Developing and Fixing

Part of Photography

Developing and fixing are the two chemical stages that transform an exposed plate’s invisible latent image into a stable, permanent, visible photograph.

Why This Matters

An exposed photographic plate is useless until it is processed. The exposure creates only a latent image — a chemical change so small it is utterly invisible to the eye. Development amplifies this invisible change into a visible silver image. Fixing removes the remaining light-sensitive material so the image becomes stable in light. Together these two steps are what make photography work.

Most photographic failures that seem like camera or lighting problems are actually developing and fixing problems. An image that “didn’t come out” may have been correctly exposed but incorrectly developed. An image that fades after days or weeks was not properly fixed or washed. Understanding these two steps — the chemistry involved, the physical procedure, and the variables that affect the result — gives you control over the entire photographic process. A competent developer can rescue underexposed plates, tame overexposed ones, and produce consistent results under highly variable field conditions.

The Latent Image

When light strikes a silver halide crystal, a photochemical reaction creates a cluster of metallic silver atoms on the crystal surface. This cluster — typically 4 to 10 silver atoms — is the latent image. It is invisible: the crystal still looks exactly like an unexposed crystal. But chemically, these exposed crystals are now distinguished from unexposed ones.

The latent image is unstable. Leave the plate for too long before development and the tiny silver clusters dissolve back into the crystal structure — the image disappears. Most gelatin emulsions are stable for hours to days; thinner or less well-preserved emulsions may lose the latent image in hours. Develop promptly after exposure.

Development in Detail

The developer solution contains a reducing agent — a molecule that can donate electrons to silver ions, reducing them to metallic silver. Silver ions that receive electrons become silver atoms that join the latent image cluster, building it larger and larger. This is catalytic: the initial silver cluster attracts and activates the reducing agent, so the growth is exponential early in development.

What happens in the developer:

1. Developer solution penetrates the gelatin layer, reaching silver halide crystals
2. At exposed crystals, the latent image nuclei catalyze rapid reduction: silver ions → silver atoms
3. At unexposed crystals, the same reaction occurs far more slowly — effectively not at all during the development window
4. Silver builds up on exposed crystals: each becomes a silver grain visible to the eye
5. Together, millions of silver grains form the visible image

Developing agents available:

• Gallic acid (from oak galls): Slow but reliable. Physical developer when combined with silver nitrate.
• Pyrogallol: Fast and high contrast. Chemical developer with sodium carbonate activator.
• Catechol (from pine bark, catechin from tea): Works similarly to pyrogallol.
• Hydroquinone (synthesized from phenol if available): Slower but very consistent.

The activator: Developer reducing agents work best in alkaline conditions. Sodium carbonate (washing soda) is the standard activator. It also swells the gelatin slightly, allowing the developer to penetrate more easily. Concentration: 5-20 g per liter of developer solution.

The restrainer (optional): Potassium bromide (from mineral deposits or wood ash) added in small amounts (1-2 g/L) slows the development of unexposed crystals more than exposed ones, increasing the selectivity and reducing fog. Useful when plates have aged or when development is running too fast.

Standard Development Procedure

Before entering the darkroom:

• Mix developer fresh (or take from stock prepared that day)
• Fill three trays: developer, stop bath (water), fixer
• Check that all trays are in correct positions
• Ensure safelight is functioning and properly filtered
• Gather tongs (separate for each tray)

In the darkroom:

1. Remove exposed plate from its light-tight holder in total darkness or deep safelight
2. Slide the plate smoothly into developer, emulsion face up
3. Start timing
4. Rock the tray gently every 30-60 seconds — side to side, then front to back
5. Observe image emergence under safelight (deep red light only)
6. Watch for: image appearing first in shadow areas (bright in scene), then building in midtones, then finally in highlight areas (dark in scene)
7. Judge development completion by midtone density and shadow detail
8. Transfer plate to stop bath tray at correct time; rock for 30 seconds
9. Transfer to fixer; rock occasionally

Signs of correct development:

• Good shadow density: the darkest areas of the negative should be truly dense silver, not gray
• Midtone gradation: a range of gray values between clear and dense
• Highlight separation: areas that were dark in the scene show as light but not completely clear
• No overall fog: background areas (should be clear) show only minimal grayness

Fixing in Detail

After development, the plate contains two things: the silver image you want, and the unexposed silver halide you want to remove. Fixer dissolves the silver halide while leaving the metallic silver image intact.

The fixer reaction: Sodium thiosulfate reacts with silver halide to form a soluble silver-thiosulfate complex that washes out of the gelatin layer. The reaction is:

• AgBr + 2 Na2S2O3 → Na3[Ag(S2O3)2] + NaBr

The silver-thiosulfate complex is soluble in water. The metallic silver image (from development) does not react significantly with sodium thiosulfate under normal fixing conditions.

Fixer formula:

• Sodium thiosulfate: 200 g per liter of water
• Optional hardener: chrome alum (potassium chromium sulfate) 5 g per liter. This crosslinks the gelatin, making it harder and more scratch-resistant after fixing.

Fixing time: Immerse developed plate for 5-10 minutes. To test completeness: look at the plate against a white background under white light after 3 minutes. Milky areas indicate unfixed silver halide. Continue fixing until completely clear. Add 50% extra time beyond this “clearing point.”

Warning — over-fixing: Leaving plates in fixer longer than about 15-20 minutes begins to bleach the silver image itself, dissolving silver from the image. The image thins and loses density. Fix for the correct time only.

Exhausted fixer: Fixer gradually becomes saturated with dissolved silver. Test by adding a few drops to a small amount of fresh fixer — if it turns milky immediately, the fixer is exhausted. Exhausted fixer fixes slowly and incompletely. The silver can be recovered by adding zinc dust or iron nails to the exhausted fixer, which precipitates metallic silver.

Washing

Washing removes the fixer and dissolved silver complexes from the gelatin. This is critical for permanence.

Sodium thiosulfate left in the gelatin continues to react slowly with the silver image over months and years, producing yellow-brown sulfide stains and eventually destroying the image. A properly washed photograph lasts centuries; a poorly washed one begins to fail within years.

Washing procedure:

1. After fixing, transfer plate to a tray of clean water
2. Change the water 6-8 times over 30-40 minutes, or use running water at a slow flow rate
3. Test completeness (hypo test): drop a small amount of wash water onto a piece of silver-nitrate-coated paper. If it turns yellow, thiosulfate is still present. Continue washing until no color change.
4. Alternatively, immerse a strip of starch paper in the wash water — thiosulfate turns starch blue in the presence of iodine. Absence of color change means washing complete.

Final rinse: A final rinse in slightly alkaline water (add a small amount of baking soda — sodium bicarbonate) helps neutralize any residual acidity from stop bath or acid fixer.

The Complete Processing Sequence at a Glance

Stage	Solution	Time	Temperature
Development	Developer (pyrogallol/gallic acid)	5-15 min	18-22°C
Stop bath	Water or 1% acetic acid	30 sec	Any
Fixing	Sodium thiosulfate 200 g/L	5-10 min	Any
Washing	Running or changed water	30-40 min	Any
Drying	Hang face up	1-4 hours	Cool, dry, dustless

Common Problems and Diagnosis

Problem	Likely Cause	Solution
No image at all	No exposure; developer completely exhausted; latent image decayed	Check camera loaded correctly; use fresh developer
Very faint image	Underexposed or underdeveloped	Increase exposure time or development time
Overall dark fog	Light leak; outdated emulsion; over-development	Seal darkroom; coat fresh plates; reduce development
Streaks on plate	Uneven development; air bubbles	Agitate more regularly; tap plate on entry to release bubbles
Milky patches after fixing	Fixer exhausted or time too short	Refix in fresh solution
Image fades over weeks	Insufficient washing; exhausted fixer	Wash longer; always use fresh fixer
Image lifts from glass	Glass not subbed; overly warm developer	Apply subbing layer; keep developer below 25°C