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Can you develop a way to improve it? Or does better equipment later equal more durability? 

Maybe make it so that the more you make one thing, eventually you can make that one thing more durable as time goes on?

Better equipment has more durability. The idea behind the equipment system in Soulash is to eventually replace anything so that a single great drop doesn't lock a slot for most of the game. You can always craft more items as you learn recipes, including the best rarity items.