The makeup of the ferrule portion of a brush is critical for some processes, but is not as critical for most silver gelatin emulsions, at least the ones I make and teach. You see, processes like cyanotype or VDB have relatively free metal ions in solution that can react with the metals in the ferrule. With Silver Halides though, there is little metal to interact with. So, almost any brush will work as long as the hairs are suitable for application.