Most children are aware of the form of snowflakes, delicate patterned lattices of regularly arranged crystals. They may also have watched the formation of ice ferns on glass windows in freezing weather. What they may not have realised is that these are the solid state of something that can also be found in liquid form, in this case, water.
Crystals form when the liquid state is no longer viable, because of cooling temperatures, or because the solution that holds them in liquid form is drying out. Science describes such solutions as super-saturated. In other words, there is too much of the chemical for the liquid that surrounds it.
In Nature, the process of crystallisation can be seen in the short-term formation of snow and icicles. Longer term it can be seen in the calcite stalactites and stalagmites formed by dripping water, saturated with mineral salts, in underground caves. Over even longer periods we can see the formation of rose quartz or amethyst geodes and other precious stones, rubies, emeralds and sapphires.
Crystals form in regular geometric shapes reflecting the chemical bonds that hold the substances together. For this reasons, different chemicals have differently shaped crystals. Some crystals are very durable, such as diamonds, because they do not become liquid except under very high temperatures and high pressure. Others change easily from one form to another, like ice, salt , and sugar.
These are the easiest crystals to make, and sugar crystals, or rock candy , are fun for small children to make and eat afterwards. But do warn them that most other crystals can be poisonous.
Sugar crystals are made by stirring sugar into warm water until it no longer dissolves and a sweet sludge is seen in the bottom of the glass. The water is then supersaturated with sugar. Dip a piece of rough string into the solution and leave it to dry for a few hours, until it feels stiff and gritty. These little grains are seed crystals which will grow larger when we tie the string round a pencil and let it dangle into a glass jar containing the sugar solution. Over the next few days, sugar crystals will grow all along the string which acts like a wick, drawing up the solution and depositing it on the growing crystals as it dries.
Salt crystals can be made by dripping a strong salt solution onto a porous stone and letting it dry out. Food colouring can be added to make pretty coloured films of crystals like lichen. Larger crystals can be made from salt or alum by preventing them from drying out too quickly.
Older children can melt flowers of sulphur, a yellow powder, in a glass container over boiling water. As the liquid cools, they can watch the formation of bright yellow sticks of fragile crystals. This is often done in the chemistry laboratory to demonstrate the different forms or isotopes of the same chemical substance. In the same way, soot and diamonds are isotopes of carbon.
There is plenty of information on the Internet about growing crystals, from the simplest pursuits for small children to complex and demanding projects to demonstrate geological processes or produce works of art.
You can buy crystal growing kits for children. For younger children, these consist of shaped, cut out cards which have been soaked in coloured solutions and dried. When they are stood for a few days with their bases in water, pastel coloured crystals blossom all over the cards. In this way the children can learn about basic chemistry and geological processes whilst they make gardens of crystal flowering trees and bushes.
For older children, solutions of thick, syrupy isinglass, once used to preserve eggs, can be used to fill jars into which only one or two coloured crystals such as blue copper sulphate, purple potassium permanganate , white alum or green ferrous sulphate are dropped. Over a few days, these will form growing chains of crystals, like angular seaweeds rising from the bottom of the jar to form beautiful coloured forests.
Amongst many other educational crystal craft sets, The Smithsonian, National Geographic and Natural History Museum crystal growing kits are all excellent for children over ten years old.