Fixing Process

Part of Photography

Fixing is the chemical step that removes unexposed silver halide from a developed photographic image, making it permanent and stable in normal light.

Why This Matters

Development reveals the photographic image by building up metallic silver where light struck the plate. But after development, the plate still contains all the unexposed silver halide crystals — the same light-sensitive material that was in the emulsion before exposure. If you bring the developed plate into normal light, these remaining crystals darken by direct photoreduction, destroying the image within seconds.

Fixing is what makes the image permanent. The fixer dissolves and removes all the unexposed silver halide while leaving the developed metallic silver image intact. Once fixed and washed, the plate can be handled in normal light indefinitely — the image is stabilized.

Choosing the right fixer chemistry, mixing it correctly, using it properly, and knowing when to discard exhausted fixer are all practical skills with direct consequences. Inadequate fixing is one of the most common causes of long-term photographic deterioration — images that look perfect when made but fade, yellow, or stain within months or years because residual silver halide continued reacting slowly with light and atmospheric sulfur.

The Chemistry of Fixing

Sodium thiosulfate (hypo) is the standard fixer. The name “hypo” is historical — early photographers called it “hyposulfite of soda.” It works by forming soluble complex ions with silver:

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

The silver-thiosulfate complex dissolves easily in water and washes out of the gelatin during the washing step. The metallic silver in the image does not react significantly with dilute thiosulfate at the fixing concentrations used.

Why sodium thiosulfate specifically? It is a strong enough complexing agent to dissolve silver halide rapidly, but not so strong that it attacks the metallic silver image at normal concentrations and temperatures. Salt water (sodium chloride) will also partially fix silver images but is far slower and less complete. Strong sodium thiosulfate is the practical choice.

Sources of sodium thiosulfate: In a rebuilding scenario:

• Mineral deposits: Sodium thiosulfate occurs naturally in some evaporite deposits, often associated with other sodium compounds
• Chemical synthesis: Boil sodium sulfite (from sulfur dioxide and soda ash) with elemental sulfur. The sulfur dissolves in the sulfite to form thiosulfate: Na₂SO₃ + S → Na₂S₂O₃
• From photography supplies: If any commercial photographic materials survived from before the collapse, the fixing compounds they contain are exactly this material

Making Fixer Solution

Standard fixer (acid hardening type is best):

• Sodium thiosulfate: 200 g
• Water: 1 liter (warm to dissolve, then cool)
• Acetic acid (optional, for acid fixer): 15 mL of 5% solution (or 5 mL glacial acetic acid)
• Chrome alum / potassium chromium sulfate (optional hardener): 5 g

Mixing procedure:

1. Weigh 200 g sodium thiosulfate crystals. If your crystals are large, crush slightly for faster dissolving.
2. Dissolve in 800 mL of warm water (40°C) — the reaction is endothermic, so the solution becomes cold as it dissolves. Continue stirring until fully dissolved.
3. Add cold water to bring to 1 liter total volume
4. If using acid and hardener: dissolve chrome alum separately in 100 mL warm water. Add acetic acid to the alum solution, then add this slowly to the thiosulfate solution while stirring. Never add them in reverse order — adding acid to concentrated thiosulfate causes decomposition.

Checking fixer strength: The solution should be clear to slightly yellow. A strong sulfur smell suggests decomposition has begun — use fresh. Test activity by placing a small piece of undeveloped coated paper in the fixer — it should clear completely within 2-3 minutes for fresh fixer. If clearing takes 5+ minutes, fixer is weakening.

The Fixing Procedure

Setup:

• Fixer tray: glass or glazed ceramic, slightly larger than your plates
• Tongs: dedicated for fixer use only — not shared with developer
• Wash water: a tray or running water for subsequent washing

Procedure:

1. After the stop bath rinse, transfer the plate to the fixer tray without touching the emulsion surface
2. Submerge fully so the fixer covers the plate by at least 1 cm
3. Rock the tray gently to ensure fresh fixer contacts the emulsion continuously
4. In normal conditions (fresh fixer, correct temperature), fixing takes 5-10 minutes
5. After 2-3 minutes, you can briefly turn on white light to inspect — look for milky areas that indicate unfixed silver halide
6. The plate is fixed when all milky areas have cleared completely
7. Continue fixing for 50% more time beyond the clearing point (if it clears in 4 minutes, fix for 6 minutes total)
8. Remove and transfer immediately to the washing tray

Clearing time test: Place a small piece of unused sensitized paper in fresh fixer. The time it takes to go completely clear is the “clearing time.” Double this for the minimum fixing time. For a plate, add 50% safety margin beyond that.

Over-fixing

Leaving plates in fixer more than 15-20 minutes begins to bleach the silver image itself. The dense shadow areas thin and lose density; the entire image weakens. Fix for the calculated time and no longer. Use a timer or counting method.

Fixer Exhaustion

Fixer does not last indefinitely. As it processes plates, the dissolved silver accumulates in the solution, and the thiosulfate is gradually consumed. Exhausted fixer:

• Fixes slowly (long clearing times)
• Leaves milky residue that does not clear completely
• Produces prints that appear fixed but contain insufficient removal of silver halide — these will deteriorate rapidly

Testing for exhaustion:

1. Clearing time test: when the clearing time doubles from fresh, fixer is at 50% capacity. When it triples, discard.
2. Silver saturation test: add a few drops of potassium iodide solution to a small amount of fixer. A yellow precipitate (silver iodide) indicates high silver content and impending exhaustion.
3. Odor: Fresh sodium thiosulfate fixer smells mildly of sulfur. Exhausted fixer smells strongly of hydrogen sulfide (rotten egg odor) — this indicates the thiosulfate has decomposed. Discard immediately.

Silver recovery from exhausted fixer: The dissolved silver in exhausted fixer has real value. Recover it:

1. Add zinc dust or iron filings (iron nails) to the exhausted fixer
2. Stir well and let stand for 30 minutes
3. The zinc or iron displaces the silver, which precipitates as a metallic sludge
4. Decant the liquid and collect the gray-black silver sludge
5. Dry the sludge; it can be melted to recover metallic silver for reuse in silver nitrate preparation

Archival Fixing Practice

For photographs intended to last decades or centuries, a two-bath fixing procedure dramatically improves permanence:

Two-bath fixing:

1. Fix in a first fixer bath for half the normal time
2. Transfer to a second, fresh fixer bath for the remaining half of the normal time
3. The first bath does most of the work and exhausts more quickly; the second bath ensures complete fixing
4. When the first bath shows extended clearing times, it becomes the new second bath; mix a fresh first bath

This technique ensures the final fixer the plate sits in is always relatively fresh, minimizing silver contamination of the final fix and maximizing the thoroughness of halide removal.

Washing after fixing: Fixing is only half the permanence equation. The dissolved silver-thiosulfate complexes and residual sodium thiosulfate remaining in the gelatin must be completely removed by washing. Thiosulfate left in the gelatin slowly attacks the silver image over months and years, producing brown sulfide stains and image loss.

Minimum washing: 30 minutes in slowly running water, or 6 complete changes of water over 30 minutes.

Testing washing completeness (hypo test):

1. After washing, let a drop of wash water fall onto a strip of silver nitrate-coated paper
2. If a yellow stain appears, thiosulfate is still present — continue washing
3. No color change means washing is complete

Plates and prints washed to this standard and stored properly can last for centuries.