Re: Why have I been unable to grow my own stalactites/stalagmites?

The basic concept of your project does seem like it might work (but notice 
that word MIGHT -  as you have been discovering, experimental design is 
sometimes a very tricky business).

Stalactites and stalagmites grow when minerals (usually calcium carbonate, 
which forms the minerals calcite and aragonite) that have been dissolved in 
groundwater are precipitated as that water drips into an open space and 
evaporates.  As I am sure you are aware, they form in caves, but they can 
also form in mines and other tunnels and I have even seen them on the 
undersides of bridges and inside parking garages where rain and road salt 
(here in the snowy Northeast) drip down through concrete.  If you 
understand how they form, you can start to trouble-shoot your experiment.  

First, you need something dissolved into your water that will be able to 
come back out of solution as the water evaporates.  The Epson salts should 
be good for that.  You could also try alum (available in some supermarkets, 
with the spices, or from pharmacies) or even table salt, but it doesn't 
sound like this is the problem.  One possible problem might be that the 
salts are crystallizing before they get to the yarn.  As the water cools, 
the Epson salts will want to crystallize, if they have the opportunity.  If 
you have a crust forming on the sides of the glass, then much of your 
stactite raw material may be out of the water before it even gets to the 
yarn.  If that is the case, you need to try to keep the Epson salts from 
crystallizing in the glass.  Use a very clean glass, no dust, no scratches. 
 Be very careful not to pour even a single grain of the undissolved Epson 
salts into the glasses.  You might also want to loosely cover the glasses 
to keep any dust from falling in and to slow evaporation.

Next you need a way to get the water to an open space.  The yarn in your 
experiment is designed to do this.  It should act as a wick, pulling the 
water along by capilary forces between the fibers.  Check your yarn - it 
should be wet all the way along.  If it is not, this is the problem (which 
is my guess).  If it is not wet all the way to the low point where the 
pieces cross each other, then no water (and no salt) will make it far 
enough to drip down and leave Epsom salts behind to form stalactites.  If 
this is the case, you can try several fixes.  First, be sure the glasses 
you use as reservoirs are almost completely full.  Capilary forces will 
only pull the water uphill a short distance and so if the glass is not full 
enough, the water may never make it over the rim.  You might want to cover 
the glasses to keep the liquid from evaporating from them.  Next, check the 
yarn you are using.  Color should not matter, but diameter and composition 
will.  Some synthetic materials may repel water rather than wicking it up 
and natural wool may have oils that do the same.  The best material is 
plain cotton.  Use a large diameter yarn.  If the yarn is too thin, the 
water may evaporate completely before it gets to the center point (if your 
yarn is crusted with salts up to a certain point and no farther, this may 
be the problem).  You could even used strips of an old cotton t-shirt in 
place of yarn.  Make sure your yarn hangs down all the way to the point 
where the two strands cross.  Gravity will help the water flow down to the 
central point.

Once you are getting water to the central point, it needs to evaporate in 
order to leave salts behind.  To encourage evaporation you need warm 
temperatures and low humidities.  If you are getting drips off the yarn and 
a big puddle between the glasses, but no crystals, then a lack of salt or a 
lack of evaporation are the likely causes.  You have a bit of a paradox 
here because you want your water to evaporate at the central point, but not 
before that.  If you make it too dry, then the water will evaporate too 
fast and not get to the central point, too wet and it won't evaporate at 
all.  You may want to experiment with "microclimates" and do something like 
shining a small, but warm (such as a halogen) lamp onto the central spot 
only or somehow shielding the yarn from evaporation (I thought of pulling 
it through a straw to do this, but I have no idea if that would cause other 
problems).  You could also try creating a gentle breeze (most fans would be 
too much, but a small fan on its slowest setting might work) to aid 
evaporation.  

The last thing your stalactites and stalagmites will need is time, and lots 
of it.  Cave formations take thousands of years to develop and many of them 
are only a few inches in size.  If everything is going well, it may still 
take days to make even a tiny stalactite.  The last thing you will need is 
persistence.  Stalactites form only in certain places in nature where the 
conditions are just right - you may need to try many different methods 
before you get one to form, if you are ever successful.

I don't want to discourage you and I don't know how your science fair is 
set up, but any scientist knows that she often learns as much from trying 
to explain and correct failed experiments as she does from the ones that 
work right and that the vast majority of experiments do not work right the 
first time.  A lab scientist might work for a year or more to get an 
experiment to work properly before ever collecting any data.  By the time 
the experiment is running, it might look very different from her original 
idea.  You will have more fun, or at least less frustration, if you can 
focus on the puzzles along the way rather than on the end goal.

Enjoy, and write back if you have more questions,

David Smith
Geology and Environmental Science
La Salle University, Philadelphia, PA