Growing large crystals of Copper (II) Sulphate
|My shiny |
As a kid I grew crystals as a hobby, mostly from copper (II) sulphate and alum (potassium aluminium sulphate). I tried it again a couple of weeks ago, did it properly this time, and grew the one pictured to the right over about 4-5 days. Wayne's This and That has some good first-hand info on hobbyist crystal growing, and I have come up with a variation on "slow cooling" which I shall call "fast cooling", to drastically speed up crystal growth at the expense of clarity. The post describes my method, which only required about a R50 bill of materials.
- 2 small glass beakers (empty spice jars)
- Distilled water (R7 from Dischem)
- 50g copper sulphate (R7 from Dischem)
- 0.2mm nylon fish line (R6 from AllSports)
- Small funnel (R9 from Crazy Store)
- Glass ash tray (R10 from Crazy Store)
- Plastic spoon, toothpick, blob of prestik, cloth to cover jars.
Copper sulphate is poisonous, tastes horrible, and the solution stains everything porous. It even precipitates a copper coating onto steel spoons. Copper sulphate stains can be removed from cotton with "peroxy"-type stain removers, and take weeks to come out from under your fingernails. On the plus side, copper sulphate is anti-fungal which is why pharmacies stock it.
- Grow a seed crystal
- Make a small amount of super-saturated solution (steps 2.1-2.5 with smaller quantities)
- Fill the glass ashtray about 5mm deep with solution
- Use the fridge to cool it down
- After 3-12 hours pick out a seed crystal with tweezers: either a single 2-4mm crystal, or a clump grown overnight.
- Tie the crystal to a length of nylon fishing line using a slip knot, or a reef knot and trimming excess length.
- Make a supersaturated solution
- Heat a mostly-full spice jar of distilled water in the microwave at medium power, watching like a hawk to avoid boiling it.
- Add about 25g of copper sulphate (add more if it all dissolved)
- Stir with plastic spoon till no more dissolves (takes about 2 minutes)
- Pour solution into the second jar, leaving the dregs behind.
- Place solution in the freezer for 15-20 minutes (or fridge for longer) to cool it down.
- Suspend the seed in the jar: hang it over the toothpick, secure with prestik, and adjust so that the crystal doesn't touch the jar, then cover with a cloth.
- Grow the crystal: takes 10 minutes every 12 hours
- The seed should have grown and there should also be a mat of crystals at the bottom of the jar. The solution is now merely saturated.
- Remove the seed and pour about 75% of the saturated solution into a clean jar.
- Supersaturate the remaining 25% by microwaving on low power, stirring to dissolve residue, and maybe cool it in the freezer for a bit. If there was no mat of crystals on the bottom to dissolve, add a teaspoon of copper sulphate.
- Add the supersaturated solution to the saturated solution, leaving the dregs behind.
- Suspend the seed crystal again and wait 8-12 hours.
- Nylon line prevents new crystals forming, and needs to be thin (0.2mm) to tie up a seed crystal. According to Wayne one can form a seed crystal directly on a thicker nylon line to avoid having a fuzzy core in an otherwise clear, slow-grown crystal.
- Copper sulphate solubility varies greatly with temperature, and takes ages to evaporate unless you have warm, filtered air blowing over a large-diameter beaker, preferably sitting in a warm thermostat-controlled bath. Too much effort to set up, so I supersaturated the solution every 12 hours instead.
- The surface of a finished crystal dehydrates after a while, making it lose its lustre. Complete dehydration would turn the CuS04.5H2O crystal to white powder. May as well store the crystal suspended in solution, or in an airtight jar with copper sulphate powder on the bottom to stabilise the humidity.