There may have been also another reason for additional gum prints on top of a
platinum: Like today it is somewhat difficult to get a deep black with platinum
only and the developing process and this may have been even more difficult in
earlier times with a less clean ferric oxalate. Also the matt paper surface
does not support a Dmax over 1.5. But the gum added a glossy surface to the
shadows and made them deeper.
Klaus Pollmeier