Iron and Magic

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 (>1 kg) -1

iron handcuffs -2

iron collar -2

iron frontlet -4

helmet -2

heavy helmet -3

light iron armour -3

heavy iron armour (>12 kg) -4

Captured magicians are normally bound with an iron frontlet, an iron collar, and hand- and foodcuffs (SL-10).

To bring this in line with with unhindred Magery one could access a -10% limitation "Hindred by Iron" on Magery

Druids and Metal

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".