Field guide

Card Marking (Juice, Daub, Block-Out, Scratch Work)

How it works

The practice of altering playing cards to create secret identifiers of suit and/or rank visible only to the cheater. Methods range from subtle physical alterations to invisible inks requiring special readers or lenses. Cheaters use “juice” (a tinted liquid that subtly changes pigment shades on card backs), daub/paste (substances applied during play), block-out work (inking or painting to alter back designs), scratch work (using needles or abrasives to alter patterns), or luminous/infrared inks visible only with special contact lenses or glasses. N-Dust technology allows marking on plastic cards.

On the record

The 2025 federal indictment against an alleged mob-linked gambling ring described the use of “invisibly marked cards, electronic poker chip trays, phones that could secretly read cards’ markings, and even specially designed glasses and contact lenses.”

Published as a detection reference for surveillance, compliance and gaming-operations professionals. Thresholds and tuning are set by the property. Nothing here is instruction — the method is described so it can be recognised.