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Back in 1961, the Chicago River was not something that local residents were proud of; in fact, it was a receptacle for trash from many local businesses. It is unclear how the river dyeing tradition began in 1962, whether it was inspired by an accidental spill of dye on white plumbers coveralls or by using pipe leak detection dye in the river itself for Mayor Richard Daley to crack down on polluters, but by St Patrick’s Day that year, 100 lbs of fluorescein dye in oil was added and lasted a full week (J.M. Wood, 2022).

Eco-conscious minds of today likely understand oil-based solutions should not be mixed in high concentrations into water, but the oil dilutant was not the only issue.

The colorant in the dye itself actually causes adverse allergic reactions in humans, as fluorescein is a xanthene-based organic compound belonging to the triarylmethane dye family.

SIDE NOTE: A similar dye compound with the same safety hazard labeling was researched by Dr. CC for 2 years during her Chemistry Masters synthesizing nanoparticles to detect heavy metal ions in water.

By 1966 local residents with the environment on their mind petitioned for a change in compound to a vegetable-based dye, to ensure a low ecological impact with this yearly tradition.

Tough luck for us curious readers, the local plumbers union feels that “revealing the formula would be akin to ‘telling where the leprechaun hides its gold.'”

Let’s follow the rainbow to deduce this secret using a favorite science tool: a hypothesis.

A hypothesis can sound intimidating, but can be fun if you see it as your loose boundaries and goal.

At CArtLab Solutions, we like to test hypotheses as working question(s) to guide the research journey and scope.

In the case of this mystery formula, we can start by asking a set of questions like how does the orange powder go from a solid to a liquid and then change color from orange to green?

Let’s follow one of these at a time:

1. Solid to Liquid: It is a pigment but is dispensed in the picture above in what looks like a bright yellow green solution. Usually, pigments are dissolved into a medium used to aid in spreading the color to different surfaces. In the case of the original application of leak detection in water, an oil solution helped locate a trail through the interface formed between these different materials. In the context of dying the river, dispensing an aqueous (water) solution helps it dissolve to a bigger but similar body with less danger to the life of the Chicago River, and follows the old chemistry adage: “like dissolves like.” This also helps us understand that the colorant in question is soluble in water.
2. Color change from orange to ‘Chicago St. Patty’s Day green: This clue helps exclude a simple green food coloring dye, such as FD&C Green No. 3 (Fast Green). Therefore, it is also extremely unlikely it would be a mixture of the other food dye options, which are proven to be hazardous toward human neurobiological growth, not to mention the ecological impact on local wildlife.

At this point in solving our hypothesis, a little art theory is useful.

The color wheel represents how colors mixed together creates other colors; however, there are different color wheels for absorption of light versus transmission of light.

If a material (or powder) absorbs a certain color, then the final color seen by us will be it’s complementary color; whereas if a material has no color (like a body of water) the transmission of color will be it’s true color. (https://www.acs.org/content/acs/en/education/resources/highschool/chemmatters/past-issues/2015-2016/october-2015/food-colorings.html).

For example, a blue natural dye like anthocyanin in transmitted light will appear blue, but with absorbed light or in an absorbing medium like a powder, it will appear orange (https://sites.middlebury.edu/chem103lab/2017/01/23/colors-part-ii-absorption/).

This helps us understand some of this puzzle, but the water turns bright green not blue. So if we have an orange powder, what else could be in it that helps it look green in water?

Making green with subtractive color theory (yellow-blue-red) includes the mixing of blue and yellow pigments, so the blue pigment would appear orange in powder, but wouldn’t the added yellow pigment look purple and thus appear as a brown powder mixture? However, if a yellow colorant like the water soluble carthamidin from safflowers was added already in water, then this proposed combination would also help explain why the dispensing liquid in the picture looks so incredibly yellow not green until it is in the water (https://formulabotanica.com/38-natural-colourants-skincare/).

CArtLab Solutions proposes that Chicago’s St. Patty’s Day river dyeing secret sauce is likely the combination of powdered pigment vegetable dyes such as anthocyanin with diluted safflower colorant for the yellow to create a bright green aqueous river solution!

Hope everyone continues to dye safely with vegetable-based pigments for all future St. Patrick’s Day traditions, and please reach out if you have any other information to verify or disprove our hypothesis!