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I've read elsewhere (from... let's say a "less than flawless" source) that the Craft Skill Hirelings always send the highest-level materials you can use. How true is this?
You will receive the highest level raw materials based on your crafting first passive skill points unlocked.
For improvement tempers (Dreugh Wax, Kutas, Tempering Alloys, etc.) those drop random ... but you can improve your chances by having the hirelings passive itself fully unlocked.