You'll Need:

a fresh "dressed" fish (scaled and gutted)
a plastic container several inches longer, wider, and deeper than the fish
a large box or two of baking soda (sodium bicarbonate)
a small scale, like a postage scale (optional)

Try This!

1. Fill the plastic container about 2 inches deep with baking soda. (If you have a scale, weigh the fish now and jot down how much it weighs. You may also want to note what the fish looks like--the colors on its skin, whether it looks shiny or dull, and so on.)

2. Once the fish is prepared and weighed, pack the gut cavity of the fish with baking soda. Place it on top of the baking soda in the container and cover it with more baking soda, so that it's a couple of inches thick on the top and sides.

3. After a week, remove the fish and brush off the excess baking soda. What does the fish look like now? If you have a scale, try weighing the fish again.

4. Throw away the used baking soda (inside the fish and out), and repack the fish in clean baking soda, just as you did a week earlier.

5. Wait another week to ten days and remove the fish again. By now you should have a stiff, leathery, fish mummy!

What's Going On?

You've probably discovered that, after two weeks in baking soda, your fish has been transformed into a stiff, dry mummy. It may look dull, feel rough, and have lost half its body weight (or more), but it sure doesn't smell like a two-week-old fish!

These dramatic changes are direct consequences of dehydration--removing water from the fish's body. The baking soda draws the water out of the fish's cells and tissues. But how does this happen?

Baking soda is the common name for the salt sodium bicarbonate. Because of its molecular structure, water can be incorporated into the sodium bicarbonate molecules, but the molecules remain dry--up to a point.

In your experiment, most of the baking soda remained dry, even though it absorbed a lot of water. However, you may have found that the baking soda packed closest to the fish--and exposed to the most water--was somewhat moist. The fresh baking soda used for the second week of mummification increased the efficiency of dehydration.

If you'd just left your fish out on a shelf, exposed to the air, bacteria and fungi would have begun to decay the fish, creating strong, unappetizing odors. Since all living things require water to survive, removing the water from the fish greatly inhibited the growth of these organisms, decreasing the unpleasant effects of rotting.

So What?

The Ancient Egyptians used a naturally-occurring salt from the banks of the Nile River, called natron, to mummify their dead. Natron is made up primarily of sodium carbonate (a very efficient, but relatively expensive, dehydrating material), with about 17% sodium bicarbonate (the baking soda you used), and very small amounts of sodium sulfate and sodium chloride (table salt).

Long before refrigeration and freezing of foods were available, drying, pickling, salting, and smoking were common means of food preservation. Dehydrated foods are still commonly used by backpackers and astronauts in space, and home dehydrators are popular appliances.

This activity, created by Karen Kalumuck of the Exploratorium Teacher Institute, is presented in conjunction with Revealing Bodies, a major new exhibition on view until September 4, 2000