Wood, and other organic remains, can survive quite well in an environment that is either very wet or very dry. They can also be preserved in a carbonized state, which is what we find at Dust Cave. Usually a combination of techniques that best fits an individual site is used to understand the botanical record because not all are appropriate at every site due to preservation and other considerations. They looked at charcoal remains from Maiden Castle and were not only able to reconstruct the paleoenvironment but also used the diameter of the burned wood to determine where the wood came from. Softwood is usually recognized as having longitudinal tracheids in a filar arrangement, the absence of vessels and fibres, and specific ray patterns.
Hardwoods usually have vascular tracheids as well as vessels and fibres.
Fortunately charcoal does not need to be chemically treated in any way in order for it to be examined under the microscope. If the charcoal is in extremely bad condition it can be soaked in resin, allowed to dry, and ground down like a rock sample. This only need be done at last resort.