This house rules originated from desire to separate weapons made from different materials.
In a setting were the process of manufacturing steel weapons is not known by every culture or there is no iron mining in the region and trading is scarce, there has to be some substitute material, which is more common.
I assume that the weapon and armor stats in the original edition and every subsequent edition are created for steel equipment. Steel is durable, hard to break and hard to bend in comparison to other metals or alloys. If other materials are used as substitute there should be a difference in play. The following rules are a simple approach to create that difference between steel(standard weapon rules from the system rule book), bronze and stone.
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EDIT:
After I shared my blog post on reddit, I received some nice and interesting comments about bronze. You should check them out. Further one user shared one idea for more attrition. It is implemented as optional rule for arsenic bronze melee weapons.
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Bronze Weapons
An attack roll of 1 with a bronze melee weapon decreases the weapon damage permanently.
If rules for natural 20s are used, an attack roll of 20 the player can choose to deal normal damage or double damage, but decreasing the weapon damage permanently. The decrease is applied after the damage is dealt.
Depending on what kind of weapon damage is used, the decrease is applied differently. If all weapons use 1d6, the damage is permanently modified by -1. This effect is cumulative.
For variable weapon damage the same method with the negative modifier can be used. Or the damage die is demoted to the next smaller die [d10 -> d8 -> d6 -> d4 -> unarmed damage].
Optional: The above rules apply to tin bronze. For arsenic bronze each time maximum damage is rolled, the damage is decreased. This applies additionally to the rules above.
When the maximum damage of a melee weapon is equal or less than unarmed damage, the weapon is totally broken and cannot be repaired.
Missiles with bronze tips can only be used once. Reuse for a later combat is not possible.
The Armour Class of Bronze Armour is decrease by 1 point compared to their steel equivalent.
Stone Weapons
An attack roll of 1 with a stone melee weapon destroys the weapon.
If rules for natural 20s are used, an attack roll of 20 the player can
choose to deal normal damage or double damage, but destroying the weapon.
Missiles with stone tips can only be used once. Reuse for a later combat is not possible. The damage from stone missile weapons (except slings) is modified by -1.
