Many people are unaware, but indigo dye is almost as old as civilization itself. Archeologists have even found it in cloth wrapped around Egyptian mummies and on Babylonian tablets. But getting the deep blue out of these plants is no easy task! Our predecessors devised quite the recipe to extract the dye—one that started with 3.5 gallons of stale urine and 4.5 ounces of salt.

That’s right, your eyes are not deceiving you—traditional indigo dye recipes called for vast quantities of urine. In some parts of the world, dyers even paid their neighbors for bowls of urine in order to create the dye that was called “blue gold,” for more reasons than one.

Today, however, indigo dye is produced artificially. A typical characteristic of indigo is that it does not seep into the fiber during the dyeing process; it actually envelops it like a coat. To achieve a saturated and deep blue, the cotton is put through a series of dye baths. These immersions determine the final look. The inside of the thread keeps its white color, and is revealed by abrasion or bleeding during the washing process, which gives the denim its typical mottled look.

The classic blue jean remains the ultimate favorite around the world. Perhaps that’s because blue jeans are suitable for everyone from blue collar workers to blue blooded royalty.