At the risk of prolonging his “most ridiculous argument,” I will point out the following cite from the OED, hat, n.:
OE Antwerp-London Gloss. (2011) 57 Galerus uel pilleus, fellen hæt.
The Latin pilleus means “felt cap or hat,” and galerus means a “conical cap of leather, fur cap.” The OE fellen literally means “made of skins,” and may refer to felt. So the existence of felt hats goes back to antiquity, although they weren’t made with mercury until the seventeenth century.