GLASS CUTTING

by Steve Ackman

While not actually necessary, it might be helpful to know just a little about the structure of glass before talking about cutting it. Glass is an amorphous solid. It has no internal crystalline structure like steel, or ice, or most any other inorganic substance you can think of. Many people will even argue, with very good reason, that it's not really a solid at all, but a very stiff liquid.

I'm not going to delve into a lot of physics to show how it's both a thick liquid and a solid at the same time, but the fact that glass has no crystalline structure means it has no cleavage planes (like gemstones, for instance). Glass isn't really cut in the normal sense of the word, but is actually subjected to a controlled break. Since glass is equally strong in any direction, we normally don't have to worry about direction of grain like woodworkers do. Within certain limits, wherever we score the glass is where it will break.
(If you would like to know about the structure of glass at the molecular level, it's covered very well in an article by Philip Gibbs )

If you take a regular piece of window pane between your hands and try to break it in half, what happens? The glass actually bends through a very small arc - about 1 degree. That's when the tensile strength of the surface of the glass is exceeded, and SNAP! You have two pieces of irregular shaped glass in your hands.

By scoring (scratching) the surface of the glass, you can control exactly where that lapse in tensile strength will occur (most of the time anyway, but for now, we're living in a totally theoretical world)

So, cutting glass boils down to two operations:

• score the glass where you want the cut, and
• "run" the score

Scoring the glass is disrupting the surface integrity along a thin line. Any way you can or want to do that will work.

If you plan on making a living at cutting glass, you'll probably want to get a self-oiling cutter with a carbide wheel, which will set you back about $30. These have the advantage of lasting a long time. Many have replacement wheels available, so when the wheel eventually does wear out, that's the only part you have to replace.
You can also get a carbide cutter without the self-oiling feature for around $20.

Next is the steel wheel cutter. This is much less expensive, but to tell the truth, works every bit as well as the carbide wheel. Steel wheels do get dull a lot quicker than carbide, but if you store them with the wheel in oil, they too can last a good long time. You can get a steel-wheel cutter in just about any hardware or building supply store for around $3 or $4.

When you use wheel cutters, it's important to clean the glass first. Not only will dirt and grime dull the cutter, but it can also cause very small interruptions in your score which can cause chips or voids in an otherwise clean cut. If one of these spots is at a critical point on a curve, it can cause the glass to break unpredictably.

When using a wheel cutter, it should be vertical. (like car wheels on the road, not a motorcycle going around a bend) A moderate downward pressure as you roll the cutter along the glass is required, and you should hear a noise that falls somewhere on the mild side of a screech. Use too little pressure, and you won't score the glass, (and you won't hear the noise) use too much, and you throw tiny chips off the score which can then get under the wheel, resulting in accelerated dulling of your cutter. After you do it a couple times, you'll know the sound. It's best to practice on single strength window glass at first - it's the easiest to cut, and it's relatively inexpensive.

Note that some types of art glass, particularly the opalescent varieties, may not make any noise as you score them. Don't worry about the sound on these types of glass. If you use the same amount of pressure as on successfully cut clear or cathedrals, it will be scored and will run just fine.

Some glasses are heavily textured. In those cases, be sure to score the smoother of the two sides.

The next option is not one ever used by people who regularly cut glass, but if you only want to cut a few straight lines, and already have a carbide scribe around, (you know, the thing they sell for scratching serial numbers on your belongings) that will work, too.

And, in an emergency, you can always use your wedding ring...


RUNNING THE SCORE

Running the score is when you bend the glass through that 1 degree arc to get it to break.

This can be as simple as taking the glass in your hands, and just "breaking" it. Put your hands into fists, thumbs up. Now, the glass will go between your curled index fingers on the bottom, and your thumbs on top. Get a good grip, knuckles nearly touching, and bend and pull apart at the same time. The glass will snap right along your score line. This method is good for pieces that are large enough to get a good grip on, and that have a straight, or only mildly curved score.

Another way to run the score is with "running pliers." These have (as you're looking in from the business end) a convex surface on the bottom jaw, and a concave on top. As you center these pliers on your score, the bottom jaw makes contact only at the score. The top jaw makes contact about 1/2 inch on either side of the score (the jaws are about an inch wide). The jaw placement looks like this:

As you squeeze, the pliers separate and widen the score, and the "fissure" created then follows right along the score. This gives a much more controlled break than the previous method, and is almost a necessity for tight or multiple curves.

There are a couple other methods that I'll just touch on here:

• For straight scores, you can extend the glass off the edge of a table. With the score just off the edge, hold pressure on the tabled section, and just snap down on the free piece.
• If you score a piece of glass, and turn it upside down on a piece of cardboard or other springy material, you can exert thumb pressure directly over the score, and it will run. This method is helpful sometimes when you have multiple curves, you've used the running pliers from both ends, but the run just won't quite make it all the way to the middle... A little pressure like this will usually finish the job.
• When necessary, you can also cause a score to run by tapping its reverse side. This method is rather inconsistent and should only be used as a last resort.
• There is a glass cutting "system" put out which uses plastic buttons, and a piece of concave plastic. Here you put the score directly over the button, and use the concave piece above. It's basically free-form running pliers, only there are no handles or a hinge.
• The last way to shape a piece of glass isn't so much running a score as it is nibbling small bits away up to the score. It's called grozing, and is used primarily on tight inside curves. You can use regular pliers for this, but there are specially designed grozing pliers that do it better. Before glass grinders, grozing was used for final shaping of pieces that needed "just a hair more off here, and just a touch over there."

All you need now is a piece of glass, a cutter, and a couple of practice runs! :)


MIRROR

Cutting mirror is really just cutting glass. There are a couple of things to bear in mind. Always score on the "clean" side. Generally this would be the side that you can see your reflection in, however for front silvered mirror such as you'd typically use in a kaleidoscope, it's the side without the protective plastic film on.

Once cut, the edges of the silver are exposed, so are prone to oxidation, usually known as "mirror rot." You'll want to seal the edges to prevent that. There are commercially available sprays such as Sprayway's (No.209) Industrial Mirror Edge Sealant, but really, any kind of hermetic coating will do. Clear fingernail polish has been used to good effect on small projects.


SAFETY GLASS

There are two kinds of safety glass:

The first type is tempered. This means that the glass has been cooled in such a way to induce tension in the exterior skin, while the interior is in compression; much like a balloon.

The bottom line is that cutting tempered glass is like cutting a balloon. It just can't be done. (unless you first anneal the glass, cut, and then re-temper, in which case it would be cheaper and easier to just start with annealed glass in the first place.)

The second kind of safety glass is laminated. This is actually a sandwich of two sheets of glass with a sheet of plastic between them. In order to cut this kind of glass, you need to score one side, and run the score by tapping. Then flip it over and do the same thing on the opposite side. Apply a flammable liquid such as alcohol or lighter fluid and light it. As the plastic begins to warm and soften, you can pull the pieces apart. Pull directly apart and don't let them "hinge" if you care about what the edges will look like.


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