STUFF MATTERS: Chapter 7

CHAPTER 7: INVISIBLE
• On glass
• Sand is a heterogenous mixture of different substances, a large proportion of which are quartz made from silicon dioxide crystals.  When this quartz crystal is heated to its melting point, the silicon dioxide molecules break apart and dissociate.  Unlike water, however, as it starts to cool down, the silicon dioxide molecules do not rearrange themselves in the same crystalline pattern but rather in an amorphous solid material that has the “molecular structure of a chaotic liquid, a glass”.
• The Romans developed much of the process for the practical, cheaper production of glass.  They invented the use of flux, a substance mixed with a solid to lower its melting point, in the form of natron or naturally occurring sodium carbonate.  Although not a new process (the Egyptians and the Greeks have developed it before the Romans), the Romans found a way to make it cheaper to be accessible by the masses.  The Romans invented the use of glass for:
o windows
o mirrors by layering a thinner mass of metal over transparent glass
o thin-walled wine glasses developing the technique of glass blowing (blowing air into glass while red hot)
• In discussing why some materials are transparent while some are not, the author first starts off with a good analogy of the structure of the atom and its sub-atomic particles:
o “Inside an atomic stadium, to continue the analogy, the electrons are only allowed to inhabit certain parts of the stands. It is as if most of the seats have been removed and there are only certain rows of seats left, with each electron restricted to its allotted row. If an electron wants to upgrade to a better row, it has to pay more—the currency being energy. When light passes through an atom it provides a burst of energy, and if the amount of energy provided is enough, an electron will use that energy to move into a better seat. In doing so, it absorbs the light, preventing it from passing through the material.”
o But there is a catch. The energy of the light has to match exactly that required for the electron to move from its seat to a seat in the available row. If it’s too small, or to put it another way, if there are no seats available in the row above (i.e., the energy required to get to them is too large), then the electron cannot upgrade and the light will not be absorbed. This idea of electrons not being able to move between rows (or energy states, as they are called) unless the energy exactly matches is the theory that governs the atomic world, called quantum mechanics. The gaps between rows correspond to specific quantities of energy, or quanta. The way these quanta are arranged in glass is such that moving to a free row requires much more energy than is available in visible light. Consequently, visible light does not have enough energy to allow the electrons to upgrade their seats and has no choice but to pass straight through the atoms. This is why glass is transparent.  Higher-energy light, on the other hand, such as UV light, can upgrade the electrons in glass to the better seats, and so glass is opaque to UV light. This is why you can’t get a suntan through glass, since the UV light never reaches you. Opaque materials like wood and stone effectively have lots of cheap seats available and so visible light and UV are easily absorbed by them.”
• “Chemistry was transformed by glass perhaps more than any other discipline.”  Glassware allowed them to run reactions in sturdy but inert vessels that also allowed them to see the material inside for mixing and monitoring.  The use of pyrex glass was a further improvement as it introduced a higher degree of thermal tolerance.
• Addition of boron oxide to regular glass as an additive reduces the tendency of glass to expand and contract under temperature changes allowing pyrex glass (borosilicate glass) to maintain structural integrity and strength even after a series of thermal changes.
• Toughened glass, the kind used in cars, was designed to fracture into smaller pieces upon impact.  It is made by cooling the outside of the glass fast enough to allow a compression state to develop.
• Laminated glass shatters but remains in one piece because of a layer of plastic in the middle acting as glue to keep the shards together.
• Bulletproof glass uses the same technology as laminated glass, just many more layers of plastic alternatively imbedded in the glass. Hitting and breaking an outer layer of glass absorbs some of the energy of the bullet and “blunts” its yip.  The plastic layers act as a damper that absorbs and redistributes the force over a larger surface area.  The higher the number of laminate layers, the more energy can be absorbed upon bullet impact:  one laminate layer will stop a 9mm pistol bullet, 3 a 0.44 magnum pistol bullet, and 8 layers can stop AK-47 bullets.  Plastic used must have the same refractive index as the glass so that transparency is maintained uniformly as the glass gets thicker.
• Cultural significance:
• The use of glass, Pyrex in particular, allowed the field of chemistry to progress much faster as an experimental science; it is the “workhorse” in the chemists’ lab.
• The author suggests that because the Chinese and the Japanese ignored the use of glass for a thousand years, this may have caused them to miss out on valuable technological innovations and contributions while glass “revolutionized one of Europe’s most treasured customs”.
• Use of glass in lenses has allowed us to explore the microworld and the astronomical world, allowing humans “to transcend scale”.
• It has allowed us to live inside and be protected from the elements but still be able to see outside.
• “Perhaps it is because we look through it rather than at it that glass has not become part of the treasured fabric of our lives”: it is INVISIBLE, “not just optically, but culturally”.