Fir, pine? Bark is bark. Not good for your serpent. Of course cedar was always off limits, but the bark of just about any tree is not that good. I personally believe that cypress mulch is OK and it has some bark in it, but the conifers like pine and fir are too strong. Aspen bedding is the best of the wood products to use.
That soil/sand mix you were talking about is bad. Both soil and sand are hard to keep clean and parasite free. I've tried nuking soil in the microwave. It just gets mad. I got out the magnifying glass and after wetting the soil and nuking it for over 10 minutes, there were still little white bugs in it. Presumably mites of some kind. Same with oven baking it. I guess they have such little mass and moisture content, they resist conventional baking. Sheesh. If I had an autoclave, I'd discover their flash points.
I'm not sure about the coconut bark stuff. It appears to be fairly safe, but I wouldn't recommend it 'till I personally had a chance to test it.
Good luck,
Don
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South Mountain Reptiles