@Le_Doctor_Bones
It wasn't so much as half-burned as it was just...y'know, singed.
That said, as someone who used to play with fire and paper...it takes a surprising amount of effort to get paper to ignite, and given that the fireplace had no fire (and the relative competence of the guy) we can be pretty sure that he didn't pay enough attention to the actual disposal of the artifact in question. Probably just assumed that anything tossed into embers will just burn immediately without ever actually watching it.
Also, this is the first time in the entire series where Mathias made a mistake. 27-month-long chapters in.