Artifact
Nonartifact spells you cast have improvise. (Your artifacts can help cast those spells. Each artifact you tap after you're done activating mana abilities pays for

.)
If you cast a nonartifact spell that requires you to sacrifice a permanent as an additional cost, you may tap that permanent (if it's an artifact) for the spell's improvise ability before you sacrifice it to pay that cost.
Multiple instances of improvise are redundant.