"Felt" is actually a process rather than a specific
fabric. Traditional felt implies
sheep wool, but the process really means a matting of fibers through friction, rather than a knitted
fabric or a
fabric woven so that the fiber ends are exposed (which is called velvet when
very fine.) So "felt" can come in various materials and degrees of smoothness. Basic craft
shop felt these days is usually plastic/polyester/acrylic fibers and probably not ideal for coins. Cotton felt is probably most inert.
Sheep wool is probably more durable and traditional and has a tiny amount of residual lanolin, which would have a mild reactive aspect, partly as a moisture barrier and perhaps leading to "cabinet
toning" over time. Wool is what is found in most traditional coin cabinets. If you
had a
fine wool felt, such as merino, you'd have little risk to coins and a traditional look. But more expensive than acrylic.
As to putty or wax, there is both Renaissance Wax and Quakehold, both of which purport to be inert. But there are probably methods to make photos without needing any substance on the coin at all. Most photo set ups keep the coin horizontal.
