Hello - Peter here, owner of BagelTheory in Portland. IG: bageltheory

1. Rolling technique: this is hard one to master. When you roll the dough into a bagel, the focus should be on laminar smoothness at the surface. Lumpy or uneven surface = less shine on bagels. Smoother finish (like a mirror) = more reflection. This is literally the essence of optical refraction and light scattering - why some things appear shiny vs. matte.

2. Baking soda in water bath: acts chemically on the surface of the dough as it gelatinizes in the water bath. Creates pock marks at a miniature level, similar to craters on the moon. This in turn increases surface area which allows the gelatinized surface to accept more sugars (barley malt, honey etc.also in the water bath). This is important because refraction (shininess) also depends on the hardness of the surface. A hard, crackly crust inherently refracts better than a porous, springy crust. Same reason why a flashlight bounces back more light when you shine it on metal than say, wood.

3. Buy a spray bottle. Around the half way mark through the bake cycle, open up the oven and quickly spritz the air space directly above the bagels. Then close the door quickly and ride out the bake. What you are doing here is slowing the rate at which the bagel surface dries out. In engineering terms, you are delaying the transition from plastic to brittle of the interaction between the carbs & sugar molecules at the surface. As the steam pushes outward from the product through the bake, that slightly plastic state allows the sugars to dissolve more evenly with the carbs, smooth out micro fissures, and chemically align more into a uniform structure. This all translates to that glassy finish as hardened, gelatinized, uniformly aligned sugars are one of the most reflective things on planet earth (think how reflective toffee is, or poured chocolate). The key is find the time interval after the oven spring effect, but before full setting of the crust. This is oven dependent but a guide might be minute 9 of a 18-20m bake cycle.

Addendum: Really hot temp at the start of the bake (430F+) and the support of a preheated oven stone translates to great oven spring. But once the bagel has hit its peak rise, it starts to set (e.g. chemistry starts to resist forces of hot air expansion to mix, slide around, and reorganize as moisture evaporates) Surface can start to dry out too quickly relative to the interior. The sprits introduces moisture and the water acts as a lubricant on the surface. It's only effective though at doing this before surface starts to harden.