by Ana Wang, Graduate Student at The Scripps Research Institute
It’s that time of year when we can’t walk into a store without seeing displays of chocolate bunnies, marshmallow chicks, and vibrant bouquets of flowers. Along with all these springtime treats, perhaps the most memorable and engaging tradition (and my personal favorite) is the dyeing of Easter eggs.
Decorating eggs is a deeply rooted, international tradition. The oldest examples of this activity are the engraved ostrich eggs found in Africa 60,000 years ago. Eggs across many countries and cultures are celebrated, and whether these activities are based on religion or the coming of spring, eggs represent rebirth and life.
For those who have participated in egg dyeing, you may have wondered: Why is vinegar added to the dye solution? This can be explained through simple chemistry. You've probably heard of acids and bases - it turns out that all solutions have a degree of acidity or basicity. Acids chemically react with bases to create solutions that are more neutral – that is, closer to pure water.
So, back to the initial question – why is vinegar needed to dye an egg? The answer is that most egg dyes need acid to bind the dye to the eggshell.* Vinegar is an acid, and eggshells are bound together by a basic molecule called calcium carbonate. The eggshells base molecules assure that neutral and basic solutions will not change the eggshell. However, vinegar will react with the calcium carbonate shell to dissolve it slightly, allowing the dye molecules to stick to the eggshell, giving you vibrantly colored eggs.* Without adding vinegar to your dye solution, you will likely get very faint coloring.
Have you ever tried changing the amount of vinegar added to your dye solution? Because acid is needed for the dye molecule to stick to the egg, you might predict that adding more vinegar to the solution will give you more colorful eggs. This may be true to an extent, but be careful! The acid reacts with calcium carbonate to produce carbon dioxide gas, which will float out of the solution, like carbonation in soda. This gas forming on the surface of the eggshell can leave behind streaks, causing a blotchy dye job. That is why there is an optimal amount of vinegar recommended for the dye solution. Too much vinegar or leaving the egg in the solution too long will eventually dissolve the eggshell. For a neat science experiment exploring this, click here ; you can also find this experiment in Dr. Olivia Mullins' book here!
Have you ever seen someone dye eggs with a silk tie? (If not, watch this clip from Martha Stewart!) The patterns on a silk tie can be transferred from the tie onto the egg by wrapping the egg in the tie and submerging it in water with vinegar. Eventually, some of the dye from the tie will be transferred to the eggshell. This is because silk ties are usually dyed using dyes that require an acid to bind – the same reason that dyeing eggs requires vinegar. By adding vinegar to the water, the tie’s dye molecules can be transferred over to the eggshell.
Now that you know the chemical principles behind this tradition in addition to some tips and tricks for optimal coloring, there is no reason for not making this Easter your most colorful yet.
*Going further: Egg dye molecules are typically sodium salts of a negatively charged molecule called a phenolic acid. In an acidic solution, it gains a H+, allowing it to interact with the surface of the eggshell. In particular, dye molecules interact with slightly negative parts of the eggshell, including the calcium carbonate and some parts of proteins.
Other fun egg experiments
· “Silky Science: Tie-Dyeing Eggs.” Scientific American. 21 March 2013. https://www.scientificamerican.com/article/bring-science-home-silk-egg-dyeing/. 26 March 2017.
· Stewart, Brian. “Egg Cetera #6: Hunting for the world’s oldest decorated eggs.” University of Cambridge. 10 April 2010. http://www.cam.ac.uk/research/news/egg-cetera-6-hunting-for-the-worlds-oldest-decorated-eggs . 26 March 2017.
Bunny and eggs: Superbass, CC BY-SA 4.0, via wikimedia commons
Basket of Bulgarian Orthodox Easter Eggs: lkonact via wikimedia commons
Egg in Vinegar: Yat-Long Poon, in Experimenting with Science, Wiley Publishing