Darklands Puzzle #4

(Yes, #4, I didn't say I was doing them all.)

Here is a grim iron door inscribed:
ONLY ONE KNOB OPENS THE DOOR.
EACH METAL'S STATEMENTS ARE EITHER BOTH TRUE OR BOTH FALSE.
One the floor are six statements and three metal knobs, each of a
different metal.
Gold: Tifsyn's only son is Hardgrep.
Gold: The gold knob opens the door.
Silver: The gold knob opens the door.
Silver: The silver knob does not open the door.
Copper: Tifsyn's only son is Gymer.
Copper: The gold statements are both true.
You press...
...gold.
...silver.
...copper.

(Source: Darklands cluebook by MicroProse Software)

Well let's see here: Copper claims that Gold's statements are both true. This mean's that Tifsyn's only son is Hardgrep. But Copper also claims that Tifsyn's only son is Gymer. We have a contradiction! Because what Copper is saying in one instance cannot possibly be true, as stipulated by the puzzle, both of Copper's statements are false. This means that Gold's statements are both also false. Where does that put Silver? Well Gold and Silver both claim the gold knob opens the door. That means that everyone here is a liar. Very sad! The silver knob opens the door.