The plates make up Earth's outer shell, called the lithosphere. (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a ...

As if all of that wasn't enough, every time you decided to take a picture, you'd have to set up your darkroom tent, prepare a glass plate, then expose and develop the plate while it was still wet.

To make up this non-exhaustive but still-extensive ... both point out that each plate, even among a set, is slightly unique. This gives them a “less mass-manufactured quality,” Rellosa says.