Understanding the Working Titanium
Because titanium behaves somewhat differently than the standard metals in the goldsmith’s workshop some special attention to its working is in order here. When sawing, begin the cut with a very light stroke, and increase the pressure only when the blade has securely caught. The sawblade can be protected with a lubricating grease, but even with this precaution it will dull quickly.
Titanium can be worked with standard files, but a light pressure is recommended. The file has a tendency to clog, loading itself with particles of titanium that will ÒsmearÓ the workpiece. Because of this, the file must be cleaned frequently. When drilling, use sharp bits and a lubricant. You will notice that even quality bits will dull after a few holes are made. If a drill bit is broken off in the metal, it can be etched out with nitric acid. When using a rotating bur, cool the action with oil and work at a slow rotational speed (RPM). Titanium responds well to diamond and ceramic abrasive points.
Titanium can be plastically deformed, but it must be annealed often because it becomes hard quickly. When rolling, considerable pressure is required. To draw wire, anneal it first; the lubricant sticks better to the oxide layer. Instead of the usual drawing wax, oil or soap are recommended as lubricants. The metal toughens quickly so you can expect to anneal after about three passes through the drawplate. It’s possible to hot forge titanium at temperatures between 650-950°C (1200-1740°F), but the metal can also be worked cold. You will find that it is more amenable to stretching than compressing. Titanium cannot be soldered with either soft or hard solders; welding can only be done under a protective gas.
SAFETY NOTE: There is a hazard of spontaneous combustion when titanium is heated at its surface, as might be the case in turning a sample on a lathe.
Because of the difficulty of welding, titanium is frequently joined with mechanical methods such as riveting or setting. Relatively low-cost spot welders (one brand is called Sparkie¨) have made heat joins possible, particularly for attaching findings. It is generally impractical for the goldsmith to create a seam in the conventional sense. Titanium can be glued like any other metal if the connecting surface is large enough.
Titanium has established itself as a jewelry metal because of the beautiful colors that can be predictably achieved. When the metal is heated it shows the same colors associated with the tempering of steel, plus a few others. Unlike the fleeting effects on steel however, titanium colors are completely stable. When created by heat they are difficult to locate, but when the colors are achieved through the use of electricity, both the location and color can be precisely controlled.
The color phenomenon is caused by the development of an oxide layer in which a certain part of the light is absorbed and only the remainder is reflected to be perceived by a viewer. As the temperature increases, the oxide layer becomes thicker and absorbs more light. There is a clear series of these colors, beginning with a bright yellow, which is created by an oxide layer of sufficient thickness to cause constructive interference to give this appearance. As the oxide layer grows thicker, other colors are produced because the interference removes various parts of the white light striking the surface. The colors run through green, violet, bright blue to a dark blue. If heat is continued, a second yellow color will result as the interference causes the same effect as in the first yellow.
As mentioned, titanium is a tough, wear-resistant metal. To clean the surface in preparation for coloring, it is first sanded with various grits of abrasive paper. Standard abrasive compounds cannot be used for polishing on the machine. The best shine seems to come with nickel oxide paste or with compounds intended for polishing steel. To prepare the surface for color treatment, lightly etch the surface to create a matte texture.
SAFETY NOTE: It is imperative to use great care when dealing with hydrofluoric acid. Because the skin will be attacked instantly upon exposure, rubber gloves and air-tight goggles are an absolute must.
To prepare the surface, dip the object in a 2% hydrofluoric acid solution, then rinse it well in running water. A commercially available proprietary enamel or glass etching paste will also achieve the desired matte surface and, because of its consistency, it is safer. To prevent spotting while the piece is waiting to be colored, leave it submerged in a standard pickle such as Sparex¨.
Wearing rubber gloves, splashproof goggles and a rubber apron, and using an extremely well-ventilated workspace, combine the following ingredients:
20% concentrated nitric acid
20% hydrochloric acid
20% lactic acid
40% distilled water
Only a plastic container may be used, and care should be taken that the environment is not too warm because hydrofluoric acid evaporates at 16°C (60°F, room temperature).
Dip the object into the solution with plastic tweezers, then immediately dip it, without rinsing, into concentrated nitric acid. To avoid grease spots, from this point on the titanium is no longer touched with the fingers.
A welding torch is particularly well suited to achieving color effects on titanium because of its high temperature and focused flame. The torch can also be used to melt into the metal, throwing the surface up into relief, thickening the edges and generally developing organic textures like those associated with reticulation. The opportunity for contrasting smooth and textured areas offers immediate and intriguing design possibilities. Only generalized colors and effects can be created with a torch. Because titanium is a poor conductor of heat, colors crawl slowly and it is difficult to achieve a specific uniform color on a large surface.
Because of the exactly defined relationship between the applied current and the thickness of the oxide layer that is created, titanium coloring is most precise with a process called electrolytic anodizing. In this technique, household electrical current is passed through a rectifier to reduce its voltage. An anodizing unit with a voltmeter can then be used to control the specific amounts of electrical current that are relayed to the titanium. This current will create the growth of a layer of oxidation of a specific thickness, which in turn translates to the perception of a specific color.
SAFETY NOTE: Because a dangerously high current is used, all connections and contacts outside the bath must be well insulated. Always wear rubber gloves and, because hydrogen gas is evolved in the process, ensure sufficient ventilation.
The piece being colored is submerged in a bath to evenly conduct the electrical current. This bath is called the electrolyte, and may be made of a wide range of solutions. A 10% solution of sodium sulfide is used commercially, but solutions as disparate as a weak liver of sulfur solution, sodium carbonate, ammonium sulfate, cyanide salt baths and Sparex¨ (sodium bisulfate) have also been used.
The workpiece is held with stainless steel clamps and attached to stainless steel cathodes with an insulated copper wire. Use a current density of 5A/dm2. As the current in the bath increases, thicker layers of oxide are formed. These in turn create the various color tones:
16V – yellow
22V – dark blue
27V – bright blue
58V – violet
70V – bluish-green
To create several colors on the same piece, the plate is first treated with the lowest voltage. The area on which this color is to be retained is then covered up (insulated) and the piece is taken to the next voltage. It is also possible to proceed in the reverse manner: first the highest voltage is used, then this region is covered up and all the remaining color everywhere else is removed by sandblasting. A lower voltage is applied to the metallic surface and a thinner oxide layer is formed, creating another color. To make specific areas metallically blank (white) use resists with sandblasting or cover the colored areas and etch selectively with hydrofluoric acid.
The fact that colors are considerably more pronounced on an etched surface opens the possibility for a special effect. If the unprepared metal is covered with a resist and a pattern is scraped away then etched, the exposed metal will take the color in a vibrant hue, which will contrast with a duller color in the adjacent area. Again, use hydrofluoric acid to achieve this, and again, only when proper safety precautions have been followed meticulously. Another interesting effect is created by leaving the resist in place while the metal is electrolytically colored. In this way it is possible to decorate the colored surface with a gray design.