An hour or so before Chicago’s annual St. Patrick’s Day parade last Saturday, members of Chicago’s Journeymen Plumbers Union added about 40 pounds of dye to the Chicago River, transforming a stretch of the waterway into a vivid green.

The irony is that the dye is orange. But once mixed in water, it takes on a bright emerald hue. The parade committee’s website comments “This spectacular transformation, ranks right up there with the parting of the sea by Moses and the Pyramids of Egypt.” Given the origins the word “blarney” springs to mind.

This tradition of dyeing the river dates back 47 years to 1961, (some say 43) when Stephen Bailey, a business manager for the plumber’s union, noticed a visiting plumber with vivid green stains on his boiler suit. The stains were caused by a special dye used to detect leaks in water tube boilers and condensers on ships amongst other general uses. Chicago had just begun enforcing pollution controls that year and the plumber was using the dye to locate sources of illegal waste disposal into the river.

The following year, Mr. Bailey, with the consent of city officials dropped 100 pounds of a disodium salt called Fluorescein into the river. It worked a little too well, turning the water green for a week.

Eventually they hit upon an amount that would turn the river green for just one day.

Fluorescein can be toxic, and Mr Bailey had to face environmentalists, concerned about the welfare of the river’s goldfish. They lobbied to have the dye replaced. In 1966, the parade committee agreed to switch to a vegetable-based dye. The dye’s exact ingredients are a closely guarded secret. The parade committee compares the formula to that of Coca-Cola. In a 2003 interview with the Columbia Chronicle, a student newspaper,a parade organizer compared revealing the dye’s composition to “telling where the leprechaun hides its gold.”

Despite the secret, the parade committee assures all that the dye is non toxic, and claim that “the formula has been thoroughly tested by independent chemists and has been proven safe for the environment.”

Environmental regulators in other cities have rejected plans to dye their rivers for the Irishholiday. In 2005, environmental regulators in Broward County, Fla. rejected plans to dye Fort Lauderdale’s New River. And this year Michigan’s Department of Environmental Quality rejected plans

A site sponsored by parade organizers quotes Bailey:
“The Chicago River will dye the Illinois, which will dye the Mississippi, which will dye the Gulf of Mexico, which will send green dye up the gulf stream across the North Atlantic into the Irish Sea, a sea of green surrounding the land will appear as a greeting to all Irishmen of the Emerald Isle from the men of Erin in Chicago land, USA.”

The place where leprechauns hide their gold in is known as a crock.