I have a friend with a home wood shop, and he said I can get his clean sawdust for free. Will this make good casing to keep things from drying out? Hay is a common casing for wine cap, so why not sawdust?
In this article on mushrooms on sawdust, the authors claim that sawdust is great for some mushrooms, but it is helpful to choose sawdust from hardwoods. In addition the casing requires special sterilization attention, the use of particular grow bags and the addition of nitrogen.
The answer, once again, is that it depends. If the sawdust comes from treated wood then certainly not. If the saw dust comes from untreated wood then it might be alright. Also, what type of mushroom are you talking about? For about 100 different species of mushroom I would say no but for many varieties I would say yes. I am assuming your talking about wincaps? If that's true then give it a shot. Its a very healthy variety and is quite stubborn once it spawns. So experiment and feel free to come back to this post at a later date in the future to divulge your results.