No weapons as such can be made with Raw Ash, but you can make arrows. By combining 10 Branches, 10 Raw Ash, 1 Black Blood, and 1 Feather, you can craft some Smoke Arrows if you've unlocked the Specialist Ammunition IV feat. These arrows will create a large cloud of obscuring smoke on impact. I wouldn't say these are useful, but if you're trying to desperately escape from a PvP encounter, they may provide just a little bit of cover that you can use to escape. They're far from essential though.