The Wash Question: Where Darkroom Water Actually Goes

Part 8 of 13 in the Sustainable Darkroom series | ← Previous: Part 7 | Next: Part 9 →

Film washing is a solved problem. The Ilford method—fill, invert 5 times, drain; fill, invert 10 times, drain; fill, invert 20 times, drain—uses 1.5–2 litres per roll and achieves archival permanence. I described this in Part 3. In a rotary drum, it's even simpler: the same sequential water changes, the same geometric progression, the same result. My film development uses less water than a single toilet flush.

Print washing is different.

A single 8×10 fibre-based print washed using traditional continuous-flow methods can consume 40–60 litres of water.¹ A printing session producing five exhibition prints might use 200–300 litres—more water than many households use for daily showering, cooking, and cleaning combined. This isn't speculation; it's what happens when you run a tap at 2–4 litres per minute for 30–60 minutes.

Washing is the single largest water consumer in darkroom practice. It's also non-negotiable for archival permanence. The sustainability challenge is therefore specific: we need washing that's thorough enough for prints to last, but efficient enough to justify in an era of water scarcity.

This post examines the options and works toward an answer.

Why Washing Can't Be Shortened Carelessly

Before discussing efficiency, I need to establish why washing matters at all. The temptation exists—particularly when facing water costs or sustainability pressure—to simply fix prints and move on. This is a mistake.

Residual thiosulfate causes irreversible image degradation. Thiosulfate ions remaining in the paper base after fixing react slowly with silver in the image, forming silver sulfide. The result: yellowing in highlights, brownish staining in midtones, eventual loss of image density.² The degradation takes 5–20 years to become visible, but once it starts, it cannot be reversed.

ISO 18917 sets archival standards for residual thiosulfate at 0.03–0.40 μg/cm² depending on intended storage conditions.³ For prints intended for exhibition or long-term preservation, the lower end of this range is appropriate. Achieving these levels requires systematic washing; rinsing a print under a tap for a few minutes leaves residual thiosulfate an order of magnitude too high.

Incomplete washing also affects print handling. Residual fixer makes paper slightly acidic and chemically active. This accelerates deterioration of mounting materials, causes adhesive failure in dry mounting, and can create a cloudy “bloom” on framing glass in contact with the print surface.

The consequence of inadequate washing is that you make prints that deteriorate. You then discard and remake them, consuming more chemistry, more paper, more water. Inadequate washing isn't sustainable—it just defers the resource consumption.

But over-washing is also wasteful. Running water through a tray for two hours “because that's what archival requires” is a different kind of careless: care without understanding. The water after the first 15 minutes isn't doing anything useful. You've achieved equilibrium, and you're just diluting already-clean water while waiting.

The goal is the narrow path: archival longevity achieved with minimal resource consumption.

The Physics of Washing: What's Actually Happening

Understanding diffusion changes how you wash. This isn't just academic—it's the difference between using 50 litres and using 5.

Diffusion and Equilibrium

When a fixed print sits in fresh water, thiosulfate ions diffuse from the paper (high concentration) into the surrounding water (zero concentration). The rate of diffusion is proportional to this concentration gradient—the bigger the difference, the faster ions move.⁴

As the surrounding water becomes enriched with thiosulfate, the gradient weakens. Eventually, the thiosulfate concentration in the water equals the concentration remaining in the paper, and diffusion stops. You've reached equilibrium.

At equilibrium, continued water flow helps—you remove thiosulfate-enriched water and replace it with fresh water, re-establishing the gradient. But this only works if you're continuously exchanging water. Static soaking doesn't help once equilibrium is reached, no matter how long you wait.

Agitation accelerates the process. Every time you move the print or disturb the water, you disrupt the thin layer of thiosulfate-enriched water clinging to the print surface. This exposes fresh emulsion to the concentration gradient.

Temperature matters. Diffusion is thermally dependent. At 20°C, it proceeds at a certain rate. At 10°C, it's slower. At 24°C, it's noticeably faster—though above ~25°C you risk emulsion softening on fibre paper.⁵

The implication: washing is fundamentally limited by diffusion kinetics, not by water volume. More water per minute doesn't accelerate diffusion once the surrounding water is cleaner than the paper.

Why the Ilford Method Works

The Ilford film washing method exploits this physics. Instead of continuous flow, you use sequential fill-and-dump cycles:

1. Fill tank, invert 5 times, drain completely
2. Fill tank, invert 10 times, drain completely
3. Fill tank, invert 20 times, drain completely
4. Final rinse with wetting agent

Each cycle starts with fresh water (maximum concentration gradient). Each dump removes the thiosulfate-laden water before you've wasted time waiting at equilibrium. The geometric progression—5, 10, 20—reflects the exponential decrease in remaining thiosulfate: each cycle removes less than the previous because there's less left to remove, so you agitate longer to compensate.

Total water: ~2 litres. Archival result: identical to or better than 20 minutes of continuous flow using 40+ litres.⁶

I've used this method for years. Negatives washed this way are now over a decade old with no signs of degradation.

Print Washing: Why It's Harder Than Film

The Ilford method works beautifully for film because film emulsions are thin, uniformly fixed, and processed in a contained tank where agitation is simple. Prints present different challenges.

Fibre Paper's Absorption Problem

Fibre-based papers have a paper base—pure cellulose—that wicks processing chemicals into itself, not just the emulsion layer on top. When you fix a fibre print, thiosulfate doesn't just stay in the emulsion; it soaks into the paper fibres beneath. These fibres are less chemically permeable than gelatin, so diffusion out is slower.

Additionally, fibre paper absorbs water itself, which affects the diffusion gradient. The paper becomes saturated; thiosulfate must diffuse through water-filled cellulose, not just free water.

This is why fibre paper requires 30–60 minutes of washing compared to ~1 minute for film. The geometry is different, and the diffusion path is longer.

RC Paper's Advantage

Resin-coated (RC) papers don't have this problem. The plastic coating waterproofs the paper substrate, so chemicals stay in the thin emulsion layer. RC paper washes in 30 seconds to 2 minutes—the thiosulfate has nowhere to hide.⁷

For purely environmental considerations, RC paper is dramatically more sustainable for print washing. The trade-off is that fibre paper has qualities—handling characteristics, toning response, archival reputation—that RC doesn't replicate. I use both, for different purposes.

The Geometry Problem

Unlike film in a spiral tank, prints in a tray don't receive uniform water exchange. The bottom print in a stack washes differently than the top print. Corners trap water differently than centres. Agitation is manual and inconsistent.

This is why continuous-flow washing became standard: it's crude but uniform. Running water constantly refreshes the tray, avoiding dead spots where equilibrium stalls progress.

But crude doesn't mean efficient.

The Options: From Worst to Best

Let me compare the available approaches, with actual water consumption figures.

Option 1: Continuous Running Water (Traditional)

Method: Place prints in tray, run tap at 2–4 L/min for 30–60 minutes (fibre) or 2–5 minutes (RC).

Water consumption:

• Fibre paper: 60–240 litres per session
• RC paper: 4–20 litres per session

Advantages: Simple, requires no special equipment, uniform washing if flow is adequate.

Disadvantages: Massively wasteful. Most of the water is doing nothing—you're paying for continuous dilution when you only need it intermittently. The tap runs whether the print needs it or not.

This is the baseline against which improvements are measured.

Option 2: Hypo Clearing Agent (HCA)

Method: After initial rinse (5 min running water), immerse prints in HCA solution (sodium sulfite) for 5–10 minutes with agitation, then final wash (5–15 min running water).

Water consumption:

• Fibre paper: 20–40 litres per session
• RC paper: Minimal benefit (RC doesn't need HCA)

How HCA works: Sulfite ions (SO₃²⁻) displace thiosulfate ions (S₂O₃²⁻) from binding sites in the paper fibres.⁸ This “unlocks” deeply-absorbed thiosulfate, allowing it to diffuse out more rapidly. The effect is dramatic for fibre paper—wash time can be reduced by 60–80%.

Advantages: Significantly reduces water use and time. Cheap—a lifetime supply of sodium sulfite costs a few euros. Works with existing equipment (just add one tray).

Disadvantages: Still uses continuous flow in the initial and final wash phases. Improvement over baseline, but not optimal.

My current practice: I use HCA for all fibre printing. It's too cheap and effective not to.

Option 3: Sequential Water Changes + HCA

Method: Apply the Ilford method logic to prints. After HCA bath, instead of continuous flow, use sequential tray fills with agitation:

1. Fix (1 min in rapid fixer 1+4)
2. Rinse in running water (5 min) or two tray changes
3. HCA bath (10 min with intermittent agitation)
4. First wash tray: fill with fresh water, agitate 2 min, drain
5. Second wash tray: fill, agitate 2 min, drain
6. Third wash tray: fill, agitate 5 min, drain

Water consumption:

• Fibre paper: 8–12 litres per session
• Per-print (5 prints in session): ~2 litres

Advantages: Dramatically lower water use. Applies the physics correctly—each tray starts at maximum concentration gradient. No special equipment required.

Disadvantages: Labour-intensive. You're manually filling and draining trays, shuffling prints, timing agitation. In a busy printing session, this becomes tedious.

Archival result: Testing with residual hypo indicators shows this method achieves archival permanence equivalent to continuous flow.⁹ The physics is the same; only the water volume differs.

Option 4: Archival Washing Tank

Method: Place prints in a washing tank designed for the purpose—either a vertical slot washer (Nova, Cachet) or a rocking-basket washer (Paterson, Zone VI). Water flows through continuously but at very low rates, with mechanical agitation ensuring water exchange at print surfaces.

Water consumption:

• Fibre paper: 15–25 litres per session for 6–12 prints
• Per-print (with HCA pre-treatment): ~2–4 litres

How they work:

Slot washers hold prints vertically in individual slots. Water enters at the bottom, rises through the slots, and overflows at the top. Each print is isolated, preventing prints from sticking together or blocking each other's water exchange.

Rocking-basket washers (like the Paterson) hold prints in a basket that rocks continuously, driven by water flow. The motion ensures constant agitation without manual intervention. Water flow can be very low—a gentle trickle is sufficient because the rocking does the work.

Advantages:

• Batch washing: 6–12 prints washed simultaneously with no additional labour per print
• Consistent results: every print receives identical treatment
• Hands-off: set it running and return in 20–30 minutes
• Lower per-print water use than any manual method at scale

Disadvantages:

• Equipment cost: €250–600 depending on size and type
• Space requirement: needs permanent or semi-permanent setup
• Only economical at volume: one print per month doesn't justify the investment

Comparison Table

Method	Water/session (fibre, 5 prints)	Water/print	Labour	Equipment cost
Continuous flow, no HCA	120–240 L	24–48 L	Low	€0
Continuous flow + HCA	40–60 L	8–12 L	Low	~€10
Sequential changes + HCA	8–12 L	1.5–2.5 L	High	~€10
Archival washer + HCA	15–25 L	2–4 L	Very low	€250–600

The sequential-changes method uses the least water but requires the most attention. The archival washer uses slightly more water but frees you to do other things—and its per-print efficiency improves as batch size increases.

Practical Recommendations

For RC Paper

RC paper doesn't need elaborate washing. The resin coating prevents fixer absorption into the paper base.

Method: Three-tray sequence, 15 seconds per tray with agitation, or 30 seconds running water.⁷

Water consumption: 4–6 litres per session regardless of print count.

Don't overwash RC paper. Extended washing doesn't help and can cause edge penetration and curl. 2 minutes is the maximum recommended wet time after fixing.

For Fibre Paper (Manual Method)

Minimum recommended protocol:

1. Fix in rapid fixer (1+4), 1 minute—don't overfix
2. Rinse: running water or two tray changes, 5 minutes
3. HCA bath: 10 minutes with intermittent agitation
4. Final wash: three sequential tray fills with 2-minute agitation each

Water consumption: ~10–15 litres per session.

Testing: Periodically verify your method with residual hypo test solution (€15–20, lasts years). Apply to print margin, wait 2 minutes, blot. Distinct staining indicates incomplete washing.

For Fibre Paper + Archival Washer

If you have access to a washing tank (slot washer or rocking-basket type):

Protocol:

1. Fix prints normally
2. Brief rinse (2–5 minutes running water or two tray changes)
3. HCA bath: 10 minutes with intermittent agitation
4. Load into washing tank; set timer for 20–30 minutes
5. Remove, squeegee gently, dry

Water consumption: ~20–25 litres per full load (6–12 prints), or ~2–4 litres per print.

The washer's advantage is labour efficiency—you load it and walk away. The per-print water consumption is comparable to sequential tray changes, but without the manual work.

Making Your Own HCA

Commercial washing aids (Ilford Washaid, Kodak Hypo Clearing Agent) work well, but HCA is trivially cheap to make:

Formula:

• 20–30g sodium sulfite (food-grade is fine)
• Optional: 2–3 teaspoons table salt (accelerates ion exchange)
• 1 litre water

Cost: ~€0.10 per litre versus ~€0.50 for commercial products.

Shelf life: 6 months in a sealed bottle. Discard when solution turns yellow (oxidation).

Usage: Identical to commercial HCA. 10 minutes with intermittent agitation.

The Hierarchy of Print Washing Interventions

Analogous to the chemistry hierarchy from earlier posts (silver » water » chemistry » developer choice), here's the washing hierarchy:

1. Use HCA for all fibre printing — reduces water consumption 50–70% with €10 investment
2. Apply sequential-changes logic — dump-and-refill is more efficient than continuous flow
3. Consider an archival washer — especially for high-volume printing or when labour is a constraint
4. Test your results — residual hypo test confirms you've achieved archival permanence
5. Match method to paper — don't overwash RC; don't underwash fibre

For most practitioners, steps 1 and 2 provide the biggest improvement at lowest cost. Step 3 becomes compelling at higher volumes or when you want hands-off operation.

Conclusion

Film washing is solved: the Ilford method in a rotary drum uses 1.5–2 litres per roll with perfect archival results.

Print washing is where the water actually goes. Traditional continuous-flow methods consume 50+ litres per fibre print—more water than film development by a factor of 25–50. This is the largest single water expense in darkroom practice.

But print washing can be reduced by 80–90% without compromising archival permanence:

• HCA cuts fibre wash time and water by half or more
• Sequential water changes apply diffusion physics efficiently
• Archival washing tanks automate the process at scale

The physics is clear: washing is diffusion-limited, not flow-limited. Understanding this means you stop wasting water on continuous dilution and start using water strategically—maximum concentration gradient, efficient agitation, no time wasted at equilibrium.

For high-volume printing, an archival washing tank makes both environmental and practical sense. The equipment cost is recovered in labour savings, and the per-print efficiency improves with batch size.

Inadequate washing produces prints that deteriorate—the least sustainable outcome. Over-washing produces prints that survive, but wastes resources achieving what could be done with less. The goal is archival permanence at minimal environmental cost.

The methods exist. The physics is understood. The only question is whether we use them.

---

References