The mail bombs were wrapped in brown paper with similar address and advertising labels. Inside, wrapped in bright green paper and stamped "Gimbel Brothers-Novelty Samples", was a cardboard box containing a six-inch by three-inch block of hollowed wood about one inch in thickness, packed with a stick of dynamite. A small vial of sulfuric acid was fastened to the wood block, along with three fulminate-of-mercury blasting caps. Opening one end of the box (the end marked "open") released a coil spring that caused the acid to drip from its vial onto the blasting caps; the acid ate through the caps, igniting them and detonating the dynamite.