Abstract:
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Introduction:
A couple of years ago, I learned that it was possible to dye eggs blue using red cabbage.¹ Typically, we have used a variety of artificial coloring options for our egg-dying needs, whether liquid food coloring or the store-bought Paas-type kits. Last year I was determined to try my hand at doing some natural dyes with vegetables. In the end, I gave up on my plans for using onion skins or artichokes. (The water from steaming artichokes is often an intense bright blue-green, but not from the particular ones I made that day). But I followed through with the cabbage.

I had forgotten how long it took to dye the eggs, but looking back at the photos, I see that it did indeed take a lot longer than the food coloring. So be warned: The eggs took a good couple of hours of soaking to get blue.

Methodology:
I started by cutting up some red cabbage and boiling it in some water.²

The resulting juice was quite purple, and I was doubtful that it would produce blue. It was, however, quite pretty. (6:18 p.m.)

We dunked the first egg and let it soak. 16 minutes later, a peek showed the egg looking somewhat lilac-colored. (6:34 p.m.)

At some point, I added a bit of vinegar to the cabbage juice, inspired by the instructions for dying eggs on the box of food coloring. The purple cabbage juice turned even redder, which made me even more doubtful of achieving blueness. So I poured some more cabbage juice into another glass to have one without vinegar, and dunked another egg to soak.

Here we are, almost an hour after first dunk. Getting to be the kids’ bedtime. Time to break out the chemicals. Here’s Phoebe, squeezing out some blue food coloring. (7:22 p.m.)

I don’t have a time for when the first egg (from the vinegar mixture) came out, but it did indeed come out blue eventually. Having read up a bit on red cabbage (as one is wont to do), I had learned that red cabbage juice changes color based on pH levels. Acid leads to redder colors, and adding something alkaline, and raising the pH, should make it bluer. I then tried adding baking soda to the cabbage juice with the vinegar. The change was instant and dramatic, turning from red to greenish blue.

Here we are, hours after the first dunk. (11:27 p.m.) The two glasses show “neutral” cabbage juice (left), and alkaline cabbage juice (right). In the background are the rest of the completed eggs, mostly dyed with food coloring. (I think the first cabbage dyed one is there in the photo, too. Second row, left, behind a yellow egg.)

Results:
Here are the chemically-dyed (top) and cabbagely-dyed (bottom) blue eggs arranged together. The lighter-colored leftmost cabbage-dyed egg is the one from the baking soda solution. (Blotchiness is due to condensation that happened from putting the previously-refrigerated eggs outside for the egg hunt.)

3 of one, a half half dozen of the other.

Discussion
In the process, I realized why it is that it helps to add vinegar to dye eggs. Egg shells are composed primarily of calcium carbonate. Calcium carbonate is commonly used to neutralize acidity and raise the pH level: it is the main ingredient of antacids such as Tums, as well as agricultural lime. Acids can dissolve calcium carbonate. I’m guessing that adding vinegar starts to break down the egg shell, allowing the color to permeate and bond more quickly to the shell.

This would explain why the redder cabbage juice with added acidity led to a bluer shell (or got there faster) than the bluer-appearing cabbage juice with baking soda added.

Future study:
This year, I’m hoping to try the cabbage dye again, and also to experiment with beets, carrots, berries, and turmeric. I also may play around with acidity levels of the dye solutions again, as well as using brown eggs in addition to white. I wonder if pre-soaking an egg in vinegar would make it more permeable to dyes. (Did you know that you can dissolve the shell off an egg with vinegar? That’s another science experiment for us to do.)

Conclusions:
Can you tell I’ve been wrapped up in academic writing? I need to get to bed.³

References:
More resources on using natural food dyes for eggs can be found at various places around the web:
Natural Easter Egg Dyes on about.com, Making natural Easter egg dye, Three ways to dye eggs, Natural Easter Egg Dyes

Appendix:
Here are all of the photos from above, plus a few more.

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¹ I think it was from NotSoSage, who sadly, has purged her blog archives. I’m pretty sure she also made red/purple eggs using red onion skins.
² That’s not entirely true, I started by buying a red cabbage. And there were steps leading up to that as well. I had to get up in the morning, for example. Sometimes that is the hardest step.
³ Seriously, I need to get to bed.