Você pode lançar o feitiço Graxa em um item (como uma arma)?

In 5th Edição, isso não é possível, de acordo com a descrição do Feitiço de graxa no 5e:

Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the Duration.

When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.

Você deve estar se lembrando de que edições anteriores deram Graxa a capacidade de segmentar um item. No Descobridor e em 3.5 por exemplo. Aqui está a descrição do Feitiço de graxa em PF:

The spell can also be used to create a greasy coating on an item. Material objects not in use are always affected by this spell, while an object wielded or employed by a creature requires its bearer to make a Reflex saving throw to avoid the effect. If the initial saving throw fails, the creature immediately drops the item. A saving throw must be made in each round that the creature attempts to pick up or use the greased item. A creature wearing greased armor or clothing gains a +10 circumstance bonus on Escape Artist checks and combat maneuver checks made to escape a grapple, and to their CMD to avoid being grappled.