I don't give my birds anything to bathe in. No special mixes of dirt/sand, or any thought given, to the ratios of said mix. They take care of that, themselves. They just dig a hole, under the coop, out in their yard, where ever, and do what they want. Unless you are keeping them on concrete, or penned inside the coop, they will take care of it.
So in the winter time they will be kept in their pen that is covered and has sides on it. Too much snow outside. But in the summer their favourite place to dust bath was under a big evergreen tree. So I just need some kind of set up for winter time. They do dig down in the wood chips to the smaller particles at the bottom, but there's really not much for them to actual bath in. And I've noticed they haven't been as clean lately.