Geology Home Page
last edited: 12/22/15
My favorite material for growing crystals is alum. This is my recommended
material for teaching students because the chemical is safe (it is very bitter, so
who would want to eat it?). It grows nicely either by the evaporation method or
by the supersaturated method and it produces nice crystals. The chemical is
relatively inexpensive. It used to be commonly found in grocery stores by the
canning supplies because it was used in making certain types of pickles. More
recently, I have found it in the spice section in the grocery stores.
Ammonium alum is used as an astringent and as a septic pen for stopping
bleeding when you cut yourself while shaving.
The actual chemical name of alum is ammonium aluminum sulfate
The name alum is also applied to other aluminum sulfates, potassium alum
and chrome (chromium) alum. Chrome alum produces dark purple crystals, but
it can mixed with either ammonium alum or potassium alum to produce light
purple colored crystals. The dark purple crystals are unstable and eventually
dry out and decompose. The lighter colored mixtures are more stable.
chrome alum mixed with ammonium alum to produce a purple crystal
An interesting experiment would be grow a purple chrome alum crystal and
then grow a clear, transparent coating of ammonium alum around it. The two
types of alum are compatible and produce the same shape of crystal.
purple alum crystal surrounded by a clear alum crystal
Chrome alum is poisonous, so be careful!
The evaporation technique is a slow process, but it produces satisfying clear
transparent sharp crystals. This process consists of two parts: first growing
seed crystals and then the main growing event.
The first step is to produce some seed crystals. If you are unsure of how
alum to make, make two separate batches. Heat some water using an electric drip
coffee maker (don't use any coffee). Using disposable plastic cups, place two
heaping able spoons of powdered alum to the first cup and four heaping
tablespoons of powdered alum to the second cup. Add approximately 8 ounces
of hot water to each cup, stir until all of the alum is dissolved. Place cups were
they will not be disturbed for at least one week.
Cup #1 had two heaping tablespoons of Alum dissolved in 8 oz. of hot water.
Cup #2 had four heaping teaspoons of alum dissolved in 8 oz. of hot water.
The results shown are after 1 week of sitting and evaporating.
Cup #1 produced some nice seed crystals.
The triangular shapes are the result of crystals lying on octahedral faces as
they grew. The cup was turned over on a paper towel and the crystals were
Cup #2 had too much alum.
The crystals grew too rapidly are were clouded and overgrown.
Consequently, they were not used for seed crystals.
alum seed crystals from cup #1, nice and clear
overlapping seed crystals from cup #1.
The crystals need to be separated if you wish to grow a single nice crystal.
There are still two individual intergrown alum crystals in this specimen.
They need to be separated before being placed in a growing solution.
Tie a thread lasso around a single alum crystal.
Tie the other end of the thread around a popsicle stick.
Yes, this was a used popsicle stick; why waste them?
The saturated solution from cup#1 was poured into a new clean cup,
so that no additional seed crystals would start to grow. The prepared
seed crystal was then hung in the middle of the growing solution.
Place the solution where it can sit undisturbed and evaporate. The
location should be fairly dust-free because dust can trigger the
development of unwanted seed crystals.
Seed crystal is now covered by clear alum after 1 week of evaporation.
A clear octahedron of alum crystallized to an almost perfect octahedron form.
Food dye can be added to growing
solutions of alum in an attempt to add
color to the crystals. However, since crystallization is a purification process the
growing alum crystals tend to reject the food dye unless the dye is highly
The following shows an attempt to add coloring to alum crystals by using
too many seed crystals
The crystals of alum were
distorted by growing so close together.
Here is an ammonium alum crystal that started growth while lying on an
octahedral face. The pink color is due to a small amount of chrome alum
added to the growing solution.