Sylvania light bulb glass strength demonstration

Tom's thesis advisor Bob (Uhlmann/Rose — see editorial register) would obtain freshly-made light bulbs from the Sylvania North Shore plant and demonstrate in 3.091 that a bulb thrown against the wall would bounce when fresh and shatter two days later, illustrating humidity-driven Griffith-criterion surface flaw formation.

When the glass is first formed, it has very perfect surfaces and extremely high strength. When we make fiberglass — if we make the fiberglass and then within minutes coat it with plastic and keep the moisture off — it turns out the humidity in the air will slightly attack the glass on the atomic scale, and create flaws that will embrittle the glass per the Griffith criterion. I've never done it, but my thesis advisor lives up on the North Shore, he's retired now. Sylvania had a glass factory up on the North Shore where they made light bulbs, and Bob would stop in the morning — he'd worked out a deal with the guy there — and he'd get a freshly made light bulb, and he'd bring it in when he was lecturing 3.091. He could take that light bulb and throw it across and hit the wall, and it would bounce, because it was freshly made. If he did that two days later, after it sat in the humid Cambridge atmosphere, it would shatter. But freshly made, it had no flaws on the surface, it had not been corroded by the moisture in the air, it had tremendous strength.

Combined with the Bob Rose anecdote; explains why fiberglass is coated with epoxy immediately after drawing — to preserve the 200 ksi surface strength before moisture corrosion creates micron-scale flaws that destroy fracture toughness.

The other story on glass is freshly made glass, as it's blown, has a very perfect surface. Glass actually corrodes in the moisture in the air — it's a silicate, and it forms silicate hydroxides. The strength of glass will change dramatically within a few days after its manufacture. One of the secrets of fiberglass is they draw the fibers and coat them with epoxy or whatever plastic resin, and they keep the moisture away from attacking the surface of the glass, so those fibers maintain fantastic strength — 200 ksi if you could actually measure them.

Same as above; Sylvania's Beverly, MA plant was the source of the freshly-made bulbs. ## Figures referenced

The story I know is, my old thesis advisor Bob Rose lives up on the North Shore. Sylvania had a glass light bulb manufacturing plant up there in Beverly. He used to lecture 3.091 back in the days when I was a student, and he would stop and get freshly made light bulbs on the way in before his lecture. He would take the light bulbs in 10-250 and throw them across the room, and they'd bounce. They wouldn't shatter, because they were freshly made. They did not have surface imperfections that caused them to fracture.

Tom's old thesis advisor (3.091 lecturer, North Shore resident) would stop at the Sylvania lightbulb plant in Danvers to pick up freshly-blown bulbs that could be bounced across a classroom. Used to teach that brittle materials are flaw-controlled — freshly-made glass with no surface imperfections is extraordinarily strong.

When I talk about glass being strong when it's freshly made: glass has no imperfections on the surface if it's made properly. And it's the imperfections on the surface of a brittle material that weaken it. My old thesis advisor used to teach 3.091, and he still lives up on the North Shore. They had a Sylvania lightbulb plant up there in Danvers, and he used to stop in the morning before his lecture and get freshly made lightbulbs — fresh-blown glass — and bring them in for his 10 o'clock lecture. He could throw them across the room and they would bounce, because they were freshly made. The glass had not been attacked by the humidity in the air. After a few days you throw it across the room and it'll shatter just like any lightbulb you know of. But freshly made glass, before it starts to get etched by the moisture in the air — it actually does on a microscopic scale, but it's so brittle these submicron imperfections destroy its strength.