I'm pretty sure the effect here is because the paper was so old. Prints on new paper just have more of a toned effect. Also, I suppose it makes a difference how thoroughly you mix the ingredients. I'm a musician, and I'm often drawn to John Cage's philosophy in that random/unpredictable elements can be effective artistic tools. That being said, if you want absolute control with repeatable results, use professional chemistry in your trays and drink your coffee.
I'm glad you like the shot. I'll see if I have something available online to show caffenol on regular papers.
Tom, on Point Pelee, Canada