Chromoskedasic Sabattier: The Workshop Sludge Problem

Part 9 of 13 in the Sustainable Darkroom series | ← Previous: Part 8 | Next: Part 10 →

Chromoskedasic sabattier—“chromo” for short—produces some of the most visually striking prints in darkroom photography. Metallic sheens, iridescent colours emerging from black-and-white paper, golds and coppers and blues that shift depending on viewing angle. The effect relies on Mie scattering: silver particles of varying sizes scatter different wavelengths of light, producing colour without any dyes or pigments.

It's also the process that's produced the most disturbing waste I've encountered in a darkroom workshop.

This post examines chromo through the sustainability lens I've applied to everything else in this series. The conclusions are mixed—the chemistry is concerning, but the method of application matters enormously.

What Chromoskedasic Sabattier Actually Involves

The process was developed in the 1990s, building on work by Dominic Man-Kit Lam (who published in Scientific American in 1991), William Jolly, and Alan Bean. It combines the Sabattier effect (exposure to light during processing) with specific chemistry that causes silver particles to form at varying sizes on the print surface.

The chemistry:

Two solutions are required:

• Activator: A dilute potassium hydroxide solution (typically 5–10% KOH plus sodium sulfite)
• Stabilizer: An acetate-buffered ammonium thiocyanate solution (15–20% ammonium thiocyanate, plus sodium metabisulfite and acetic acid)

The process works on unfixed silver gelatin prints. After partial development, you bring the print into room light and apply the stabilizer and activator in sequence (or selectively with brushes). The thiocyanate complexes with silver; the potassium hydroxide modifies pH and silver particle formation. The room light continues developing the print while the chemistry manipulates how silver aggregates on the surface.

The result: metallic colours in the highlight areas, ranging from yellows and golds through blues and greens depending on chemistry ratios, pH, and exposure.

The Environmental Profile

Let me break down the two main chemicals:

Ammonium Thiocyanate

This is the primary component of the stabilizer—typically 15–20% ammonium thiocyanate in the stock solution as sold.

Environmental classification: H412 (Harmful to aquatic life with long lasting effects) according to multiple SDS sources. Some sources list it as H411 (Toxic to aquatic life with long lasting effects).¹

Aquatic toxicity: LC50 for rainbow trout is 65 mg/L over 96 hours.² This is moderately toxic—more concerning than standard darkroom chemicals like sodium thiosulfate, though less acutely toxic than heavy metals.

Persistence: The good news is that ammonium thiocyanate is water-soluble and not expected to bioaccumulate. It's also biodegradable under aerobic conditions. The bad news is that it's mobile in soil and can contaminate groundwater if improperly disposed.

Disposal requirement: Should not be released to environment. Hazardous waste disposal recommended.¹

Potassium Hydroxide

The activator is essentially dilute caustic potash—a strong base.

Environmental classification: Corrosive, but not persistent. Once neutralized, potassium hydroxide becomes potassium salts that are essentially benign.

The real issue: Concentrated KOH is dangerous to handle (causes severe burns), but environmentally it's the least concerning component of the process. It can be neutralized before disposal with weak acid.

The Combined Effect

When activator and stabilizer mix—which happens on the print surface and in any tray where both are used—the combination produces a strong ammonia odour. This is the ammonium thiocyanate decomposing slightly and releasing ammonia gas.

Ventilation is essential. Freestyle Photo specifically recommends a vapour-reduction mask when using their Arista chromo chemistry.³

Tray Method vs. Brush Application: A Massive Difference

Here's where the sustainability analysis becomes interesting.

The tray method (which I've seen in workshops) involves:

• A tray of stabilizer solution
• A tray of activator solution
• Dunking prints, moving between trays, allowing chemistry to mix on the print
• Contaminated trays that become “sludge”—a grey-brown mixture of silver, thiocyanate, and hydroxide

The sludge at the end of a workshop session is genuinely alarming. It contains:

• Ammonium thiocyanate (aquatic toxin)
• Dissolved silver (heavy metal)
• Potassium hydroxide (caustic)
• Decomposing gelatin
• Precipitated silver compounds of unknown composition

This is unambiguously hazardous waste. I've attended workshops where this mixture was simply poured down the sink—a practice that should never happen.

The brush method (which I use) involves:

• Very small quantities of each solution in separate containers
• Brushing stabilizer onto specific areas of the print
• Brushing activator onto other areas
• Minimal cross-contamination; most chemistry ends up on the print itself

My working solution volumes: perhaps 20–30ml of each (diluted 1:4 from stock) for a printing session. The print absorbs most of it. Leftover chemistry is minimal and can be collected for proper disposal.

The difference in waste generation is roughly 10:1. A tray session might use 500ml–1L of each solution and generate significant contaminated waste. A brush session uses 40–60ml total of dilute working solution and generates almost none—the chemistry is on the prints.

My Practice

I work with chromo occasionally—perhaps 10–15 prints per year. My approach:

1. Brush application only. I use Japanese calligraphy brushes to apply stabilizer and activator to specific areas. This gives creative control and minimises chemistry use.

2. Dilute working solutions. I dilute stock solutions approximately 1:4 with water for brush work. The colours are more subtle but the waste is negligible.

3. Minimal mixing. I apply stabilizer first, let it work, then apply activator to different areas. The colours develop where the two meet on the print surface, not in a contaminated tray.

4. No drainage disposal. The small amount of leftover chemistry goes into labelled containers for hazardous waste collection. In practice, this means I've accumulated perhaps 50ml of mixed chromo waste over two years—less than a single tray session would produce.

Per-print chemistry use: Approximately 2–5ml of each solution per 8×10 print.

Per-print environmental burden: Minimal. The chemistry is on the print (which is kept), the waste volume is tiny, and proper disposal is feasible because the quantities are small.

The Workshop Problem

The sustainability issue with chromoskedasic sabattier isn't inherent to the process—it's about how it's typically taught and practiced.

A workshop with 10 participants, each making 5–10 prints over a day, using the tray method, generates:

• 2–5 litres of contaminated stabilizer
• 2–5 litres of contaminated activator
• Significant cross-contaminated “sludge”
• Multiple litres of silver-laden wash water

If this goes down the drain—as I've seen happen—the environmental impact is substantial. Ammonium thiocyanate at those volumes, plus dissolved silver, represents genuine aquatic toxicity.

Workshop best practice should include:

• Teaching brush method alongside (or instead of) tray method
• Providing proper waste collection containers
• Arranging hazardous waste disposal for contaminated solutions
• Honest discussion of the environmental profile

The fact that chromo produces visually stunning results doesn't exempt it from responsible practice.

Honest Assessment

Is chromoskedasic sabattier sustainable?

With brush application and responsible disposal: Yes, marginally. The chemistry is concerning but the volumes are tiny. Per-print impact is comparable to toning processes.

With tray method and drain disposal: No. The waste volumes and aquatic toxicity make this genuinely harmful.

Comparison to other processes:

Process	Primary concern	Per-print impact	Disposal complexity
Standard B&W	Silver in fixer	Moderate	Low (silver recovery)
Selenium toning	Selenium compounds	Low (dilute)	Moderate
Mordançage	Copper chloride	Low (reusable)	High (hazardous waste)
Chromo (brush)	Ammonium thiocyanate	Low	Moderate
Chromo (tray)	Ammonium thiocyanate + silver	High	High

The brush method puts chromo in the same category as selenium toning or mordançage—processes with concerning chemistry but manageable impact when practiced carefully. The tray method puts it in a worse category altogether.

Why I Continue (Occasionally)

Chromoskedasic sabattier produces effects that can't be achieved any other way. The metallic colours, the iridescence, the way prints shift in different lighting—these are genuinely unique aesthetic qualities.

I make perhaps 10–15 chromo prints per year. At brush-application volumes, my annual consumption is roughly 30–50ml of stock solution total (diluted for use). This is environmentally meaningful only if I dispose of it improperly—which I don't.

The honest position: chromo has a concerning chemical profile, but the process can be practiced sustainably if you:

• Use brush application
• Work at dilute concentrations
• Minimise waste generation
• Dispose of residues properly

What I can't defend is the workshop culture that treats the sludge as normal, pours it down the drain, and never discusses the environmental implications. That's not sustainable practice—it's just convenient ignorance.

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Practical Notes

Brush Application Setup

Materials:

• Two small glass containers (50–100ml each)
• Japanese calligraphy brushes or soft watercolour brushes (separate for each solution)
• Diluted stabilizer (stock diluted approximately 1:4 with water)
• Diluted activator (stock diluted approximately 1:4 with water)
• Partially developed, unfixed print
• Good ventilation

Process:

1. Develop print normally until highlights have detail (slightly underdeveloped)
2. Rinse briefly in water
3. Bring into room light (print will begin to darken)
4. Apply stabilizer to areas where you want light yellow/green tones
5. Apply activator to areas where you want warm brown/red tones
6. Where the two meet on the print, colours emerge
7. Continue until satisfied with effect
8. Rinse thoroughly, then fix briefly (30 seconds–1 minute to preserve colours)
9. Final wash as normal

Waste Management

Leftover solutions: Collect in labelled containers. Do not mix. Dispose through hazardous waste collection.

Contaminated brushes: Rinse thoroughly in water; the small amount of chemistry washed from brushes is negligible. Or dedicate brushes to chromo and rinse minimally.

The print itself: Once fixed and washed, the print is stable. The silver is metallic and bound in gelatin. Handle normally.

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References