When working in copper or its alloys, it is sometimes desirable to get the effect of age. This is especially true when matching a part to another, older piece, or when making vintage looking jewelry. On occasion, it becomes valuable with instruments as well, though many techniques are designed for smaller parts.
What is a Patina
Copper, brass, and bronze all are constituted primarily of copper. Copper, as a rule, reacts with the elements around it to first form a dull brown haze, and with significant exposure, a green verdigris cover. That antiqued look is referred to as a patina, while it is actually a tarnish. Depending on your goals, you may want to achieve the brown or green states, or anywhere in between, without having to wait for years for natural tarnish to appear. Brass and bronze will also tarnish in much the same way.
Getting your Patina at Home
In each of these cases, the rust is a reaction with the atmosphere and minor chemicals in the environment. If we want to, we can speed along the processes using a few basic tools at our disposal. First, household chemicals contain the sulphur, chlorine, or acetate to react with the copper. Second, heat can be used to free up bonds or for affect. After that point, it is up to us as artists to control the patina.
Chemical Patinas
Depending on our goals, we may want the brown stage or the green stage of patina. The green stage is easiest to achieve with distilled vinegar (acetic acid), as copper acetate is that pretty green color. Other options for brown states are mixing vinegar, water, and ammonia in varying quantities. It takes experimentation, and your effect might be best achieved through starting with a very dilute mix and working your way up when results aren’t strong enough.
If you get too rich of a brown patina, you can polish it away like silver tarnish with a polishing cloth and/or solution. Green verdigris is much harder to take off, and typically requires some real effort, possibly by buffing.
Typically, all you need to do is either paint your solution on to the material (hard to control) or let the vapors soak in. Placing the item in a box of non-clumping cat litter and soaking the litter in the solution is a good way to get an even coat. Sawdust works too. Alternately, you can put the solution in a garbage bag with the item, placing the item on a platform to prevent it from contacting the solution, and then letting the vapors soak in that way. Since condensation can occur, you should use pins, pegs, or something else to suspend the bag over the item.
As always, don’t work with harmful chemicals indoors, especially with the windows closed. The right place to do any chemical work is outside.
Heat Tarnishing
If your item won’t be hurt by heat, you can place it in a hot oven for a while. This will accelerate tarnishing and typically give it a nice color. Alternately, you can go over the entire surface in strokes with a blow torch. Being careful not to melt or damage your item, the heat will not only cause the darker colors, but you will get an interesting mix of blue and green patinas, like an oil sheen, with the blowtorch. This is another dangerous option though, and you should experiment with this knowing that you might ruin your item if you hold it for too long.
Sources:
Science Company: Patina Formulas
Finishing.com: Copper Patina
JewelryLessons.com: Creating Color on Metals: Patinas