Annealing

Part of Wire Drawing

Heat-treating wire between drawing passes to restore ductility.

Why This Matters

Every time wire is pulled through a draw plate, the metal gets harder. This work hardening is a predictable consequence of cold-working — the crystal structure inside the metal deforms, grains elongate, and dislocations pile up. After a few passes, the wire becomes too brittle to draw further. Try to force it and it snaps.

Annealing reverses this. By heating the wire to a specific temperature and holding it there, you allow the deformed crystal grains to recrystallize into new, unstressed grains. The metal becomes soft and ductile again, ready for more drawing passes. Without annealing, you could only reduce wire diameter by about 30-40% before failure. With annealing between pass sets, you can draw wire from thick rod down to hair-fine gauge — as many reductions as you have patience for.

In a rebuilding scenario, annealing is the difference between making a few sizes of thick wire and making the full range of gauges needed for electrical work, springs, fasteners, musical instruments, and fine metalwork. It is a simple process — heat, hold, cool — but the temperature, atmosphere, and cooling rate all matter, and getting them wrong ruins the wire.

The Science of Work Hardening and Recovery

What Happens During Drawing

When wire is pulled through a die:

1. Elastic deformation — the metal stretches slightly and would spring back if released
2. Plastic deformation — the metal flows permanently, reducing in diameter
3. Dislocation multiplication — defects in the crystal lattice multiply by orders of magnitude
4. Grain elongation — originally round grains stretch into long, thin shapes aligned with the drawing direction

The accumulated dislocations act like traffic jams in the crystal structure. Each new dislocation makes it harder for others to move, and since metal deformation requires dislocation movement, the metal becomes progressively harder and more brittle.

What Happens During Annealing

Heating reverses the damage in three stages:

Stage	Temperature Range	What Happens
Recovery	150-300°C (for copper)	Internal stresses relieve, some dislocations annihilate. Metal softens slightly but retains most of its hardness.
Recrystallization	300-500°C (for copper)	New, stress-free grains nucleate and grow, consuming the deformed structure. Ductility returns dramatically.
Grain growth	Above 500°C or prolonged time	Recrystallized grains continue growing larger. Very large grains make the metal soft but weak — avoid this.

The goal of annealing is to reach full recrystallization without excessive grain growth.

Annealing Temperatures by Metal

Getting the temperature right is the single most important variable. Too low and the wire stays hard. Too high and you waste fuel, risk melting, or grow the grains too large.

Metal	Annealing Temperature	Hold Time	Color Indicator
Copper	400-650°C	15-30 min	Dull red glow
Brass (70/30)	425-600°C	15-30 min	Dull red, just visible in dim light
Iron/mild steel	700-900°C	30-60 min	Cherry red to bright red
Silver	600-700°C	5-15 min	Dull red glow
Gold	650-750°C	5-15 min	Dull red glow
Bronze	450-600°C	15-30 min	Dull red, just visible

Judging Temperature by Color

In a dim workspace (not outdoors in bright sun), heated metal glows at characteristic colors:

• Black heat (below 400°C): No visible glow — too low for most metals
• Faint red (400-500°C): Just barely visible in the dark — copper annealing range
• Dark cherry (500-600°C): Clearly visible red — brass and bronze range
• Cherry red (600-700°C): Bright, obvious red — silver, gold
• Bright cherry to orange (700-900°C): Iron and steel range

Heating Methods

Charcoal Forge

The most accessible method. A standard blacksmith’s forge with bellows or blower easily reaches annealing temperatures for all common metals.

1. Build a deep bed of charcoal — at least 15 cm
2. Coil the wire loosely into a flat spiral or figure-eight that fits in the fire
3. Bury the coil in the charcoal — it must be surrounded, not just sitting on top
4. Apply gentle airflow — you want even heat, not a welding-temperature blast
5. Watch for the target color on the wire — check by parting the charcoal briefly
6. Hold at temperature for the required time

Open Fire

Workable but less controlled. Best for copper and other low-temperature metals.

1. Build a hot, concentrated fire with hardwood coals
2. Place the wire coil directly in the coal bed
3. Monitor closely — open fires have hot spots and cool zones
4. Rotate the coil periodically for even heating

Muffle Kiln or Oven

If you have a pottery kiln or bread oven that reaches adequate temperature, this provides the most uniform heating:

1. Place the wire coil in a covered ceramic container (a saggar) to protect from direct flame
2. Load into the kiln after it reaches operating temperature
3. Hold for the required time
4. This method produces the cleanest results — no fire scale or oxidation

Atmosphere and Oxidation Control

When metal is heated in air, it oxidizes. For wire, this means a layer of scale (oxide) forms on the surface that must be removed before the next drawing pass. Scale is abrasive and will damage your draw plates.

Reducing Oxidation

Method	Effectiveness	Complexity
Charcoal burial	Good — charcoal consumes oxygen locally	Simple, just bury the coil deep
Sealed container	Very good — limited oxygen available	Seal wire in a steel or clay box with charcoal dust
Wrapping in wet paper	Moderate — steam displaces some oxygen	Wrap coil in soaked paper before placing in fire
Borax coating	Good — flux coating protects surface	Dip wire in borax solution before heating

The Charcoal Box Method

Pack the wire coil into a steel pipe or clay pot, fill all air spaces with fine charcoal powder, and seal the opening with clay. The charcoal consumes available oxygen, and any CO2 produced creates a reducing atmosphere. This produces bright, clean wire with minimal scale.

Removing Scale After Annealing

If oxidation occurs (and it usually does to some degree):

1. Pickle in acid — dilute vinegar (acetic acid) or a weak citric acid solution dissolves copper and brass oxides in 15-30 minutes
2. Mechanical cleaning — pull the wire through a bundle of fine steel wire or abrasive cloth
3. Tumbling — if you have a rotating barrel, tumble with sand or fine gravel

For iron and steel wire, scale is harder to remove. A dilute acid bath (vinegar or fermented fruit juice) followed by wire brushing is effective.

Cooling After Annealing

How you cool the wire after heating affects the final properties.

Copper and Brass

• Quench in water: Perfectly acceptable and actually preferred. Unlike steel, copper gets softer when quenched. Rapid cooling also minimizes oxide formation.
• Air cool: Also fine. Takes longer but produces the same softness.
• Never slow-cool copper in the 200-400°C range for extended periods — this can cause “temper embrittlement” in certain copper alloys.

Iron and Mild Steel

• Slow cool in the forge: Let the fire die naturally with the wire still buried in coals. Cooling over several hours produces the softest result.
• Never quench steel after annealing — this hardens it (the opposite of what you want). Quenching is for hardening, not annealing.
• Ideal cooling rate: 20-30°C per hour through the transformation range (700-400°C).

Silver and Gold

• Quench or air cool — both produce soft, ductile results.
• Quench in water is the standard practice in jewelry work.

Annealing Schedule for Wire Drawing

A systematic approach to when and how often to anneal:

The 30-40% Rule

Anneal after every 30-40% reduction in cross-sectional area. For a round wire, this corresponds to about 20-25% reduction in diameter.

Example — drawing copper from 4 mm to 1 mm:

Pass Set	Start Diameter	End Diameter	Reduction	Action After
1-3	4.0 mm	3.2 mm	20% diameter (36% area)	Anneal
4-6	3.2 mm	2.5 mm	22% diameter (39% area)	Anneal
7-9	2.5 mm	2.0 mm	20% diameter (36% area)	Anneal
10-12	2.0 mm	1.6 mm	20% diameter (36% area)	Anneal
13-15	1.6 mm	1.25 mm	22% diameter (39% area)	Anneal
16-17	1.25 mm	1.0 mm	20% diameter (36% area)	Final anneal (optional)

Total: 17 drawing passes, 5-6 annealing cycles, starting from 4 mm rod to 1 mm wire.

Signs You Need to Anneal

Even without calculating reductions, the wire tells you:

• Springback increases — the wire holds its shape less willingly after drawing
• Drawing force increases noticeably — you need more effort for each pass
• Surface marks appear — fine cracks or a rough texture on the wire surface
• Wire breaks during drawing — you waited too long; anneal the remaining stock before continuing

Troubleshooting

Problem	Cause	Solution
Wire still hard after annealing	Temperature too low or hold time too short	Increase temperature by one color step; hold longer
Wire very soft but weak	Grain growth from overheating	Reduce temperature or hold time; do a few light drawing passes to refine grain, then re-anneal at lower temperature
Heavy scale on wire	Heated in open air without protection	Use charcoal burial or sealed container; pickle after
Wire breaks on first pass after annealing	Thermal shock from uneven heating, or localized overheating	Heat more slowly and uniformly; rotate coil in the fire
Wire is soft in some sections, hard in others	Uneven heating	Use a deeper coal bed; coil wire uniformly; rotate during heating
Copper turns black after annealing	Normal copper oxide formation	Pickle in dilute vinegar for 15-30 minutes
Brass turns pink after annealing	Zinc loss (dezincification) at surface	Annealing temperature was too high; reduce by 50°C