Large Original Fused Glass

Making wonderful glass mosaic tile art is easy! Let me show you how.

Ever wonder what glass actually is? Do we cut it or break it (is there a difference)? Why and how does scoring a line allow us to control the way glass breaks? To fully understand why we cut glass the way we do, we should first understand a bit about glass itself.

There are two types of solids: amorphous and crystalline. (Huh? Amorphous? Wasn’t he a character in the movie series The Matrix? No, that was Morpheus.) As you’ll learn in my eBook in the chapter on Tessera Types, glass is an amorphous solid (the chapter explains what glass is to help the mosaic artist understand how to make better cuts and breaks, and to help reduce wasted glass). An amorphous solid, such as glass and plastic, has molecules arranged randomly in no particular pattern. On the contrary, a crystalline solid has molecules arranged in fixed patterns, sometimes called lattices. Most solids are crystalline, such as metal, ice, and diamonds.

Glass can break in a controlled manner (e.g., along its score line) because it doesn’t have a specific molecular structure. For example, a diamond breaks cleanly along its fixed molecular structure or cleavage (more commonly understood as “grain”). If you don’t properly align your breaking tool along the grain, the diamond can shatter. However, because glass doesn’t have a grain, you can break it in any direction without it shattering. The question is how do we get it to break the way we want?

Depending on the tools used, glass can either break from exceeding its tensile strength or cut from exceeding its compressive or shear strength. To control the fracture, we must define where to exceed the tensile, compressive, or shear strength to result in a controlled fracture. We do this by scoring the glass when we want to break it by applying a tensile stress or properly aligning the cutters when we want to cut it by applying a compressive or shear stress. For example, the glass’ tensile strength along a tiny score line is less than anywhere else on the glass, so it tends to break cleanly along that line (i.e., the break follows the path of least resistance).

When a separation occurs because of tensile stress, the separation is called a “break.” When a separation occurs because of shear stress or compressive stress, the separation is called a “cut.”

When using a scoring tool and running pliers on stained glass, you apply tensile stress to break the glass. When using wheeled cutters with the two wheels aligned (or other tool with the cutting edges aligned, such as nippers), you apply compressive stress to cut the glass. When using wheeled cutters with the two wheels misaligned (e.g., because you dropped the tool and bent the jaws out of alignment), you apply shear stress to cut the glass. (The most familiar example of a cutting tool with misaligned cutting edges is a pair of scissors where the two cutting edges are side by side instead of aligned.)