I tried Suminagashi paper marbling yesterday, you have to be very patient. The process reminded me of making a zebra cake, which having made once, I promised myself never again.
I started with quink ink, and then thought I might get better results with calligraphy ink.
I have to say the quink ink worked best, but I still don't believe it was the best ink to use so any tips would be gratefully received.
In this process you use two soft brushes, one for ink and one for soapy water, alternating between the two, gently adding them to the surface of a tray of water. when you have made a detailed tree ring of concentric circles you can use a cocktail stick or other sharp thin implement to draw a pattern through it and then print onto paper by laying the paper on the surface.