Part 2: Why Does Glass Break?

Last week we started to look at the question of why optical fiber bends so much. We learned that glass is rigid at an atomic level and metal has more flexibility. In order to really understand why things do not break, we need to know why and when they do break.

Materials break, or give, at the weakest point. In metal, usually the bonds are the weakest point, and since the bonds are flexible, the metal can flex without breaking. Flexibility can add a lot of strength because it can withstand force by giving a little rather than breaking. Think of a willow tree vs an oak tree.

In glass, however, the bonds are not the weakest point. The bonds are very strong, rigid, and reinforced by the poorly ordered placement of molecules, so any microscopic crack, scratch or impurity in the glass becomes the weak point. Any cracks on the surface split wide open as the glass is pulled on the outer curve, and the glass breaks. Any bubbles (called seeds), impurities, or internal stresses will cause breaking as well. This foam block is a good representation of flawed glass-any cracks are worsened due to stretching on the outer curve, causing glass to break.

Any crack, when split by bending, will cause the glass to completely break. Thus, even if it is thin enough, typical glass such as soda-lime or borosilicate will only bend a little bit before it breaks. So how can we bend optical fiber enough to wrap it around a pencil? Well, part of that is again due to the thinness, but here is where the manufacturing process and the purity of the glass come in.

Soda-lime and borosilicate glass are usually very impure and physically flawed; the glass is full of tiny bubbles, inclusions, impurities and internal stresses and the surface is full of tiny cracks and defects. When these glasses are made, they are made in factories which are not carefully controlled for dust and humidity. Dust in the air, faster cooling, and contact with other surfaces when hot will cause microscopic cracks and stress in the surface of the glass. When pressure is put on the glass by an attempt to bend it, those cracks immediately give way and the glass breaks.

Moisture or dust in the air can interact with the raw materials of the glass which can cause impurities. The processes of making glass often leave tiny imperfections within the glass such as bubbles or tiny un-melted grains of raw ingredients. The raw materials also contain a lot of impurities to begin with, such as metal oxides. The main ingredient of glass is silica, mined out of sandstone along with a lot of these impurities. You can try to purify the glass partially, but some impurities will still be left. The fact that glass is greenish when very thick, like a glass shelf or table, is usually due to iron impurities in the glass.

When you try to bend this type of glass, even if the surface is pristine, those impurities will cause the stress to concentrate in one place around that impurity and that will become the weakest point. Thus the glass will break.

Optical fiber is made out of a special glass called High Purity Fused Silica. It is made in a laboratory-like, clean-room environment in a patented process combusting a very pure silicon-containing flammable gas, called silane. The manufacturing process of the glass is carefully controlled from start to finish. There are no impurities in this glass because it doesn’t start off as sand, or another ‘dirty’ product mined from the earth. Silicon tetrachloride gas is burned and the soot is then collected and melted into glass. This creates an absolutely pure glass with no bubbles, inclusions, or impurities.

Once the glass is made, the manufacturing process of the fiber is also carefully controlled in a clean-room type environment. The blank, a solid cylinder of glass with two layers, is melted and pulled very quickly. Other than the very tip where the pull is started, the glass touches nothing while it is pulled. The moment the glass is cool enough, which is a few seconds, the glass is coated with a layer of plastic. This protects that perfect surface. So long as the plastic remains on the glass, it has no surface imperfections.

Looking at the foam block again, a pristine surface and pure material means the material can stretch enough to bend without breaking.

So if we look back to where glass breaks, at its weakest point, we can see that in optical fiber there are no impurities, imperfections, or tiny micro-fractures where it would break. Both inside and out, this glass is near-perfect, and there are no weak points. So the weakest point becomes the molecular bonding, which we know is incredibly tough due to that less ordered structure. The glass can then withstand that stretch on the outer curve to accommodate the bending of the glass.

The ability of glass to bend has been a wonderful trait which is used every day to make lots of different products. Today we have optical fiber, which is used every day to communicate and in medical implements such as gastroscopies. There are bendable flat glasses which cover cell phones and televisions. In the near future it may be new technologies currently being developed (for example bendable touch pad devices.) In any case, bending glass will certainly play into the development of our communication, electronic, medical, and many other industries.