I have these large collage pieces on panels that I made a few years ago. My work has changed a little since then, at least in technique, so rather than throw these older pieces away, I tried to bury the existing surface in a mixture of gesso and piles of acrylic gel medium. When it dried, it left this interesting surface. The Prussian Blue areas were originally a steel grey, which seems to have interacted with the gel to make this colour. The result reminded me of the cyanotypes I've been making, so I taped up a few of those, and they seem to match: Hmmm....food for thought.