This is an explanation for passage 4 of LSAT preptest 75, the June 2015 LSAT. This passage is about why some old glass windows are thicker at the bottom. The author debunks the common myth that solid glass flows downwards. New research from Edgar Dutra Zanotto adds empirical data to confirm this debunking.
This section has paragraph summaries and an analysis of the passage, links to the explanations for the questions are below.
- Despite myths to the contrary, glass won’t flow downwards unless heated to the glass transition temperature.
- A new study confirms that glass flow is not the cause of thicker windows in medieval cathedrals. Glass will very slowly flow downwards – but on a timescale beyond the age of the universe.
- Even the glass that flows most easily, germanium oxide, would take trillions of years to produce the effects seen in medieval windows.
- Window thickness differences probably come from old glass manufacturing methods. Old methods used to produce thicker ends. These ends were placed at the bottom for stability.
This passage has a lot of details which I couldn’t include in the paragraph summaries. You don’t need to know all these details or even necessarily understand them. But you should know where to find them.
I know a bit more than what I wrote about each paragraph. For instance if someone mentions “impurities in glass” I remember that’s in paragraph 3, when the author was talking about composition of medieval church windows.
The more of the science you understand, the better you’ll do. But you don’t need to understand everything to get all the questions.
My strategy on these passages is to read things I don’t understand over again. Maybe you won’t get a perfect understanding, but you’ll at least understand more than before, and this will help you make sense of the rest of the passage + the questions.
The gist of this passage is that regular people falsely believe that glass flows like a liquid. Important: Glass researchers do not share this belief. See lines 1-4 for both facts.
So Zanotto’s research is not proving anything that glass experts didn’t already know.
Zanotto’s research is interesting, and it provides new data to support existing beliefs, but it hasn’t changed our view of glass.
I’ll explain the physical structure of glass, then cover Zanotto’s research.
Some key facts about glass:
- Liquid and solid glass have similar structures. Their atoms aren’t in a crystal structure. (Lines 9-12)
- Liquid and solid glass are thermodynamically different. This means they flow differently.
- Glass doesn’t freeze from liquid to solid. Instead, it has a transition temperature.
- When sufficiently cooled, liquid glass keeps its atomic structure, but becomes physically like a solid.
Basically liquid and solid glass have similar structures. But solid glass behaves like a solid. Heat glass hot enough, and it starts behaving like a liquid.
However, a lot of people believe in a myth: they think that solid glass flows downwards because it has a liquid structure. (Remember, glass experts do not share this belief. See lines 1-4 “To glass researchers it seems strange….”)
This belief is false, and that’s what Zanotto’s research helps confirm.
Paragraphs 2 and 3 describe Zanotto’s research. He helped conclusively confirm that it would take a long time for glass to flow downwards. You don’t really need to know the details of these paragraphs beyond that.
The final paragraph explains the real reason that glass is thicker at the bottom of many older windows: past manufacturing techniques produced uneven window panes, and the thick ends were placed downwards.
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