be added to this page, please use the contact me page and let me know.

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    Problems and Solutions

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