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.

Experiment 52: Highly Flammable Nitrocellulose

Nitrocellulose is simply cotton that has had its hydroxide groups replaced with nitrate groups.  This simple substitution makes it highly flammable and even explosive.  After seeing some YouTube videos demonstrating its highly combustible properties, I decided to make some.

First, I selected some 100% cotton string, a piece of white paper towel, and some cotton balls as my sources of cellulose.  I very carefully made a 2:1 volume mixture of concentrated sulfuric and nitric acid in a small glass beaker.  After stirring gently, I placed this beaker in a bucket of snow to cool the nitration.  I nitrated my three sources of cotton separately, one after the other.  To nitrate each batch, I placed it in the acid mixture for 15 minutes.  After the time was up, I transferred the now-nitrocellulose to a saturated baking soda solution to neutralize remaining acid.  Finally, I thoroughly washed the nitrocellulose with plain water and let it dry.

One interesting thing I noticed was that the cotton balls almost seemed to freeze in the mixture.  It was quite cold when I did the experiment, but the stiffening could also have been caused by close packing.  I had just enough acid to cover three cotton balls at a time, so they were squished.

The homemade nitrocellulose is quite inflammable.  I have even held pieces in my hand and lit them without hurting myself.  While the paper towel and the cotton seem to burn extremely quickly without any residue, the string burns slower and leaves some charred material behind.  A cotton ball that I nitrated at the end of the run also left some residue--perhaps the acid was nearly used up.  The best cotton burns so quickly that it can detonate under the right conditions.
I created these "right conditions" by compressing the nitrated cotton balls in a spent brass shell casing and initiating detonation with a homemade bridgewire detonator.  I first made an insulating sleeve out of a straw to go on the inside of the shell; this protected the bridgewire from shorting on the casing.  Then, I packed the shell half full with nitrocellulose.  I inserted the bridgewire (also stuffed with nitrocellulose) and then packed the shell the rest of the way.  I left 3/8" at the top and crimped this over with some pliers to seal the shell, creating containment for the combustion gasses.

I put the device in a cardboard box with a polycarbonate viewing window and set it off from another room using my flash capacitor bank.  I was wearing hearing protection (a must for explosions), but from family member accounts, the ensuing explosion was frighteningly loud.  The shell casing was completely blown to pieces, with brass shrapnel flying through the cardboard box and also embedding itself a centimeter into a nearby block of wood.  I was quite impressed by the detonation-it was much more powerful than I had expected.

If you read the preceding two paragraphs, please don't do anything stupid.  While exploding bridgewire detonators are incredibly safe (as they contain no primary explosives), shrapnel is much more dangerous than a simple fireball.  If you repeat this experiment, make sure you will be protected from potential shrapnel.

Laser Cutter Update: Great Improvements and Laser Art

A while ago, my church did a silent auction to raise money for students going to a youth conference.  I wanted to donate some laser art, so I found neat designs online and used Inkscape's "Trace Bitmap" and "Object to Path" tools to create a path from an image.  Then, I used a laser engraver plugin to transform the path into g-code for my laser cutter.  I tried cutting the design from black paper, but it didn't work!  My laser cutter (PiKnife) messed up the movements and cut things out in wrong places, making the entire cutout worthless.  I was quite disappointed, so I tried to figure out what the problem was.

One problem PiKnife had a lot was that the stepper motors would sometimes stick and jitter.  Eventually, I noticed that pressing on the input and output wires plugged into the ribbon cable going to the Raspberry Pi would cause this problem, so I deduced that the issue must stem from bad connections.  To fix the connections, I wrote down where everything was in the ribbon cable and then used male-to-female breadboard jumpers to wire each input and output directly into the Pi's GPIO header.  This eliminated the uncertainty of plugging wires directly into a ribbon cable.

Amazingly, that fixed the problem entirely!  I set a new piece of paper on the laser cutter, pressed enter to begin the program, and didn't have an issue again!  While it took five and a half hours to complete the design, it showed the true capability of PiKnife.  It can cut extremely complex shapes for a long duration without problems.
With mere hours to spare, I completed the laser art and entered it in the silent auction.  I also made a sped-up video (above) of the laser cutter working, which turned out well.  Perhaps now that PiKnife really works it is time to expand into the CO2 laser range...

Experiment 51: Electric Resistance Furnace

Electric resistance furnaces differ from electric arc furnaces (like the one I made in Experiment 32) in that they use resistive heating (like a toaster) instead of giant lightning bolts to melt metal.  I had a hotplate that died (because a container holding hot sodium hydroxide solution broke on it), so I decided to use its heating elements to make a small furnace.

First, I played with the resistance wire to see what sort of current made it glow.  I found that 12 volts made a 7" piece glow red hot.  However, it didn't melt the wire, which was good.  I calculated the current using Ohm's law and determined how much wire I would need to use to get the same current at 120 volts.  Actually, the math is pretty simple--I needed to use ten times as much, or 70".

I used a hollowed-out low density soft alumina-silica firebrick as the furnace body.  It had seen previous service as an arc furnace, so it was fairly beat up and I didn't care if it broke.  I also used another piece of firebrick as an insulating lid.  Fitting 70" of resistance wire in a 2" diameter hole was difficult, but coiling the wire seemed to work well.  For connecting the mains electricity to the ends of the wire, I made clamps using small nuts and bolts that sandwhiched the wire snugly.

My crucible was a cut-off soup can bottom, and my metal of choice was aluminum.  Plugging the furnace into the outlet made the heating elements glow, but the really neat thing about this furnace was that it was silent.  I even read a book while I waited for my metal to melt!  An hour later, I checked on the melt, and the aluminum was undoubtedly molten!  I wanted to grow aluminum crystals inside a blob, so I tried pouring the molten aluminum onto some glass to insulate it better.  That was a rather bad idea, because the glass exploded, and molten aluminum was dispersed about the room.  Oops!

Even though the crystal growing didn't work out, I was quite impressed that aluminum could be melted silently with just some electricity!  Of all the metal-melting methods I have tried (there are at least 14 of them), this is one of the quietest.