Graham,
Many thanks for this information.
The Arnold viewer is very similar to, if not the same as, the 3M
plastiform viewer. Our experience is the same as yours: the sponge in
the container needs to be kept wet, you can't quite see the edges of the
tape, and they work better with tapes containing high recorded levels,
although the tracks are visible enough with some lower level tapes. This
is just one diagnostic tool that we use--our ears, eyes, and the meters
on various types of tape machines are the others.
I have stayed away from the Kyread developer because I am hesitant to
place any substance on an archival original tape. I'd love to hear
arguments one way or the other about this. From your photos, it looks
like it works quite well.
Mike