Washing and Drying

Part of Photography

Washing removes residual fixer and dissolved silver from processed photographs, and proper drying prevents physical damage — together they determine whether your photographs last years or centuries.

Why This Matters

A photograph that has been correctly exposed and developed can still fail if poorly washed or dried. Sodium thiosulfate (fixer) left in the gelatin layer reacts with the silver image over months and years, producing brown stains and a progressive bleaching of the image. The photograph appears fine when made, then deteriorates slowly — an insidious failure because there is no warning until the damage is done.

The standard is non-negotiable: wash until no detectable fixer remains. This takes 20-40 minutes in running water or equivalent changed-water washing. No short cuts can substitute. A photograph that is washed for 5 minutes instead of 30 minutes may look identical when finished, but it is not the same photograph — one is an archival record, the other is a deteriorating document.

Drying matters for different reasons. Wet gelatin is soft and fragile; a fingerprint on a wet emulsion leaves a permanent impression. A print dried too fast in hot air can crack. A print dried unevenly can curl so severely it cannot be flattened. Good drying technique preserves the physical integrity of the photograph.

The Permanence Problem: What Fixer Does to Silver

Sodium thiosulfate forms soluble complexes with silver: Ag + Na₂S₂O₃ → Na[Ag(S₂O₃)] (and higher complexes)

These complexes are stable in solution and wash out of the gelatin — this is the desired reaction. But thiosulfate also slowly oxidizes in air to form sulfide compounds (tetrathionate, polysulfides). When these sulfides contact silver, they form silver sulfide (Ag₂S) — a brown-yellow compound.

The rate of this reaction is slow, so photographs with residual thiosulfate look fine initially. Over months to years, yellow-brown staining appears, usually starting in the shadows and spreading. The silver image itself is attacked: the image fades, loses shadow density, and eventually the photograph becomes an indistinct brown stain.

This deterioration is irreversible. The only prevention is complete removal of thiosulfate during washing before storage.

Washing Procedure for Plates and Prints

Target: Reduce thiosulfate concentration in the gelatin to less than 0.1 mg per cm² of emulsion. This requires extensive water exchange.

Running water washing:

1. After fixing, place the plate or print in a tray under a gentle flow of water
2. The water flow should be slow enough not to abrade the emulsion but continuous enough to carry away thiosulfate
3. A flow rate of 1-2 liters per minute is adequate for a standard tray
4. Wash time: 20-30 minutes for glass plates; 30-40 minutes for prints on paper (paper absorbs more fixer)
5. Do not pile plates — ensure water reaches every surface

Still water washing (where running water is not available): Change the water completely 8-10 times over 40 minutes. Each change should use approximately the same volume as the tray.

Scientific measurements show that a single volume exchange removes about 90% of the soluble fixer. Each subsequent change removes 90% of what remains. After 8 changes, approximately (0.1)⁸ = 1 × 10⁻⁸ of the original fixer remains — essentially zero. This requires patience; rushing the changes (allowing less than 3-4 minutes per change) reduces the efficiency of each exchange.

The Hypo Test

Use this test to confirm washing is complete before drying:

Materials:

• Prepare a test solution: 10 g silver nitrate dissolved in 100 mL distilled water, applied to strips of paper and dried. These are your test papers.

Test procedure:

1. After washing, let a drop of water from the wash tray fall onto a silver-nitrate paper strip
2. Hold the strip for 1 minute
3. Observe any discoloration

Reading the result:

• No color change: washing is complete. Proceed to drying.
• Pale yellow or cream color: trace amounts of thiosulfate remain. Continue washing 10 more minutes and retest.
• Yellow or orange color: significant thiosulfate remains. Continue washing 20 more minutes and retest.

Alternatively, use the starch-iodine test: thiosulfate in water will reduce iodine, decolorizing a blue starch-iodine solution. A strip of iodine-stained starch paper that loses its blue color in the wash water indicates thiosulfate is still present.

Hardening Before or After Washing

Gelatin softens in warm water and can be scratched or marked easily when wet. Hardening the gelatin before processing, or in a final hardening bath before washing, protects the emulsion during the washing phase.

Chrome alum hardener: Dissolve 5 g potassium chromium sulfate (chrome alum) in 500 mL water. After fixing and before the main wash, immerse plate or print for 3-5 minutes. This crosslinks the gelatin, making it tougher and less prone to scratching.

Hardened gelatin requires somewhat longer washing to reach the same thiosulfate level, because the crosslinked matrix is slightly less permeable. Add 5-10 minutes to your standard washing time if you have hardened the emulsion.

Alum-salt fixer: Including chrome alum in the fixer (5 g per liter) hardens the gelatin during fixing, eliminating the need for a separate hardening bath.

Drying Glass Plates

Vertical drying:

1. After washing, remove the plate from the water and let excess water drain from the edges for 30 seconds
2. Stand the plate vertically in a groove or frame with the emulsion side facing away from nearby walls or surfaces
3. Do not wipe the emulsion surface — this causes scratches. Blot the glass back if needed, but never touch the emulsion face.
4. Allow to dry in still, cool air (15-20°C)
5. Drying time: 1-3 hours

Accelerated drying with heat: In cold or humid conditions, very gentle warming (30-35°C maximum) accelerates drying. A rack near a warm (not hot) fire works. Above 40°C, the gelatin melts and runs, destroying the image. Never place a wet plate in direct sun or over hot coals.

Dust protection during drying: Cover the drying area with a thin cloth tent that allows air circulation but prevents dust settling on the wet emulsion surface. A single dust particle on a wet emulsion plate embeds permanently and shows as a white spot in every print made from that negative.

Drying Paper Prints

Paper prints require different handling because paper swells significantly when wet and can distort as it dries unevenly.

Face-up on a flat surface:

1. After washing, lift the print and allow excess water to drain
2. Lay face-up on clean, smooth cloth (linen or closely-woven cotton)
3. Gently blot the back with another clean cloth
4. Allow to dry in still air

Hanging by corner:

1. Attach a wooden peg or clip to one corner of the wet print
2. Hang vertically in still air
3. Slight curl develops as the print dries, but can be flattened

Preventing curl: Paper prints curl because the base paper and gelatin have different shrinkage rates as they dry. Prevention methods:

1. Press between clean glass plates while still slightly damp (not wet)
2. Place between clean, dry blotting papers under moderate weight
3. Leave under weight until fully dry (several hours)

Forced drying (heat) increases curl tendency. Cool, still air produces the flattest results.

Resin-coated prints (if you have access to resins): Paper backed with a thin resin coating is less absorbent and dries much flatter. For standard gelatin-coated rag paper, accept some curl and plan on pressing.

Archival Storage After Drying

A properly washed and dried photograph is ready for archival storage.

Negatives (glass plates):

1. Store vertically in individual slots in a box — do not stack flat, as the weight of stacked plates can crack lower plates
2. Each slot should be lined with thin cloth or folded paper so the glass does not contact the wood directly
3. The box should be in a cool, dry, dark location
4. Do not store with rubber bands, rubber gaskets, or wool — these emit sulfur vapors that attack silver

Prints:

1. Interleave with neutral tissue or clean rag paper
2. Store horizontally in a flat box with a tight-fitting lid
3. Cool, dry, dark conditions
4. Remove from storage periodically (every few years) to inspect for signs of deterioration

Numbering and cataloging: Number each negative and its corresponding prints with the same number. Keep a written log with the negative number, subject, date, location, and exposure data. Without this record, a collection of photographs is much less useful — the context is often as important as the image.