Thiourea Variable Sepia Toner
Thiourea Sepia Toner (Variable) is the modern controllable sepia alternative — uses thiourea + sodium hydroxide in the toning bath instead of sodium sulfide, producing the same silver-sulfide warm-brown image without the hydrogen sulfide gas of traditional sepia toner. The "variable" property comes from controlling toning time and dilution to produce a continuous range from light-warm-brown to deep-brown.
Key features
- Two-bath process: bleach (potassium ferricyanide) → toner (thiourea + sodium hydroxide)
- No H₂S gas concerns — odorless toner bath; safer for non-ventilated darkrooms
- Variable color shift through bleach time + toner time + toner concentration
- Modest archival benefit — same silver-sulfide chemistry as traditional sepia
- Replaces traditional sodium-sulfide sepia in modern darkrooms for safety reasons
Workflow
- Print → develop → stop → fix → wash thoroughly
- Bleach bath — partial or complete bleach controls final tone:
- Complete bleach (3-5 min) → deep brown final tone
- Partial bleach (30-90 sec) → split-tone effect (warm highlights, neutral shadows)
- Water rinse
- Thiourea toner bath: 30 sec to 5 min depending on desired depth and dilution
- Wash 10-15 min
Practical notes
- Choose thiourea sepia over traditional sodium-sulfide sepia for safety (no H₂S) and control (variable toning)
- Thiourea is moderately toxic — handle with gloves and eye protection; avoid skin contact
- Working baths are session-use only; mix fresh per session
- The partial bleach + brief tone approach is the most flexible — produces subtle warm shifts that complement rather than dominate the print
- Compatible with subsequent selenium toning for combined warm-tone + archival workflows (sepia first, then selenium)
Related recipes
- [[recipe-sepia-toner|Sepia Toner]] — traditional sodium-sulfide alternative; H₂S concern
- [[recipe-krst|Kodak Rapid Selenium Toner]] — can be applied after thiourea-sepia for combined warm + archival
- [[recipe-copper-toner|Copper Toner]] — alternative warm-color shift (red-brown)
Mixing Instructions
Two-bath process.
Bleach bath: Dissolve 50 g potassium ferricyanide and 50 g potassium bromide in 1 liter of water at 20 °C.
Toner bath: Dissolve 20 g thiourea in 1 liter of water at 20 °C.
To control tone, add sodium hydroxide to the toner bath:
- Yellow-brown tones: 1-5 ml of 10% NaOH solution per liter
- Warmer brown: 10-20 ml per liter
- Purple-brown: 20-50 ml per liter
Process:
- Bleach the print until the image is partially or fully faded.
- Rinse 2 minutes.
- Immerse in toner until fully redeveloped (1-3 minutes).
- Wash 10 minutes.
Ingredients for 1L of Working Solution
Volume:
ml
| # | Chemical | Role | Qty (1L) | Unit | Note |
|---|---|---|---|---|---|
| 1 | Potassium Ferricyanide | Toner | 50.0 | g | (Bleach bath) |
| 2 | Potassium Bromide | Restrainer | 50.0 | g | (Bleach bath) |
| 3 | Thiourea | Toner | 30.0 | g | (Toner bath) |
| 4 | Sodium Hydroxide | Accelerator | 15.0 | g | (Toner bath, variable 5-30g) |