lvl18 +6 weapon Need advice about crit

[shoto]

Ok so i'm a little new to the alchemy i have a lvl18+6 blade and it has a horrendous crit, it is 4. I wanna raise the crit to atleast 7~8. How do i go about doing this? wha are the risks involved? how can i increase my chances? Any help will be appreciated and dont be shy...you can have long drawn out explanantion

[red_baron2000]

Just use a stone of challenge. Using stones won't affect any plus you have. Also, it is all luck so you cannot increase your chances it will work.