A red patina can be developed on copper with a torch and any number of fluxes. The heat is applied to the copper from below. When the copper glows red hot sprinkle on the flux for a speckled look. Alternately, apply the flux first, then apply heat, for a smoother color. (Generally, both techniques happen at the same time because the salt bounces around.)
(If you don’t apply the flux you can get lovely tans and browns to blacks, depending on how hot you heat the metal: red hot gives the darkest colors.)
Be careful with this technique, especially with thin copper. It is very easy to blow a hole through the red hot metal.
Note that a black crust forms as you work the metal. You can’t see what is happening underneath! Thus every patina is different and a surprise. You can’t see what it looks like until you cool the metal down and wash off the crust.
Also, if you reheat an area that you’ve previously salted, the colors change, often going to oranges.
I use a rosebud tip on my oxy-acetylene setup (see image right). You can buy one of these at your welding supply store, or on-line, for about $55. This create a very hot wide flame and helps get the job done faster. Be careful not to touch the gas controls while using the redbud tip – I hold it behind the control knobs.
Note: you have to turn the oxygen and acetylene dials way up to the 5-10 range. Start with the acetylene plus a little bit of oxygen to minimize the soot floating around the room. Get the torch going, then dial in the oxygen until the yellow flame is about an inch long. Don’t go to a flat blue flame! Heating the copper releases bonded oxygen. This oxygen super-ignites the flame, creating a very loud pop and blowing out your flame. Startling! When you turn the torch off, turn off the acetylene first to avoid the loud pop. The pop sounds like a gun blast heard from a distance.
Other aspects of this technique – I support the copper on bricks (on concrete blocks standing on a board laid over sawhorses) at a height a few inches above my elbow height to avoid arm strain. The copper invariably warps and bends with the heat. I wear the thickest & longest welding gloves I can find and touch the metal as little as possible when moving it. A large spatula-like tool can help with the repositioning. I do this procedure in the garage door: not actually outside to avoid wind gusts; not inside to avoid fumes and heat and soot. Carefully sweep up the spilled salt afterwards. All salts are deliquescent (moisture attracting) and corrode concrete.
In theory, you could heat the copper very uniformly with multiple gas jets while holding it flat on a grill. That might allow more control.
For metal, I generally use 99.9% pure copper, such as roofing copper, .015 to .030 thickness. The .015 copper takes a gentle touch: don’t hold the flame too close. I used .015 copper scrap for these experiments.
Note that all the colors formed are copper oxides: copper (I) oxide, Cu2O, a red color; copper (II) oxide, CuO, a black color. Thinner layers of Cu2O appear as orange to brighter red to pink to purples; thinner layers of CuO appear as tans to browns to dark browns.
Copper Red Patinas – Experiments
The photo at the top shows the four experiments I conducted as a group. (Note that these samples are unlacquered. All colors appear stronger, deeper and more vibrant after being lacquered.) In my previous article on creating a copper red patina, I stated I was using table salt mixed with baking soda (sodium chloride with sodium bicarbonate) in a 5:1 ratio. I started doing this to get the red to adhere more to the copper. That was successful, but at the cost of the loss of many of the lighter oranges and browns. Some people liked it and some didn’t. It tended to give mostly reds and blacks, with just a few oranges. I felt like I needed to get back to basics.
Experiment 1: NaCl, sodium chloride, aka table salt. This was the flux I started off using, many years ago. Note that the hotter you get the metal, i.e. bright red hot, the less red patina that will adhere, because hot flux aggressively cleans the metal. Black areas are places that got red hot but did not get fluxed. If you can achieve a dull red color, then sprinkle on the salt, interesting effects can be achieved.
Experiment 2: NH4Cl, ammonium chloride, aka sal ammoniac. This is a flux used by stained glass window artists. It also has some applications in cooking: see the Wikipedia article. This was an incredibly unpleasant flux to use. Fortunately I was working in the open garage door and could avoid most of the fumes: it took a day for my lungs to recover. Note the beautiful yellow-green patina that formed. Yellow-green patina is a clear indication of copper chloride. This patina could be left on or scrubbed off.
Experiment 3: CaCl2, calcium chloride, aka ice-melting salt. Not bad, interesting. Note the blue crystals that formed. Fairly straight forward application. Has promise.
Experiment 4: KCl, potassium chloride, aka Salt Substitute. I used Morton’s Salt Substitute because it had the least amount of additives and no mineral oil. This gives a lighter patina than the table salt, with a lot more oranges. My favorite for this group. Also easy to use.
Other possible fluxes: zinc chloride, widely used in fluxes for soldering; borax – gives a beautiful red but the borax forms a glossy glass-like covering that takes work to remove; baking soda – reds but hard to remove; phosphoric acid (haven’t tried it); here’s a list of many more.
In general, fluxes do two things: clean the metal and block out oxygen.
Copper Red Patinas – Afterwork
1st, cool the metal. I spray the backside with water from the hose.
2nd, rinse off the black crust and the flux from the front. You will lose some red at this point so don’t rinse for more than 10 seconds or so. You will do more rinsing later.
3rd drip dry briefly as you carry it to a table
4th, gently flatten the metal with a piece of wood. You don’t need it super-flat, you just want it flat enough to dry without having large puddles. Drip dry briefly again as you carry it to its drying table.
5th, lay flat to finish completely drying (not in sun). If you dry it leaning, it will develop streaks. You might like that. The sunlight heats the metal to much, which dulls the colors.
6th, when completely dry, lay several damp towels over the metal to soften any additional flux. After 4-6 hours or overnight, remove towels and rinse off any remaining flux or white powder.
7th, Dry completely again.
At this point you could hammer it flat, glue to a substrate, lacquer, make jewelry, etc. I have found these patinas to last for many years inside, but outside, without lacquer, they turn brown and lose their vitality.
Favorite lacquers: Clear Guard (fast-drying, great adhesion, satin) or Incralac (slow drying, high gloss, excellent adhesion). I’ve heard good reports regarding Permalac and Nikolas #8321 lacquer. Avoid any nitrocellulose lacquers or water-based lacquers. Krylon clear works fine inside, degrades outside in a few years.
Copper Red Patinas – Additional Ideas
1. if you like most of the patina but not all, lay a wet towel around the area you want to keep and work on the area that needs work
2. Use different fluxes on different areas of your metal
3. Add some green acid patina to the red – the red and green patinas work well together
4. Melt brass or bronze on the metal before doing the patina
Some metal art work I’ve created with copper red patinas: