Experiment 53: Growing Large Copper Crystals

For a very long time, I have been fascinated with crystals of pure metals.  Many people have grown crystals of copper sulfate, but crystals of metallic copper are a rare thing.  After seeing The Backyard Scientist's video as well as this more crystalline crystal, I had to try it myself.
The basic principle of copper crystal growing is to very slowly electrolyze a solution of copper sulfate with two copper electrodes.  I used a normal copper wire as my cathode (-) and a flat spiral of thick copper wire as my anode (+).  The cathode just poked into the solution ~5mm, and the anode rested on the bottom of my glass jar electrolysis cell, connected to my breadboard via a soldered-on insulated wire.  In a Sciencemadness forum post, The Backyard Scientist says he used 100g/L copper sulfate and 60ppm chloride ions to prevent spindly dendrites from forming (thick crystals are more impressive).  I initially thought dilute copper sulfate would conduct less electricity and thus grow the crystals more slowly, thus helping to form larger crystals.  It turns out that if the solution is too dilute, the growing crystal will "use up" all the Cu2+ ions in the immediate vicinity and will get weird polyp-like black growths instead of shiny crystalline copper.  I ended up using 40g/L of copper sulfate for the final iteration of my experiment; concentrated solutions (or frequent stirring) are key to success.  My chloride ion source was simply table salt, and I used tap water for my electrolyte bath.  There was some gunk in the root killer copper sulfate, so I filtered my solution before use.

While a dilute solution is actually not good for making beautiful crystals, very low currents and voltages are.  My electrodes were spaced about 7cm apart, and my voltage varied throughout the experiment‐it was usually around 0.28V.  The key is to keep the current very low.  I never let the current exceed 10mA for the entire experiment.  To achieve the very low voltages necessary for low currents, I made an LM317 constant voltage circuit and put five 1N4007 diodes in series on the positive output of the circuit to drop the voltage down lower.  Basically, the LM317 can only regulate down to an output voltage of 1.25V, but I wanted the ability to go to around 0.25V, so I put a bunch of diodes on the output so that each of them dropped the voltage down a little bit.constant voltage circuit.  Oh well!

 Turning the potentiometer on the circuit still changes the voltage, only everything is shifted down a volt.  I used a 7.5V wall wart power supply to power the LM317; anything within the specified input voltage will do fine.  One odd thing I noticed was that while reading the voltage between the two electrodes, if I connected the ammeter, the measured voltage would increase.  I have no idea why this happened, since my circuit should have been a

With everything connected and my electrolyte prepared (filled to ~1/2" of top of glass jar), I turned on the power and adjusted the potentiometer until my multimeter read less than 10mA on the output.  I recorded the voltage and nearly everything I did in an experiment log which may be viewed here.  If you would like to repeat this experiment, definitely read it through.  With my electrolysis power on, I just had to wait.  Every few days, I checked on the experiment.  At times, I had to gently swirl the solution to mix the Cu2+ ions up again.  When I swirled the solution, the current would increase and the voltage would decrease.  I could tell when the solution was becoming depleted around the crystal because it would be a lighter blue than the rest of my electrolysis cell.  I also covered the jar to prevent dust from entering and added water to balance evaporation.  After waiting for six weeks (yes, 42 days), I pulled out an unbelievably shiny bloom of huge copper crystals!

Over six weeks, my crystal grew to 14 grams and a nick in the anode wire insulation allowed the wire to corrode almost through.  In a previous run of this experiment, I had noticed that the crystal dulled and darkened after exposure to air.  I wanted to preserve the extremely shiny pink color of my new crystal, so I cut the cathode wire ~1/2" from the crystal and hot-glued this to the inside of a small spice jar lid.  One note on cutting thick wire with side cutters-be careful to hold the crystal at all times!  My first crystal shot a yard away and smashed against the wall when the cutters made it through the wire.  My new crystal had already darkened some, so I dipped it in plain vinegar, which restored its ultra-shiny pink color well.  I then filled the spice jar with mineral oil and sealed the crystal in to keep its shine.

For photography, I brought the crystal display outside for the bright sunlight and set it on a microwave turntable motor connected to 120VAC.  The motor was an AC motor rated for 3rpm, so it slowly rotated the crystal for the video.  The effect was rather nice, and I was exuberantly happy to be the new owner of such a rare and amazingly beautiful crystal.