For generations, bluing has been known as an essential ingredient of a “Salt Crystal Garden”. This delightful crystalline formation became popular in the Great Depression, and is still known to some as a “Depression Flower” or “Coal Garden”.

Salt Crystal Garden Kit components


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PDF Download — Salt Crystal Garden Instructions and Scientific Explanation

By combining common ingredients (salt, water, bluing, food coloring and ammonia (optional) over a porous base material (coal, sponge, or other), through capillary action and evaporation, the salt recrystallizes into beautiful, delicate coral-like formations. Children growing one of their own have been winning blue ribbons in science fairs ever since! Elementary teachers find it invaluable in teaching their students about crystals – learn more about Mrs. Stewart's Bluing in the Classroom.

SCG Instructions Yellow Box

Tips for Successful Salt Crystal Growth

  • Good air circulation:  The drier the air, the better.  An environment where the water and the bluing can evaporate is key to the success of crystal growth.  Using a glass pie plate that is very open rather than a fish bowl style container will be helpful.  As the liquid evaporates out the top of the base the material, the salt is left behind in salt crystalline form.  The rate of growth depends largely on the humidity of the surrounding environment, taking from hours to days to begin.
  • Science Project Variations: A good science project may consist of varying the base material.  Do the crystals grow better / faster on coal or on sponge?  Do the crystals grow faster with ammonia added or without? Other variations can be imagined and accomplished.
  • Crystal Christmas Trees:  Make a stand-up tree out of blotter paper and set it into the ingredients above — watch the magic!

Note regarding the use of ammonia:

Ammonia is an optional ingredient.  It increases the rate of evaporation, thus helping the crystals to grow faster.  Ammonia as well as the fumes from ammonia can be toxic.  Children using ammonia should be supervised.

How Do the Salt Crystals Grow?

Table salt (NaCl) can be dissolved in water. The amount of salt in a particular quantity of water depends on its temperature – more being soluble in warmer water. The oceans of the world contain about 5% salt, but lakes such as The Great Salt Lake or the Dead Sea have salts to about 25%.

People who live near the ocean often transfer sea water to open pools where evaporation can take place. This is still done in the Hawaiian Islands where there is an extended dry season. To a greater extent, such pools are formed by low dikes west of the Great Salt Lake in Utah and pumps lift the lake water in substantial quantities.

As water evaporates, some of the salt cannot be retained and crystals of salt form along the edges of the pools and finally when all the water is gone there will be crystals on the bottom of the dried pool. These need only purification to be used as table salt.

The recipe for the Salt Crystal Garden calls for proportionately large amounts of salt in the presence of little liquid. Thus crystallization takes place very quickly.

Mrs. Stewart’s Bluing is a colloidal suspension of extremely minute particles of blue powder (Ferric Hexacyanoferrate). This is not a solution in the true chemical meaning of that word.

As the water from the bluing and the clear water which is first added evaporate, two things happen. The blue particles can no longer be supported and the excess salt cannot stay in solution. The salt crystallization process will take place around the blue particles as nuclei, in much the same way as silver iodide cloud seeding accelerates the formation of rain drops.

Small amounts of ammonia are added chiefly to speed up the evaporation process.

The purpose of the porous material (sponge pieces) is to provide a means for capillary action to carry the liquid containing bluing and salt up from the main source of liquid. This further speeds up evaporation and causes the crystals to form over a larger area than just the rim of the bowl.

Additions of bluing and salt on later days should be made by slipping the new liquid in below the rest of the growth. Capillary action will bring this further material up where the evaporation can cause additional formations of crystals.

No chemical reaction takes place in this process, just dissolving and recrystallization aided by the bluing particles.

PDF Download — Salt Crystal Garden Instructions and Scientific Explanation

Learn about many more uses for Mrs. Stewart's® Bluing on our Many Uses page.

If you have questions, suggestions or just want to share with us that your Salt Crystal growth science project won a blue ribbon, please Contact Us.