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. Scratch and thumbnail marks change the back of a card. Some inks can be read only with special contact lenses or glasses. Near-infrared marks described in the trade press are read with a camera.
On the record
A 2025 federal indictment in Brooklyn charged alleged organized-crime associates with rigging illegal poker games. The Justice Department said the games used altered shuffling machines, a chip tray that read cards with hidden cameras, an X-ray table, and special contact lenses or eyeglasses that could read pre-marked cards. Phones were described as relaying hand information, not as reading card markings.
Detection & preventionADVISORY
The paid halfHow a monitoring room catches card marking (juice, daub, block-out, scratch work), and what stops it recurring, written for the desk, not the reader.
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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.