Guide to Glass Fusing, Slumping, and Kiln-forming Techniques

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Devitrification, a scummy white crystallization on the glass surface, occurs when glass lingers too long in the temperature range just before it becomes molten. This is about 1400 degrees Fahrenheit for most glasses.

The cure starts with minimizing the time spent in that temperature range. If you're fusing, flash vent to quickly "crash cool" from the fusing temperature to just above the annealing zone. If you're slumping, work at lower temperatures if at all possible.

The textbook solution to this is to apply a devitrification spray ("devit" spray) prior to firing. This process involves spraying or brushing a thin, even layer of spray on the top surface of the item. Be sure to allow it to dry thoroughly before firing.

Although devit sprays are widely used, you should be aware that some people believe that it is better not to use them. This position rests on the belief that devit sprays create an "artificial layer" on the surface of the glass that will deteriorate in time. Instead, argue proponents of this view, you should control devitrification by improving your kiln procedures and using glass that is less likely to devitrify.

In addition, some glass artists believe that devitrification imparts a unique beauty to the surface of the glass. They actually encourage its formation and use it as an element of their artwork. Even if you normally work to prevent devitrification, experimenting with "controlled devitrification" can lead to some interesting pieces.

But if all else fails and you do end up with unwanted devitrification, you can attempt to remove it. You can lightly sandblast the piece or use an acid bath to remove it (take proper safety precautions, please). You can scrub the piece with pumice or and a rotary brush (or even an old toothbrush). Sometimes it even helps to apply a devit spray and re-fuse. Many of these "treatments" will change the outward appearance of the piece, so proceed with caution.



There are several reasons for kiln wash to stick to the underside of a piece. Firing with wet kiln shelves or with shelves that have picked up moisture from the air can result in the glass picking up kiln wash. Using shelves where the kiln wash has started to deteriorate can also lead to problems with kiln wash.

These kinds of problems are easily eliminated by a fresh application of kiln wash and by pre-firing the shelf to around 400-500 degrees F to make sure all moisture has been driven off.

Kiln wash can also stick if you overfire or soak for too long. It frequently happens at high temperatures, such as during combing or long soaking periods. Some brands of kiln wash are more susceptible to sticking than others, so trying an alternative brand might help.

Removing kiln wash can be an extremely difficult process. If you have the equipment, then sandblasting is fast and effective. However, it does change the appearance of the piece. This may not be a problem for the backside of a platter or similar object, but it will dramatically change the appearance of a more three-dimensional piece.

The safest way to attempt to remove baked on kiln wash is by soaking the piece for several days in a bath of white vinegar, then scraping with a paint scraper or scrubbing with a nylon scrubbing pad or piece of steel wool. Very fine steel wool - 000 or 0000 - is available from most hardware stores and is far preferable to household steel wool.

Unfortunately, gentle methods such as white vinegar don't always work. Lysol brand toilet cleaner works on some pieces.  More stubborn cases call for a commercial stain remover such as CLR or Limeaway. Preparations specifically made for glass (Wash-away is one brand) are also available. Many of these solutions can be hazardous and require the use of gloves and/or respirators, so be sure to follow the directions on the product label.

A diluted solution of muriatic acid (diluted hydrochloric acid) works when virtually everything else fails, but this is an extremely hazardous operation. It requires gloves, a respirator, plenty of ventilation, and extreme care. Do not attempt to use strong solvents such as muriatic acid unless you have the proper equipment and experience.

As with many problems, prevention is preferable to removal. Frequent reapplication of kiln wash will help, paying special attention to removing the old kiln wash thoroughly prior to applying the new. Firing the shelf to dry the kiln wash and remove all moisture also helps. And selecting the right brand can be a factor: some brands do not work as well at high temperatures.

When firing, it also helps to fire a bit slower and to a slightly lower fusing temperature than normal. Similarly, reducing soak time will often help.

Finally, you can sidestep kiln wash problems entirely by firing on fiber paper.



One of the most common complaints voiced by beginners concerns the edges of the fused or slumped glass. This falls into two categories: edges that suffer following the fusing process and edges that remain unacceptable following slumping. Each category demands its own kind of treatment.

If the edges of your fused piece are not as smooth and well rounded as you want, then you may not have fired the the piece enough. Just heat it again, this time firing a bit higher (or soaking for a bit longer). If you can, keep an eye on the piece and let it soak until it rounds the way you'd like.

If the fused piece has needle-like projections along the edge, then you've overfired. Grind the projections away and then re-fire, but this time at a lower temperature (or soaking for a bit less). (Around 1400 degrees F will often work for work that's been fused but not slumped.)

For glass that you're unfamiliar with, it's also a good idea to peek through a peephole while the piece is firing and stop soaking when the edges look the way you'd like.

Another way to improve the edges following fusing is to round them the way you'd like using a grinder or similar piece of equipment, then fire them using a more controlled slump firing. This technique, which requires a kiln controller or greater-than-usual attention to the firing, involves firing with three soak periods rather than the one typically used in slumping.

(Please note that the temperatures and soak times indicated below would probably need to be adjusted for your kiln and for the size of the work being slumped. This technique has been adapted from one discussed in Henry Halem's Glass Notes, and is attributed to Klaus Moje.)

The first soak, which occurs at around 1100 degrees Fahrenheit, is designed to give the glass a few moments to equalize temperature. This will lead to more even slumping and more controllable glass movement. This soak, which should take place at the point where the glass is just starting to melt, should last for about half an hour (longer for thicker pieces).

The second soak, which occurs at around 1200 degrees Fahrenheit, is where slumping begins. This step requires you to keep a close eye on the glass and be ready to override your controller if necessary. Watch the glass through a peephole (wear safety glasses) until it begins to slump, then continue watching until it is over half way slumped into the mold.

Once the object has slumped about two thirds of the way, quickly raise the temperature to around 1400 degrees Fahrenheit. This temperature increase should occur as quickly as your kiln can make it happen. The glass will slump fully and (if you've timed it right) almost instantly.

Once that occurs, allow the glass to soak for a few moments more. Ten minutes is usually sufficient. This third soak will not only complete the slumping, but also yield well rounded, smooth edges. As soon as this is achieved, crash cool the kiln and anneal and cool as normal.

Sometimes, however, despite our best efforts, the slumped pieces still have rough or distorted edges. The best way to improve this is to use coldworking equipment to grind and polish the edges. This can be tedious work, but it makes the difference between a second-rate work of art and one that is worthy of display.



There are few things more frustrating to the glass artist than opening the kiln to find that the piece you've fired has cracked. The only bright spot is in figuring out why the piece cracked so it doesn't happen again.

First, let's assume you didn't drop the piece. Let's also assume you didn't remove it while it was still warm and then "accidentally" put it on something cold or dip it in water.

Now that we've set those obvious (but they happen) reasons aside, let's try to figure out the less obvious reasons for glass to crack. You can learn a lot by examining the nature of the cracks. Do they go all the way across the glass? How are they shaped? Is the piece broken entirely in half? Are the pieces pie-shaped?

Analyze the kind of cracks you're experiencing to learn what caused them, then change your kiln processes to prevent their re-occurrence.

Curved cracks across the middle of the piece.

Improper annealing causes this kind of crack. It will show up as gentle curves (sometimes as straight lines) that break the pieces into two or three pieces. Often the crack will curve sharply as it nears the edge of the glass. This kind of crack is the piece's way of relieving stress. The solution is to raise the annealing temperature slightly and to spend longer in the Annealing phase.

Cracks where two different glasses come together.

Glass incompatibility causes these cracks. The cracks can be very small or they can cause the pieces to break apart,but they will always show up on the edges of the incompatible glass. Conduct your own compatibility tests or use "tested compatible" glass to keep this from reoccurring.

• Small, interconnected cracks (like a spider web).

These cracks generally extend from a single spot on the underside of the glass. They aren't usually severe enough to cause the item to split into pieces. Sometimes shelf primer will also be stuck to the underside of the glass. Most likely, this kind of crack is caused by glass sticking to the kiln shelf. A close examination of the shelf may even reveal small pieces of glass that are stuck to the shelf. The obvious solution is to scrap the shelf clean and apply fresh kiln wash.

Pie-shaped pieces, with smooth edges.

These cracks, which usually occur with such force that they split the piece into five to ten pieces, are caused by thermal shock. The edges of the pieces are often rounded because these tend to happen early in the firing cycle (around 300 to 400 degrees Fahrenheit) and the edges round during later phases of the firing. The cure for thermal shock is to fire more slowly in the early part of the Heating phase. You might also try cutting very large pieces into smaller ones before firing. Finally, it's a good idea to peek in the kiln around 400 degrees or so just to make sure the piece is still OK.


Cracks that occur long after firing.

Sometimes a glass piece will just be sitting on a table when you hear a sharp ping. It might be quite loud, and perhaps there's a second (or even a third) ping. When you check it out, you discover that the glass piece you thought was beautifully finished has cracked. (This cracking can even be severe enough to shatter the piece, leaving the artwork in pieces on the table.)

The reason for this disaster is undoubtedly stress that has built up in the glass piece. Stress can come from many factors: improper annealing, thermal shock, incompatible glass, and even "normal" wear and tear. Improper annealing is the most likely culprit. Go back to your log — you do keep a log of your firings, don't you? — and check out the firing schedule to see if you notice anything out of the ordinary.

If you used the same schedule you've always had success with, then perhaps this piece of glass was a bit thicker or larger than normal. Perhaps it was a different glass than you usually use. Perhaps your "normal" annealing schedule needs to be adjusted to anneal just a bit longer and slower. Remember that it's impossible to anneal for too long.



If your particular problem isn't listed, why not submit it for consideration and possible addition to the troubleshooting guide?  Just send an e-mail with your request.



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