From what I've discovered, it must be a Japanese Lacquer Brush. Traditionally these are made from the hair of pearl divers, "selected as the finest and densest fibre."[1] Another reference I found quotes that "They say that the salt strengthens the hair"[2]
Thanks! I guess that makes sense, but I didn't think it through. I wonder what the original author thinks of that, do they expect their audience to be aware of this? Weird.