I never met the man I was told is scientifically associated with this sample; his name is Eli A. Silver. I searched a bit and found this: “During February 1994, we carried out twenty ALVIN dives in search of indicators of fluid venting on the Costa Rica prism. As the trench off Costa Rica reaches depths of 4500m, the ALVIN went through a process of refit and Navy clearance to dive to depths of up to 4500m, and our deepest dive reached that depth.”
I’ve had the most fun asking various people who might know, “What is an accretionary prism?” I’ve heard many different answers and seen many vivid pictures drawn in air with describer’s hands. Most personally important is that I feel, through this material, a close association and robust curiosity about subduction zones.
The material, when applied thinly on leather-hard clay is plain smooth matte brown. When applied thickly on bisque-fired clay, it contracts as it dries and curls until later, in the firing, it relaxes down and forms tightly bound clumps of glaze that reveal lots about surface tension and clay wall stresses from forming. What I sense while handling it is an association with the terrific pressure of that zone of plates grinding. An example to ponder: