Annealed copper is copper that has been heated and slowly cooled to make it soft, flexible, and easy to work with. The process, called annealing, reverses the hardness and brittleness that copper develops when it’s been rolled, drawn, or hammered into shape. If you’ve ever bent a piece of copper wire back and forth until it became stiff and eventually snapped, you’ve experienced exactly the problem annealing solves.
How Annealing Works
When copper is shaped mechanically, whether by rolling it into sheets, pulling it through a die to make wire, or hammering it flat, its internal crystal structure gets compressed and distorted. The metal’s tiny grains are squeezed together and locked in place, which makes the copper harder but also more brittle. Metallurgists call this “cold working,” and it’s why a fresh piece of copper tubing feels stiffer than you’d expect.
Annealing reverses this by heating the copper to a temperature where its internal structure can reorganize. For pure copper, this typically starts around 200 to 350°C (roughly 400 to 660°F), where the distorted grains relax, release stored energy, and form new, larger, more uniform crystals. This process is called recrystallization. Above about 400 to 600°C, the grains continue growing larger in a predictable way. Heat it too aggressively, past about 700°C, and the grains can grow unevenly, which causes a rapid drop in hardness and can create weak spots in the metal.
After heating, the copper is either allowed to cool slowly in air or quenched in water. Unlike steel, where cooling speed dramatically changes the result, copper is forgiving here. Both methods produce soft, workable metal.
Softness, Strength, and Stretch
The defining trait of annealed copper is its dramatically improved ductility, meaning how far you can stretch or bend it before it breaks. Properly annealed pure copper can reach around 37% total elongation before failure. Compare that to copper that’s been heavily cold-worked or processed through advanced techniques, where elongation can drop to as low as 4 to 6%. That’s the difference between a material you can form freely and one that cracks under pressure.
The tradeoff is strength. Annealed copper typically has a yield strength around 190 MPa and tensile strength near 420 MPa. Cold-worked copper can reach 280 to 600 MPa in tensile strength, depending on the processing method. For applications where the copper needs to bend, wrap, or conform to shapes, softness is the priority. For structural applications where it needs to resist force, harder tempers are preferred.
Hardness follows the same pattern. Annealed copper measures around 38 on the Brinell hardness scale, which is quite soft as metals go. This is exactly what plumbers, electricians, and craftspeople want when they need copper to cooperate with their hands and tools.
The Standard for Electrical Conductivity
Annealed copper conducts electricity so well that it literally defines the global standard for measuring electrical conductivity. The International Annealed Copper Standard (IACS), established in 1913, sets annealed copper at 100% IACS, corresponding to a resistivity of 1.724 × 10⁻⁸ ohm-meters at 23°C. Every other conductive material, from aluminum to silver to specialized alloys, is measured as a percentage of this benchmark.
This isn’t just trivia. It’s the reason annealed copper is the default choice for electrical wiring, motor windings, and circuit board traces. The annealing process itself improves conductivity compared to cold-worked copper, because the reorganized crystal structure allows electrons to flow with less resistance. Cold-worked copper with high strength typically measures around 80% IACS, a meaningful drop in performance for electrical applications.
Annealed vs. Hard-Drawn Copper Tubing
In plumbing and HVAC work, copper tubing comes in two tempers: annealed (labeled “soft”) and drawn (labeled “hard”). The practical difference shows up immediately when you try to bend it. A half-inch annealed Type K or L tube can be bent to a radius as tight as 2.25 inches. The same size in hard-drawn temper requires a minimum radius of 2.5 inches, and it’s far more likely to kink or crack without a proper bending tool.
Annealed copper tubing is sold in coils because it’s flexible enough to wrap. Hard-drawn tubing comes in straight rigid lengths. Plumbers choose annealed tubing for running lines through walls, around obstacles, or underground where the pipe needs to follow curves. Hard-drawn tubing is used for exposed runs where rigidity and a clean appearance matter. Here’s how minimum bend radii compare across common sizes:
- 1/4 inch annealed: 3/4-inch minimum bend radius
- 3/8 inch annealed: 1.5 inches (vs. 1.75 inches for hard-drawn)
- 1/2 inch annealed: 2.25 inches (vs. 2.5 inches for hard-drawn)
- 3/4 inch annealed: 3 inches
- 1 inch annealed: 4 inches
Annealing Copper by Hand
Jewelers, metalsmiths, and wire artists regularly anneal copper themselves using a butane or propane torch. The technique is straightforward but relies on visual cues rather than a thermometer. You heat the copper with a sweeping motion, moving the flame evenly across the entire piece. As the temperature rises, the surface cycles through a range of colors: first gold, then purple and blue, and finally a dull red glow.
That red glow is the target. It indicates the copper has reached recrystallization temperature and the internal grain structure is reorganizing. Once the entire piece has glowed red, you remove the flame and either let it air-cool or pick it up with pliers and quench it in water. Both approaches yield soft, workable copper. After quenching, many artisans drop the piece into a mild acid solution (called “pickle”) to clean off the dark oxide layer that forms during heating.
Copper can be annealed repeatedly. As you work it, whether bending, hammering, or shaping with pliers, it gradually hardens again. When it starts resisting your tools or feels springy instead of soft, you anneal it again. There’s no limit to how many times copper can go through this cycle, which is one reason it’s been a preferred material for handcraft for thousands of years.
Common Uses for Annealed Copper
Annealed copper shows up wherever softness, conductivity, or formability is the priority. Electrical wiring is the single largest use, since the combination of maximum conductivity and flexibility makes it ideal for pulling through conduits and bending around corners. Plumbing and refrigeration lines rely on annealed tubing for the same bendability. In electronics manufacturing, annealed copper foil forms the conductive layers in circuit boards.
On the craft side, annealed copper wire is the material of choice for wire-wrapped jewelry, sculpture, and decorative metalwork. Its softness allows intricate shaping by hand, and its warm color develops a natural patina over time. Copper sheet metal for roofing, flashing, and architectural details is also frequently supplied in an annealed temper so it can be formed to fit irregular surfaces on site.