Lee Filters are extremely well regarded filter makers, and this video shows just what goes into one of those pieces of neutral-density resin that we stick in front of our lenses. While you might think something as seemingly straightforward as squares of clear material with a slight gradation would be easy to construct, this factory tour from 2010 proves otherwise. Each stage of the process, from setting the resin, checking it for physical abnormalities, testing its hardness, cutting it, dying it, testing its darkness, and more, are all done by hand.
It looks like a lot of these steps could be mechanized, but the amount of care and dedication that goes into doing this work manually says mountains about Lee Filters, as well the workers who have been doing it for decades. Sure, you could probably automate how long you submerge the filter in the dye bath, or something, but the touch of an experienced craftsman is always fantastic.