Once you have laid each one of those thousands of layers meticulously

Once you have laid each one of those thousands of layers meticulously on top of the previous one, and you have heated the whole thing at a precisely controlled rate to a precise temperature, when it cools you will have glass with very specific properties.

“That’s how you control the light,” Mazzali said. “That’s how you can trap the light inside, by playing with different attributes of the fiber.” He was smiling, excited. I asked for the recipe—the identity of the gases being blown through the flames to create the soot particles—but he wasn’t telling. “A little bit of germanium, a little bit of this, a little bit of that.”
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Mazzali was showing me a demonstration version of the printing-on-a-rod process that Maurer and Keck had created at Corning; the real process, he says, takes several hours. He led me to a tubby, short white tube and had me touch it.

This was what all the printing of soot I had seen ends up creating. A white chalky substance came off on my fingers. “Sorry about that,” said Mazzali. “Those are actually particles of glass. It’s not just silica. We are doping each one of those layers with different materials and different amounts of materials, to change how they reflect light.”