about glass doorknobs?

On our trip to New Orleans, we found a shop on Magazine Street selling great 20th century stuff, including a selection of glass doorknobs.

The proprietor came up to us while we were admiring the display of sparkling glass doorknobs. R. said we have porcelain doorknobs in our house, but that she prefers the look of the glass doorknobs. The store owner informed us that the porcelain doorknobs were older, that the switch to glass doorknobs came with a move towards cleanliness later on. He said that “you could tell if the maid had cleaned the doorknob” easier if it was glass.

This sounds feasible, but I couldn’t find anything online in support.