In my timed read-through of this passage I had chosen E over B because of my misinterpreting of the grammar. I had found where it mentioned the 19th century and had eliminated it based on the word "until". In my mind this meant that this glass making technique existed before but not during the 19th century so it could not differ from today.

I also misinterpreted support for E through the same sentence. The passage reads "before the 19th century the only..." which to me was suggesting that the author believes that all glass that was made before the invention of the new technique had been subjected to these impurities. Medieval glass was made before the 19th century and thus this is how this glass came to be.

Not sure why this one specifically gave me trouble over the others. Based off the analytics I seem to be the only one who psyched myself out on this question.