In Aventuria, iron hinder magic. The more iron a spellcaster wears, the more difficult is it for him to use his magical abilities. His skilllevel in the different spells sinks. Probably like this:
heavy weapon (> 2 lbs.) -1
iron handcuffs -2
iron footcuffs -2
iron collar -2
iron frontlet -4
helmet -2
heavy helmet -3
light iron armour -3
heavy iron armour (>24 lbs.) -4
Captured magicians are normally bound with an iron frontlet, an iron collar, and hand- and footcuffs (Skill-10).
To bring this in line with with unhindred Magery one could access a -10% limitation "Hindred by Iron" on Magery
Druids are even more vulnerable to metal. Any forged item negates their power.
In game terms this could be modeled with a 15% Pact limitation and the -15 point Vow "Never touch or use forged items".