Salted Paper Print
Summary
You’ll make the oldest kind of photograph: salt and silver nitrate form light-sensitive silver chloride in the paper, and sunlight reduces1 it to metallic silver, “printing out” an image with no developer. You’ll be able to explain why the salt goes on first, why you overexpose, and how thiosulfate “fixes” the print.
History
The salted paper print is the oldest surviving photographic process. William Henry Fox Talbot developed it in 1834 after failing to sketch landscapes on a camera obscura — frustrated by the gap between what the device could project and what his hand could capture, he resolved to make the image fix itself.
His method was astonishingly simple. Paper was soaked in common salt (sodium chloride), dried, then brushed with silver nitrate solution. Where the two met within the paper fibres, silver chloride formed in situ. Objects placed on the coated paper in sunlight slowly reduced the silver chloride to metallic silver — the image “printed out” directly, without any development step. Talbot called the products photogenic drawings.
The great problem was fixing. Without removing the unexposed silver chloride, the image continued to darken in light until the whole sheet turned brown. Talbot initially used a strong salt solution to suppress further reaction. John Herschel2 then demonstrated that sodium thiosulfate (hypo) was a far more effective fixer — a discovery that became central to all silver photography for the next 170 years.
Salted paper prints have a characteristic warm brown to purple-brown colour and a gentle softness, since the silver image forms within the paper fibres rather than on a smooth coating. Victorian photographers often toned them with gold chloride to give the characteristic sepia purple of 19th century portraits and to improve archival stability.
Hazards & preparation
PPE: nitrile gloves throughout; protect your work surface.
- Silver nitrate stains skin black (harmless, fades over days–weeks) and is toxic to aquatic life. Sensitize in dim light, away from sun.
- Keep the sensitized paper out of strong light until you’re ready to expose.
Disposal: don’t pour silver nitrate down the drain. Add table salt until no more precipitate forms, filter off the silver chloride and bin the solid, then flush the clear liquid. Spent fixer is not drain-safe — fixing dissolves silver into solution, so used thiosulfate fixer accumulates toxic silver. Precipitate that silver (stir in excess table salt), settle, bin the solid, and drain only the clear liquid — or hold the spent fixer for a photo/hazmat waste facility. Only unused, silver-free thiosulfate is harmless. See the Safety page.
Materials
- Silver nitrate — 12 g dissolved in 100 mL distilled water (store in a dark glass bottle)
- Table salt (NaCl) — 20 g dissolved in 1 litre distilled water
- Sodium thiosulfate — 100 g dissolved in 1 litre water (fixer; also sold as aquarium dechlorinator)
- Watercolour paper (90 lb/190 gsm or heavier) — thicker paper handles the wet treatments better
- Flat, opaque objects to print as silhouettes, or a negative printed on inkjet transparency film
- Foam brush or sponge brush
- Glass sheet to press objects flat
- Rubber or nitrile gloves
- Two trays: one for the salt soak, one for the fixer
Procedure
- Salt the paper: Soak or brush the paper with the salt solution and let it dry completely in normal room light. It is not yet light-sensitive.
- Sensitize with silver nitrate: In dim tungsten light, away from windows, brush the dried paper with silver nitrate. It reacts with the salt in the fibres to form silver chloride: \[\ce{AgNO3 + NaCl -> AgCl(in paper) + NaNO3}\] The paper turns slightly cream. Handle with gloves.
- Arrange objects or negative: While still damp, lay objects or an inkjet negative face-down and press firmly under glass.
- Expose in sunlight: Carry to direct sun. The image prints out as light reduces silver chloride to metallic silver — pale cream → brown → dark grey-brown, over 15–45 minutes in strong sun. Overexpose deliberately — it lightens during fixing.
- Rinse briefly: Indoors, remove the objects and rinse 30 seconds to slow further exposure.
- Fix: Immerse in sodium thiosulfate 5 minutes, agitating. Unexposed silver chloride dissolves and the image becomes permanent: \[\ce{AgCl(s) + 2Na2S2O3(aq) -> Na3[Ag(S2O3)2](aq) + NaCl(aq)}\]
- Wash: Wash in running water 30 minutes (residual thiosulfate causes yellowing). Dry flat.
What you should see
The freshly sensitized paper is pale cream. In the sun the exposed areas visibly darken — cream to brown to a deep grey-brown — while whatever you laid down blocks the light and stays pale. Fixing clears the milky unexposed areas and lightens the whole image to its final warm brown, with crisp silhouettes.
| Symptom | Likely cause | Fix |
|---|---|---|
| Faint image | Underexposed, or weak silver coat | Expose longer; brush a more even silver layer; overexpose |
| Whole sheet browns before you’re ready | Fogged in stray light | Sensitize and handle in dim light until exposure |
| Image fades away after fixing | Over-fixed or over-washed a thin image | Overexpose more; fix just until the milkiness clears |
| Yellow-brown staining over time | Thiosulfate not washed out | Wash a full 30 minutes in running water |
The reactions
Silver chloride printed out directly by light:
\[\ce{AgCl ->[\text{UV/visible light}] Ag^0 + \tfrac{1}{2}Cl_2}\]
light reduces silver chloride to metallic silver (the brown image); the fixer then dissolves away the unexposed silver chloride so it can’t darken further.
The metallic silver (Ag⁰) appears as a warm brown to grey-brown colour depending on the particle size. Unlike modern photographic paper, there is no developer — the image forms entirely from light exposure.
The Science
The photolytic decomposition of silver chloride is not catalysed — light simply excites electrons in the crystal, freeing them to reduce Ag⁺ ions directly to metallic silver. Because no chemical amplification step is involved, the required exposure is enormous compared to developed silver processes. This is why salted paper requires minutes to hours of sunlight rather than the fractions of a second needed by modern film.
The warm brown colour of the final image comes from very fine silver particles that scatter short wavelengths and absorb others. Larger particles (produced by slower printing or lower silver concentrations) tend toward grey; finer particles lean toward warm red-brown. Gold toning converts some silver to gold metal, shifting the colour toward cool purple-brown and improving archival stability.
Sodium thiosulfate fixes the print by forming the soluble [Ag(S₂O₃)₂]³⁻ complex, which washes out completely in water. Insufficiently washed prints retain thiosulfate, which oxidises slowly over years to sulfur compounds that stain the silver image brown.
Questions to Explore
Why apply salt before silver nitrate, not mixed together? They react instantly to form silver chloride; the process needs it inside the fibres. What if you mixed them first?
Hint / answer
Mixed in solution they’d precipitate silver chloride as a clump you couldn’t coat evenly. Salting the paper first, then adding silver, forms the light-sensitive silver chloride right where it’s needed — dispersed through the paper fibres.
Why overexpose? The image lightens during fixing. Is the fixer changing the silver or washing some away?
Hint / answer
Fixing dissolves the pale, unexposed silver chloride that was adding some overall density, so the print lightens. It doesn’t remove the exposed metallic silver, but the loss of that background is why you print darker than you want the finished image.
What does “printing out” mean versus developing? Salted paper forms silver directly by light; film needs only a few atoms then amplifies ~10⁸×. What sets the minimum silver needed?
Hint / answer
Printing-out must make all the visible silver with light alone, so it needs a huge exposure. Developing needs only a tiny catalytic “latent image” of a few atoms, which the developer then amplifies chemically — so far less light (and silver from exposure) is required.
Why does thiosulfate fix the print? It turns unexposed silver chloride into a soluble complex. What happens if you skip fixing?
Hint / answer
Unfixed, the leftover light-sensitive silver chloride keeps darkening in any light until the whole sheet goes brown and the image is lost. Fixing removes it, so only the stable metallic-silver image remains and it no longer reacts to light.
Why does gold toning shift colour and improve permanence? Gold displaces silver in the image. Why is gold more durable?
Hint / answer
Gold is far less reactive than silver — it doesn’t tarnish with atmospheric sulfur or oxidize — so replacing silver atoms with gold armours the image against the very reactions that fade old prints. The finer gold particles also shift the tone toward purple-brown.
Going further
- Gold tone it. After fixing, immerse briefly in a dilute gold-chloride solution to shift the colour cooler and greatly extend the print’s life.
- Print a photograph. Contact-print an inkjet negative on acetate for a full tonal image rather than a silhouette.
- Next in the Redox track: develop film with kitchen chemistry in Caffenol Film Development.
Footnotes
Reduction — The gain of electrons by an atom, ion, or molecule; always paired with oxidation.↩︎
John Herschel — English polymath (1792–1871) who invented the cyanotype photographic process in 1842.↩︎