What is Risograph printing?
    

Risographs are duplicator printers from Japan that create stencil/image “masters” that are applied to cylindrical ink “drums” to print spot-colors. Risographs print one color at a time in a process that is similar to offset & screen-printing.

Each ink drum contains a single color, & the masters are made from grayscale color-separated files/images. The risograph reads grayscale as ink opacity ー higher or darker opacity (black or 100%) results in highly pigmented & vibrant print with dense ink coverage. 

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Riso Process Color, Duotone, & Spot-Color Printing:

To print photographic images on riso, 4-color CMYK color-separations are replaced with different riso process color (spot-color) combinations.