TomeNET

(5.10a) Equipment damage from elemental attacks                                 
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Armour can get damaged when an elemental monster attacks you or when you get
hit by elemental magic. Blocking an attack with a shield may damage it in the
same manner. Weapons may take damage if you hit an elemental monster with them.
Armour might also rust from moving through water without a tarpaulin equipped.

Armour/shields getting damage means a reduction of their +AC bonus by 1 point
each time, while weapons getting damaged means a reduction of their +damage
bonus by 1 point each time.
AC bonus won't drop lower than half the negative amount of the base AC.
Damage bonus won't drop lower than -10.

If the player has resistance to the element, this will cut down the probability
of any equipment damage. Having double resistance (permanent + temporary) will
cut it down even further.
To completely prevent any equipment damage, you need to become immune to the
element, or the equipped item needs to ignore or resist the element.

The following table lists the elemental damage taken by various item types.

                    Acid    Fire    Water
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Wooden boomerangs   x       x
Metal boomerangs    x               x

Swords              x               x
Axes                x               x
Blunt                       x
Polearms                    x

Leather caps/Hats   x       x
Metal caps/Helmets  x               x
Iron crowns         x               x
Crowns
Leather shields     x       x
Shields             x               x

Boots               x       x
Gloves              x       x
Gauntlets           x               x
Cesti               x       x       x
Cloaks              x       x
Soft Armour         x       x
Metal/steel Armour  x               x
Mithril/adamantite
DSM Armour