It's a lobstered gauntlet. Very fine. Not the heaviest but not the sort of thing a man is going to want to be smacked in the face with.

"It was this, an alchemist's gloves - I don't think I would want any of us to touch the outside of those, I don't know what they've been fiddling with - or a blacksmith's glove. I did not think the polite gentleman would wish to be struck with something unclean." Araceli smiles as if butter wouldn't melt in her mouth, that very same smile reflected half a hundred ways in the gauntlet, all polished and glittering

Because Guillaume deserves to know what he's been smacked with after all. Actually-- "I'll need that back," she steps out of the way when he lunges, "it belongs to a very obliging guard, she could break you both."