I have a buddy living here in Austin, Jon Wolfe, who makes these beautiful little boxes from large acorns. I’ve written about them before. When TechShop recently opened its Austin-Round Rock location, Jon bought a membership, and made a deal with his wife that he would pay for it by selling stuff he made there. So, he’s been refining his acorn box design for production—highly polished surfaces, tiny intricate brass hinges, magnetic closures, flocked interiors—with an eye to selling them as classy gift boxes for rings, jewelry, and other small, precious objects. I’ve seen some of his prototypes, and they’re just gorgeous.
Anyway, this weekend, Jon came over, and we used my garage chem-lab to silver-plate some brass hinge leaves for his prototype box series. I had never electroplated anything before, and have been curious about the process since my undergraduate days. My impression, based on my survey courses, was that electroplating is messy and dangerous—one of those jobs it’s usually best to contract out to a speciality shop.
Jon came prepared with a bucketful of supplies. He had the parts themselves, a benchtop power supply, a strip of stainless steel to serve as an anode, cotton plating pen tips, a strip of 0.999 silver to wrap around the pen tip and connect it to the PSU probe, copper wire to support and ground the parts during the plating operations, and three bottles of MIDAS-brand electroplating chemicals. (MIDAS, for the record, is Rio Grande’s house electroplating products brand.) We treated the parts with each of these three chemicals in series, with water rinses after each. The instructions specify deionized water, but we used grocery store distilled water and were entirely pleased with the results.
The first treatment is called electrocleaning. The electrocleaner bath was provided as a couple tablespoons of dry white solid rattling around in the bottom of a quart plastic bottle. Following the directions, we opened the bottle and poured in distilled water in portions, swirling to dissolve, and stopped when the bottle was full. There was a noticeable exotherm. Per the MSDS, this powder is apparently just sodium hydroxide, aka lye, and if I were to do this on a regular basis, I would probably experiment with making this solution myself since I’m entirely comfortable handling and measuring out solid lye on my own.
To use the electrocleaner, we poured about 40 mL into a beaker and heated it to 150 °F (66 °C), with stirring, on a small hotplate. Jon propped a 4″ strip of stainless steel anode inside the beaker, against one wall, and connected the positive lead from his power supply (a Velleman PS3003U). One of the hinges was threaded onto a piece of copper wire and suspended in the solution, on the opposite side of the beaker. This copper wire was clipped to the PSU’s ground lead, and 9V were applied. This caused vigorous evolution of hydrogen gas from the part itself, and the slow accumulation of a brownish film on the stainless steel electrode.
This process is called cathodic electrocleaning, and its method of operation is slightly counterintuitive. The way current flows in this arrangement, positive ions in the electrocleaning solution (if there were any) would actually be deposited on the surface of the part that’s being “cleaning,” just as in an electroplating bath. In fact, the cleaning action is caused by the evolution of hydrogen bubbles at the part surface, which mechanically loosens and carries contaminants away. Because the flow of conventional current is actually toward the part to be cleaned, cathodic electrocleaning can result in the deposition of “smut” on the surface, which is why care needs to be taken that the electrocleaning solution is as free of contaminants as possible.
After about a minute, the power was turned off, the part was lifted on its supporting wire, rinsed in a beaker of clean distilled water, and then transferred to the acid dip solution. Like the electrocleaner, this was mixed up by adding water to a dry solid in a quart bottle. Per the MSDS, the active ingredient is sodium bifluoride (also a common ingredient in glass etchants) which is used to remove any intermetallic compounds, metal oxides, or other crud remaining on the part after electrocleaning. The part was agitated in a beaker of this solution for about a minute, then rinsed once more in a beaker of clean distilled water.
The actual electroplating process used MIDAS’s silver pen-plating solution. The traditional processes for electroplating silver and gold involve cyanide, which helps ionize the noble metals and keep them dissolved. Cyanide is nasty, of course, so a lot of work has been done on developing a cyanide-free process. The cyanide-free pen-plating solution we used contains silver succinimide (shown to left) and potassium hydroxide, and was developed in Japan in the 1990s. More background on the process is available in this January 1998 article from Metal Finishing.
Jon and I experimented with two different methods for performing the actual electroplating process. In the first method, called pen-plating, the part, connected by its suspension wire to the power supply’s ground, was rubbed with a small cotton swab soaked in the plating solution and connected to +4V DC. Rather than spring for a platinum-tipped plating pen to hold the swab, Jon bought a small strip of 0.999 silver which he wrapped around the swab, at one end, and clipped to the regular PSU supply lead at the other. This method was fun but time-consuming—fun to watch the silver get “painted” onto the brass, but time-consuming because you have to go over the entire surface by hand.
The remaining half-dozen hinge leaves we plated together, using a dip process. The electrocleaning, acid-dip, and washing steps were essentially the same, except we strung several parts together on a single wire. When we got to the plating step, instead of using the pen directly on the part surfaces, we submerged the parts in a beaker of plating solution, connected the grounded support wire to the PSU, and submerged the tip of the cotton plating swab on the other side of the beaker. This proces was faster, but seemed to result in dirtier surfaces. But in fact all of the parts looked great and it was impossible to tell these apart from the pen-plated leaf after polishing with a cotton Dremel wheel.
The chemicals from Rio Grande cost about $30 altogether, and we didn’t even come close to exhausting them. Not counting general-purpose equipment (the PSU, the glassware, the stirring hot plate) some other process-specific stuff was required—the stainless steel anode, the silver strip, the cotton pen tips—and Jon reports spending about $80 for materials all told. I’m still not sure I would want to do this, personally, on any kind of production scale, but altogether it was much easier than I expected going in. And it was a heckuva lot of fun.