why do you need either of those for making a hat?

When felt was made from animal fur, it made it easier and less gross the remove the hair from the hide and soften it.

Previously, urine was used.

Yikes. Knowing what we know now, is mercury the worse option?

It depends. Urine is not pleasant, but is usually safe to work with. I’d rather smell than suffer brain damage. :)

shiny when applied to silk.