Why did the knocker on the door of Scrooge's house look odd?

When he returns home on that fateful Christmas Eve, Scrooge is shocked to see the face of his late business partner, Jacob Marley, appearing on the door knocker. There's something strange and otherworldly about Marley's face; there's no indication as yet that he's a troubled spirit forced to wander round in chains for the rest of eternity. But for now the face vanishes, leaving Scrooge feeling more than a little perturbed. After all, this isn't something that happens every day.
Later on, Marley's ghost will make a more lasting appearance, dragging his chains across the floor as he urges Scrooge to change his ways before it's too late, before he ends up the same way as his former business partner. Scrooge is utterly terrified by this spooky encounter, but it won't be the last he'll have to endure on this long Christmas Eve.