This a variation on common crystal growing techniques.  Instead of growing crystals on a string, grow them inside a 'geode' made from an egg shell and plaster of paris.

Egg shells (washed)
plaster of paris
disposable cup
craft sticks or stir sticks
table salt or other crystal growing material - magnesium sulfate, borax etc.
bowl or pan
optional: food coloring for colored crystals


Step 1: Prepare 'Geode' Shell

 Carefully crack an egg in half - try to save the largest part of the shell.
Clean egg shell.  Rinse with diluted bleach water, followed by plain water.  You want to kill any bacteria on the eggshell.

Mix plaster of paris according to directions on package (This is usally about 2 parts plaster powder to one part water.  ALWAYS add plaster powder to water, this minimizes airborne plaster dust.  Stir well and thump cup a few times to force air bubbles to the surface).
When plaster starts to thicken, spread a thin layer of plaster inside the half egg shells - about 1/4 inch or 6 mm.  Try to get a thick edge of plaster along the edge of the shell for a more durable 'geode'.

Let plaster set and dry.


Step 2: Prepare Crystal Growing Solution

For one egg shell geode make about 1/4 cup of solution.  Start with hot water and dissolve as much salt as you can.  Stir well and heat water as needed to make a saturated saltwater solution.  Keep adding salt until you get a few undisolved grains of salt in the bottom of the pan.  Add food coloring if desired.

Step 3: Grow Crystals

Fill plaster egg shell with crystal solution and set in aprotected, out of the way place. 
Wait a day or more.  Crystals will form as water evaporates from the salt solution.  Vary air temperature and observe variations in crystal size.
I am wondering if there is a way to make this without breaking the shell in half, so that it would be a surprise when you open it, like a real geode. By drilling a hole in each end and blowing out the egg, for example.<br>
I don't think a blown egg would work very well. I don't know how you would remove the membrane lining the egg shell, and I think the membrane would interfere with crystal formation. Please share your results if you try a blown egg.<br>Enjoy!
Is the plaster just for reinforcement of the egg shell? Could other materials be used to reinforce the shell? Does the porosity of the plaster help the crystals grow?
I've not tried other materials for lining the shell. I think plaster is useful because it is porous and provides good matrix for crystals to grow onto. Plaster also provides strength. I suppose you could some kind of cement mixture.
BTW, this is a really nice science/family project. Great job.<br>
would Epson salts work faster or the normal salt ones?????
That would work just fine, as would anything that would crystalize out of a liquid water solution
In fact, pretty much any <a href="http://en.wikipedia.org/wiki/Evaporite">evaporite</a> could be done I think.<br>
I don't think it matters what kind of salt you use. Crystals will form faster if the area is warmer and drier - under an incandescent bulb or near a heat source. But, faster evaporation will also produce smaller cystals.
Hmm, This sounds like a neat project. I'd do it with copper sulphate (with some iron included for contrast) and bake it in an autoclave for a few days to try to get the egg to pyritize (with the iron). OOhh, I could mix iron filings or other metal nodules into the plaster to see if that would make interesting artificial minerals.
So this is like the same as rock candy solution, but with salt(or other crystal-forming substance), right?
Yep! You can also dilute sugar in a cup and then tie a string to a pencil and grow a EDIBLE sugar crystal!
Copper Sulphate works well and forms really deep blue rhombic crystals. I used to buy it over the counter at my village pharmacy.
you can also buy copper sulphate at the big box home improvement stores. it usually goes buy septic root killer its usually 99% copper sulphate
Yes, though I've only used salt and magnesium sulfate (epsom salt). I think sugar should also work, but I wouldn't eat sugar crystals grown on plaster, who knows what trace chemicals might leach out into the sugar.

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