Are you having trouble with your stained glass not break right? There are many things that go into a good score line; the pressure, your technique, glass cutter and type of glass.
When you score, the line is creating a vent crack of micro fractures in the glass surface. As the wheel tip wedges the crack open, the wheel pushes on alternate sides of the crack making a score line. The most common mistake after a score line doesn’t break the way you want is to press to harder (just force it to go the right way). When you press too hard you create more lateral fractures and chipping in the surface, so when you break your score line it may choose one of those other fracture lines that you did not intend it to make. Just remember that you are not trying to cut through the glass with your cutter but trying to guide the glass in the way you want it to break.
(Scoring too hard)
(Comparing too hard vs. normal scoring)
When scoring, try and stay as perpendicular to the glass a physically possible. If you score at an angle your glass will break at an angle. Staying perpendicular will give you less to grind and less unintentional breakage. When I cut, I try and use my body instead of moving only my hand. I move my hips and shoulders, which helps me stay perpendicular to the glass especially when I’m going around curves. Also, pay attention to where you are on the table. Make it easy for yourself, move your pattern around to make each piece the most reachable for cutting.
Spend the money and get a carbide wheeled cutter, you’ll save money on glass breakage and your sanity. Steel gets dull fast and your line won’t be as precise. If you find yourself still pressing too hard try using a pistol grip cutter. The pistol grip’s shape will make it tougher to press too hard and make you use a more uniform pressure. I also use my right index finger (I’m left-handed) to guide my cutter along the path I need. Cutting oil is a must but not for the reason you might think. Cutting oil gives you a better score line by helping your cutter wheel not age and get dull as quickly. Plus it helps the wheel not bind in the head so you get a nice push of the cutter (would you use you sewing machine without oil?). I don’t fill the handle of my cutter it usually dispenses too much oil per cut. I use a little cup with a sponge on a container with a little oil and I run my cutter over it periodically.
Stained glass is not only different from each manufacturer but is also different from each color and texture within each company. Glass made a long time ago is hard to cut but not because it’s old but because the techniques used to make it was less reliable and more flawed. We are spoiled by all the wonderful glass manufacturers making glass today. If you are a new to stained glass try cutting new glass don’t try taking the old glass from windows and making things, you will be very frustrated.
In conclusion, use the right amount of pressure, keep your hand straight, use a good cutter and good glass. Keep in mind that even doing all these things, glass will still break the wrong way. Even the best stained glass artists have around 20% scrap. If doing this was easy, everyone would do it. But, when you see all your hard work in your finished stained glass piece you’ll know that it was worth it and know that you are one of the few that can do it. :)