Salt crystal trees

It has been snowy here recently - which has been lovely as it's given the boys a change of scenery whilst not being able to go anywhere - and I thought it was a good time to make a snowy scene from cardboard and salt.  

You may have seen 'magic' growing crystal trees for sale which are more impressive than this, but contain ammonia and bluing/prussian blue which isn't easy to come by here.  I also wasn't too keen on the idea of this with a 2 year old keen to 'help'.  It's the same idea, with crystals forming as the liquid evaporates and still looks pretty, but it is slower and you get less elaborate crystal structures.

Cardboard tree shapes

I made two tree shapes from cardboard - one with simple triangles (two pieces, with a slit in the centre starting from the bottom on one piece and the top on the other so you can slot them together to form a shape that stands up), and the other with a broader top and narrower trunk.  The simple one worked better and stood up more reliably so if we were doing it again, I'd stick with that or shape some 'branches' within the triangles.  The cardboard I used was from an Amazon envelope, but beware that there seem to be two types, one which is pretty water resistant so the cardboard doesn't soak up the liquid and you don't get pretty trees!

Assembled tree shapes

We made a saturated table salt (sodium chloride) solution, taking hot water (be careful with little people) and adding salt until no more would dissolve.  Once this had cooled to room temperature, we poured it into a plastic tray with the two trees standing up.  The water soaks upwards through the cardboard due to capillary action, carrying the dissolved salt with it.  

Adding salt solution

As the water evaporates, the solution gets more concentrated until the salt starts to form crystals.  These get larger over time - in our pretty cold kitchen, it took 3 days for all the liquid to evaporate.  Not all the liquid was soaked up by the trees, and the top of the more elaborate tree barely formed any crystals, but it left lots of crystals forming in the tray.

'snowy' salt crystal trees

My son correctly identified that the crystals are cubic, but it's fair to say that he wasn't massively impressed by the pretty trees!  It gave some nice talking points about dissolving, evaporation and crystals and I'm hoping we can try growing some other crystals soon to see how they aren't all the same shape.