What determines an items powder slots?

What determines the number of powder slots on an item? And how would I increase it?

 

The higher level the materials are, the more powder slots there are

 

The crafting materials? Like the stars next to them?

 

I meant whether it's for example oak wood (level 1) or light wood (level 70)

 

weapon sprite depends on what ings you use and what level materials you use

1-30 tier 1 sprite 1 powder slots
31-70 tier 2 sprite 2 powder slots
71+ tier 3 sprite 3 powder slots

you need to put either powders of that element or ings with specific ids (raw melee or spell for thunder, main attack% or str for earth, spell% or int for water, etc) to get your desired sprite otherwise it'll just be a t2 basic weapon