in spanish we have the phrase "más duro que el pan de ayer", what means "harder than yesterday's bread". i know it is irrelevant and incoherent but in my defense i would say that i'm just high. but i would be lying.
The counting rule? Also, "a piece of" or "a loaf of" aren't specified, it could be a bag of leftovers from breakfast, or crumbs from the world's buisiest toaster for all we know. Since there aren't any specifics, an article isn't really required.