Benson is a master printer who is extremely knowledgeable about virtually every form of photo reproduction. His wonderful book The Printed Picture should be required reading for all digital printers. Benson's singular expertise is the use of a 4-color offset web press to make one-off fine art prints. As I understand it, Benson makes his own custom color separations and uses a modified Epson printer to put down separate layers of ink to increase the sense of depth in the print. This requires disabling the sensor in the Epson printers that detects ink already on the page, which will otherwise prevent multiple pass printing.
There was an interview in View Camera magazine a couple of years ago with Craig Blacklock that discussed this. Blacklock described how to print twice on the same sheet using an Epson 7600, using the first pass just to lay down black in the image's shadows to increase shadow depth on matte rag paper. This required taping a thin piece of white paper over the leading edge of the already printed sheet to hide the first layer of ink from the printer's sensor, plus some tricks to get the paper aligned precisely in register for the second pass. I tried this and was not very impressed with the result. Laying down more matte black ink did not appear to appreciably improve depth or D-max, but it did make the print surface more fragile and prone to smudging.
