Daub is the hustlers' term for any kind of substance that can be used to smear marks on the backs of playing cards. Daub is usually used to mark the cards during a game. In a game of blackjack, for example, a daub worker would commonly mark only the 10-value cards, or sometimes the aces; in poker, the aces and kings, or the low cards, and so on. Professionals usually work in mobs, so the player that marks the cards is not the same player that takes advantage of the marked cards. A daub worker will mark the key cards very quickly and usually leave the game (usually by loosing all the money). An accomplice will come at a later time and take advantage of the work that has been put on the cards during his/her absence. If the marks are discovered, the culprit literally had nothing to do with the act of putting the work on the cards. Often the second player will even take the work off, in the later part of the evening, before leaving the game.
The daub pictured on this page is professional-grade daub, sometimes called N-daub. To protect the secret of the gaff I cannot really reveal what the "N" stands for, but I can say that it is made from a recipe that has been handed down through several generations of card cheats. The liquid shown next to the daub is called daub rejuvenator. This liquid is used as one of the ingredients in the making of the daub. This kind of daub can easily dry-up if it is not maintained. So, to avoid that, one drop of this liquid can periodically be added to the daub and left overnight to be absorbed.
The metal can is magnetic, so it can be worn at the underside of a jacket by simply placing a strong magnet into the pocket. This can is actually smaller than the size of a quarter, but it literally contains a lifetime supply of daub. Only a very small amount is used by tapping (not rubbing) the tip of a finger onto the surface of the daub, before transferring it to the back of a playing card.
Unlike luminous daub, this daub does not require the use of special glasses or contact lenses to read the work. When used properly the work is almost invisible; obviously it is not literally invisible, otherwise it would not work, but for all practical purposes it can be said that an untrained eye cannot spot the work even if told what to look for. If the daub is put-on a tad too strong it will appear as an innocent smudge. Good daub (such as this one) will have properties that allow for it to be tuned down, by rubbing some of it off, while still being able to withstand normal handling and last throughout one evening, without being rubbed-off too soon.