A dye pack is a radio-controlled incendiary device

Incendiary device

Incendiary weapons, incendiary devices or incendiary bombs are bombs designed to start fires or destroy sensitive equipment using materials such as napalm, thermite, chlorine trifluoride, or white phosphorus....

 used by some bank

Bank

A bank is a financial institution that serves as a financial intermediary. The term "bank" may refer to one of several related types of entities:...

s to preemptively foil a bank robbery

Bank robbery

Bank robbery is the crime of stealing from a bank during opening hours. According to the Federal Bureau of Investigation's Uniform Crime Reporting Program, robbery is "the taking or attempting to take anything of value from the care, custody, or control of a person or persons by force or threat of...

 by causing stolen cash

Cash

In common language cash refers to money in the physical form of currency, such as banknotes and coins.In bookkeeping and finance, cash refers to current assets comprising currency or currency equivalents that can be accessed immediately or near-immediately...

 to be permanently marked with red dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 shortly after a robbery.

In most cases, a dye pack is placed in a hollowed-out space within a stack of banknote

Banknote

A banknote is a kind of negotiable instrument, a promissory note made by a bank payable to the bearer on demand, used as money, and in many jurisdictions is legal tender. In addition to coins, banknotes make up the cash or bearer forms of all modern fiat money...

s, usually $10 or $20 bills. This stack of bills looks and feels similar to a real one, with new technology allowing for the manufacturing of flexible dye packs which are difficult to detect by a criminal handling the stack.

When the marked stack of bills is not used, it is stored next to a magnetic plate near a bank cashier

Cashier

Cashier is an occupation focused on the handling of cash money.- Retail :In a shop, a cashier is a person who scans the goods through a machine called a cash register that the consumer wishes to purchase at the retail store. After all of the goods have been scanned, the cashier then collects...

, in standby or safe mode, ready to be handed over to a potential robber by a bank employee. When it is removed from the magnetic plate, the pack is armed, and once it leaves the building and passes through the door frame, a radio transmitter located at the door will trigger a timer (typically 10 seconds), after which the dye pack will explode and release an aerosol

Aerosol

Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 (usually of Disperse Red 9

Disperse Red 9

Disperse Red 9, also called C.I. 60505 and 1-methylamino anthraquinone, is a red dye derived from anthraquinone. It has the appearance of a red powder with melting point of 170-172 °C. Its EINECS number is 201-417-2....

) and sometimes tear gas, intended to permanently stain and destroy the stolen money and mark the robber's body with a bright red color. The chemical reaction causing the explosion of the pack and the release of the dye creates high temperatures of about 400 degrees Fahrenheit (204 degrees Celsius) which further discourages a criminal from touching the pack or removing it from the bag or getaway vehicle.

The design of a modern dye pack is available via US patent #5196828.