Can the material used to print Xrays be used as an ND filter ? I would like to do some long exposure photography and since I am stuck at home , so was thinking if it is possible.
Film blackened by exposure and developing has long been used as a neutral density filter for viewing the sun during an eclipse. While this procedure produces a safe to use filter for visual use, it has its drawbacks. X-ray film generally has an emulsion on both sides, thus it has twice the density of pictorial film. This density is due to the accumulation of metallic silver. Now photo films and X-ray films are poor substitutes for a photo grade ND, so my answer is, give it a try. Why not? The drawback will be turbidity within the emulsion.
Consider using the ND filters or polarizing filter you already own. Mount one and cover with aluminum foil with a pin-hole in the center. This pin-hole overlay might just do the trick for you without any added filters.