Coloring the Reactive Metals

Q: How does one go about coloring the reactive metals like titanium, niobium, or tantalum? Aside from grinding and polishing the metal surface first, what do I need to do to get those iridescent colors I’ve seen?

A: Actually, it isn’t necessary to grind the surface. One can color any surface, whether polished or textured. However, sandblasted or matte finished surfaces may show the colors more effectively for some pieces. On others, one may prefer a polished surface. Since the colors on these metals are interference colors based on the thickness of a transparent oxide film, a polished surface can be interesting, as it allows the apparent color to shift slightly with the viewing angle. However, that said, I’ll also note that Titanium and Niobium are both somewhat slow and bothersome to polish, so the less time-consuming finish is more popular. In addition, since scratches will cut through the color, a textured finish often will hold up better over time.

Titanium can be heat-colored, but it’s not highly controllable that way. Anodizing is much more precise and controllable on titanium, and the only way to color niobium or tantalum. This requires a DC current with common values of from around 35 volts up to about 140 volts being useful. The metal piece is connected to the anodic terminal, and immersed in any common conductive liquid. A solution of TSP works nicely, helping to clean as it colors. But baking soda or even Coca-Cola have been used. A piece of stainless steel is used for the cathode. The color developed is entirely dependent on the voltage, so in essence, one dials in the color one wants when setting the voltage. Current-limiting resistors, placed in series with the power supply, will slow the development of color, and reduce “burning” effects, as well as increasing the safety factor.

Conveniently, if one uses a common variable transformer (variac), with rectifying diodes and a large filter capacitor, one gets that voltage range with household current. Note, however, that variacs are not an isolated transformer type, and one should never use one simply plugged into the wall. Use it with an isolation transformer as well, for safety. Even with this, the process requires careful attention to safety, since one is dealing with potentially lethal voltages. I’ll also note that AC can be used as well, with anodic coloring taking place during only one-half cycle. However, this wastes current, and generates heat, which can create burning effects on the metal. Often, AC-developed colors are not quite as good as DC-produced ones. Since diodes are cheap enough, rectify the current for best results.

For those wondering where to find a variac; these were often used for theater lighting control, although most have now been replaced by electronic controls. So old used powerstats and variacs are often available in electric surplus outlets. A jeweler’s commercial electroplating supply usually uses a variac whose output is then stepped down by a transformer to yield the common 12 volts (or whatever ) that these supplies usually are rated for. One can often tap off the original 0-120 volt output of those small variacs, adding an additional diode/capacitor circuit to rectify and filter that output. In this case, be sure to use current-limiting resistors to avoid the possibility of overloading the capacity of those small variacs. A short circuit will destroy them otherwise, and those small ones will blow as fast as the fuse in the machine. If modifying a plating machine in this way, be sure it’s plugged into an isolation transformer before use, and be sure to disconnect the normal plating outputs before using the high-voltage ones.

A brief safety review for anodizing reactive metals: For higher-voltage colors, the voltage across the two leads is potentially lethal. For this reason, one must be sure the output comes from an isolated source; ie a power supply run through an isolation transformer. This ensures that the only active current path is from one lead to another, instead of, for example, from one lead to the floor, a much more difficult risk to manage. Even better is to add, in addition to the isolation transformer, a “dead man switch”, which is a fairly large but flat pushbutton switch that one must press, but cannot grab, in order to keep the power on in the circuit. A shock will cause one to remove ones hand from the switch, cutting the power. In using these power supplies, never touch both leads at once, especially with different hands. Briefly running 100 volts from one part of one hand to another part of the same hand will be painful, but probably not lethal. Running it from one hand to the other, past the heart, is quite another situation.

Some artists use a setup where the metal being colored is clipped to a clipboard or other work surface, and connected to the anodic lead. The other lead is run to a brush plater dipped in electrolyte, allowing one to paint the color on selectively. This type of arrangement requires current-limiting resistors in the supply, or the brush will boil. When doing this, touch only the brush lead, which should be well constructed to insulate all but the bristles, and wear a good rubber glove. Make sure not to touch the metal being colored at all.

A tank rig, like a normal plating tank, is easier to set up and run safely. The metal is simply hung in the bath and the power dialed up until the desired color is reached. I should mention, as well, that the color series repeats several times as one increases the voltage. This means that if one overshoots the desired color and goes too far, one can continue increasing the voltage until that part of the series recurs. The colors do start to get a little less intense as one gets to really high voltages, but it’s a minor effect. And the higher-voltage colors are a little more durable, due to the thicker oxide layer.

Multicolor effects can be done in several ways. Areas can be separately masked with plater’s tape or other resists, and anodized separately. If one protects all but the highest-voltage color area, anodizes it, then removes some tape and anodizes again at a lower-voltage color, the new area will be colored without the previous higher-voltage color being affected. This can be repeated as desired. Or one can anodize the whole thing at lower voltages, then mask off areas, and increase the voltage to alter the color of still-exposed areas. Again, this can be repeated. My own work usually involves putting on a high voltage background color, through which I cut/engrave a pattern, often with a straight line engine—turning machine (guilloche work). Re-anodizing at a lower voltage colors the cuts. I repeat this several times, developing multicolored overlaid patterns on the metal. Or, as described above, one can use a brush plater like a paintbrush, painting on colors depending on the voltage one has set.

by Peter W. Rowe M.F.A., G.G.