How to Cut Glass: Using the Right Amount of Pressure
One of the first things you learn when making stained glass, is how to cut glass. It can be a strange experience because it's often intimidating yet exhilarating all at the same time.
Teaching and demonstrating how to hold the tools and what to do with them is so much easier than describing how much pressure to use when cutting glass. The latter, is best acquired through practice, guidance, and observation.
So let's start at the beginning.
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What to Use to Cut Glass
Carbide wheel cutters are the most commonly recommended when making stained glass or fused glass art. These have a smaller, harder wheel than the plain steel wheel cutters, and this allows the glass to be scored using less pressure.
Did you know, that not all cutters are made with the same angle for the wedge on the cutting wheel? For our hobby, and for the type of glass we cut, cutting wheels are usually in the neighborhood of 134 degrees to 140 degree angles.
If you purchase and use a traditional stained glass cutter such as the Toyo Pistol Grip Cutter or the Toyo Custom Grip Cutter, you needn't worry about any of these specifics as these tools are ideal for stained glass and fused glass.
But there is a science behind how these cutters are designed to work.
The most commonly asked question about glass cutting, is “ How much pressure should be used when scoring the glass?”
Let's start by diving in and learning a little bit more about about what a score really is and how it allows us to control where the glass breaks.
What is a Score?
Think of a score as an almost invisible path that we make to basically “tell” the glass where we want it to break.
If you’re new to glass cutting, you should know that we don't actually cut glass in the traditional sense of cutting something. But rather, we create a small fissure along the surface of the glass. The cleaner the fissure, the cleaner the break.
The Science Behind Breaking Glass
When placed in the jaws of running pliers, a plain piece of glass that hasn't been scored, will experience tension on the top surface and compression on the bottom. With only a slight bit of pressure on the handles of the running pliers, there isn’t enough localized stress to break the glass.
But once the glass is scored, the equation changes.
Because of this tiny fracture we call a “score,” we have created a point for localized stress to build up along the fracture. It creates a line or crevice where the thickness of the glass is thinner and this allows the glass to break under the same amount of force exerted from the running pliers, which in our last example had no effect on the glass.
So now that we understand what a score is and how it affects breaking glass, we can dive into the question of how much pressure is actually needed to create a GOOD score.
What Makes a Good Score?
If too much pressure is used, instead of causing a small fissure or separation on the surface, the glass crushes creating a dusty line that is full of little nicks and chips. Instead of having just one stress point to direct your break, each of those little nicks cause additional weakness or stress in the glass making the break less predictable and more likely to go awry.
A smooth flowing motion with consistent pressure across the piece will create the most reliable score. And, you might be surprised add how little pressure is needed to create a good score!
Are You Using the Right Pressure to Cut Glass?
The most common mistake, is applying too much pressure. Here are three methods you can use at home to test you're cutting pressure.
Method # 1
Using your bathroom scale can help you determine how hard you need to press to score the glass. Holding a pencil in your hand in a similar fashion to how you hold your cutter and with the eraser pointing downward, press down on the pencil until your scale reads about 6 lb. This will be the approximate amount of pressure you need to use in order to make a good score to cut glass.
Try this a couple of times to become familiar with the amount of force you need to use.
Method # 2
Using your bathroom scale again, place some folded newspaper (this will act as a cushion for your cutter) with a piece of glass on top. Using your cutter on the glass, practice scoring across the sheet of glass while aiming for your scale to read 6 lbs.
This will give you a feel for how hard to press while scoring and whether or not you keep consistent pressure.
Method # 3
For this method, you will need a piece of glass roughly 8 in by 4 in. Cutting the glass across the shortest distance, make a score about an inch from the edge using your normal cutting pressure. Look at your score. Do you see chips? Is it white and dusty or powdery looking? Or does it look smooth and almost invisible? Sometimes you can only see the score if you tilt the glass a little to one side. Now, use your running pliers to break the glass and then look at the 2 raw edges that you just broke. How do they look? They should appear smooth.
Now we're going to repeat this exercise and cut the glass again. Only this time, we're going to do it with a little less pressure. Inspect the score and then run the break. Look at the glass edges again. Did it break easily? Is it smooth?
Repeat the process again with even less pressure to cut the glass. Keep repeating this whole process until you get the lightest pressure for scoring that still breaks cleanly and easily. That’s the correct amount of pressure you will need to cut the glass.
Other Factors to Consider
The type of cutter you use as well as it’s age, rather the wear and tear on it, will play a factor in how hard you need to press when making your score.
Transparent glass will often cut easier and with less pressure than opalescent glass.
Even different colors will cut slightly differently, and every manufacturer and style of glass will cut differently too.
For more simple tips to improve your glass cutting skills, read: 14 Strategies to Improve Your Glass Cutting.
Is the Sound the Glass Cutter Makes a Good Indicator?
Sound can be a helpful indicator when scoring glass, however it shouldn't be your main comparison.
While transparent glass tends to make more of a clicking and popping noise, opalescent glass will often create less sound. If you were judging you're cutting pressure based solely on the noise your cutter makes, you would be pressing much too hard on opalescent glass to create the same amount of sound you would hear while scoring transparent glass.
What To Do
Grab some scrap glass and your cutter and give these exercises a try. Try all or only of the methods described in this article and let me know in the comments below what you discover about your scoring pressure when you cut glass.