Seeing inside a shell

This exploration involves making something called an endocast (that means a cast of the interior) of a snail shell. In nature, endocasts can form when a shell or a skull fills with mud before it is buried. Later, ground water sometimes removes or dissolves the shell or bone, just leaving the internal cast.

The real process takes ages, but you can make a much quicker inside cast of a shell by filling it with plaster, and dissolving the shell with acid. Plaster of Paris can be messy stuff, so be careful always to work in the middle of several large sheets of newspaper, and be sure to clean up after yourself. Better still, use a large plastic bowl as your work space.

You also need to talk with an adult about a safe place to leave the acid, and you should get some help in diluting the acid and pouring it. Remember that acid spills are best treated with lots of water, so keep water handy, and work in a place where you can splash water around if you need to.

Get a triangular or flat file, a hand drill with a fine (no more than 2 mm) bit, some snail shells and/or sea shells (try to get some variety!), plaster of Paris, an old yoghurt container, water, an old teaspoon, gardening gloves, rubber gloves, hydrochloric acid, and some safety glasses.

Endocasts of a garden snail shell (left) and a sea snail shell. I had
decided early in the book not to include this project, so I have no
shots of the process to offer, and only this one 20-year-old slide

of the results— and that had to be scanned, hence the quality.

Start by making a scratch at the top of the shell with a triangular file or the corner of a flat file, so the drill has somewhere to grip. Then working very carefully, put on a gardening glove, hold a shell and drill a small hole in the top.

This hole is there to let the air escape when you stuff the shell with wet plaster through the main opening. Drill each shell and test the holes by pouring water into each shell with a wash bottle, then put the drilled shells to one side while you get the plaster ready.

Spoon some plaster powder into the container, pour in a small amount of water, and stir gently, trying not to get any bubbles in the plaster. Mix the plaster and water carefully, until it is about as thick as cream, then try to move any air bubbles to one side, out of the way. Keep adding water until it seems about right; if you add too much water, use a dry spoon or spatula to add a small amount of extra plaster.

When the plaster is ready, use your fingers to push the wet plaster into the opening of the shell until a small 'worm' of wet plaster starts to ooze out the drill hole. Wipe the 'worm' away with a damp tissue and leave the plaster to set. Fill all the shells with plaster, and then go outside and wash the implements you used. Tip the leftover plaster into a hole in the garden or into a container to go in the bin, not down the drain!!

When the plaster is hard, you can put the shells in a beaker (a glass jar will do for this, and so will a plastic yoghurt container), and add some dilute acid. Pool acid or muriatic acid (both are really hydrochloric acid) will be fine, but remember that the acid you buy is strong, so you need to break it down, one part of acid to about nine of water. Strong acid will foam and spit dangerously. As the shell reacts with the acid, bubbles and foam may overflow, so don't fill the beaker more than half-way, and sit the beaker in a large plastic bowl.

For your hands' sake, wear the rubber gloves, for your eyes' sake, wear the safety glasses! With weak acid, you may need to wash out the container and add more dilute acid on the second and third day. Put everything in a safe place (think about pets, small brothers and sisters, unaware adults, wind gusts, and think about what the acid will spill onto if the container tips over).

Wear the rubber gloves when you take the plaster out, once all the shell has gone. What you do now with your trophy is up to you, but you may wish to look, to see if the inside is completely smooth.

Plaster of Paris is a dried-out form of gypsum, which has one molecule of water for every two calcium ions and two sulfate ions, giving us the formula (CaSO4)2.H2O. After you add the water, the plaster forms a solid mass of highly insoluble gypsum crystals, CaSO4.2H2O, with four times as much water, two waters to every calcium sulfate. Plaster expands slightly as it sets, which explains why plaster makes such accurate copies of a surrounding mould. The acid does not affect the plaster, but it does react with the shell, which is mostly calcium carbonate.

You could try making a collection of shell moulds of related species and comparing them, or you could collect some skulls, and make casts of their brain cavities. Hydrochloric acid, diluted with about six times as much water, will take away the hard calcium phosphate of the bone over several days. It is many years since I did this, but three days should reduce the bone to soft cartilage that you can scrape away carefully.

Safety note: unless you are used to handling acid, and maybe even then, give some thought to safety. You need protective goggles and rubber gloves, and anything that has been in acid should be both rinsed and soaked in large amounts of water. Keep pets and small children away!

The technical name for a cast like this is, as I sad at the start, endocast. One of the most important fossils ever found was an endocast of half of the brain of a juvenile Australopithecus africanus, found in the 1920s by (Sir) Raymond Dart. He took one look at the brain cast, and knew that the owner of that brain had walked upright, because he could see where the brain stem left the skull, underneath the brain, rather than at the back! The specimen is often referred to as the Taung baby.

This is a fascinating tale, well worth reading up on. I recommend Lucy: The Befinnings of Humankind by Donald C. Johanson and Maitland A. Edey, Penguin (my edition is 1981).  The ISBN-10 of that edition is 0-14-013935-4.