Photographic Chemistry Web Page

Early Photography-- through the Daguerreotype

Discovery of Light Sensitive Materials

• Found that chalk which had been treated with Silver Nitrate would turn dark purple when exposed to sun light. He did experiments to show the effect was caused by light and not by the heat of the sun.
• Actually made the first "photographic" image by cutting several letters out of a paper stencil, wrapping it around a beaker of treated chalk, and exposing it to light. When the stencil was removed, a dark image of the letters could be seen in the beaker.
• Of course, after a few hours of viewing, the surrounding chalk also became darkened and the image disappeared. It would be many years before someone discovered how to permanently fix such an image.
• Schultze's demonstration became widely known and actually was published as a suitable parlor trick.

• Only the blue end of the spectrum had much of an effect on the silver salt.
• Silver chloride was more sensitive to light than most other silver salts.
• Exposed silver chloride would turn dark and it would no longer dissolve in ammonia.
• (Ordinary silver chloride dissolved in ammonia readily.)
• Without realizing it, he had hit on the key to fixing photographic images but it would have to be rediscovered by a later generation.

• One critical observation was that sun light would cause some resins to harden and to lose their solubility in turpentine.

In the early 1800's several people worked on the problem of trying to produce images using the "camera obscura." Thomas Wedgewood and Sir Humphry Davy managed to produce a few weak images but were unable to produce a usable formula.

• Nicephore Niepce is often said to have the best claim as the inventor of photography.

• In 1813 Niepce, an ex army officer living in near retirement, became involved in printing by the recently developed process of lithography.
  • Since you are not probably not familiar with lithography, we will branch to that topic and then return to more photography.

Lithography-- A printing technique

• The word literally means Stone (litho-) Printing (-graphy)
• A slab of fine grained sandstone is used; the upper face is ground and polished to a flat smooth surface.
• This stone is relatively porous and it will absorb water based inks. If a piece of paper is laid on the inked stone, the ink will transfer to the paper.
• It is necessary to form an image on the stone to limit the absorption of ink to only those regions that should become inked.
  • One way is to draw on the stone with a wax pencil. The wax line prevents the absorption of ink and will result in white lines on the final print. Larger areas can be masked by painting them with a varnish which serves the same purpose.
  • One of the favorite varnishes of early lithographers was asphalt-- a tar like substance that's not much different from the asphalt (tar) used to page streets. The asphalt was dissolved in a solvent so it could be brushed; the solvent evaporated leaving the tar like protective coating.
• Once the stone had been prepared, it was inked and the ink was transferred to paper. The same stone could be used to make many copies of the same image.
  • Often several stones are used to produce a final image, each stone being used with a different colored ink.
    • In conventional lithography the stone is prepared by hand-- someone essentially draws the final image directly on the stone.
    • Lithographic stones can be cleaned and reused many times.

The word lithography is used now to refer to similar techniques even if the underlying material is not actually a stone slab. return

• Niepice was plagued by a problem-- he was a poor draftsman and was unable to produce acceptable quality drawings and his lithography was therefore not marketable. Printing generally required an artisan to copy an artist's drawing to the lithographic stone; the quality of the print depended on this person's drawing skills.
• Instead, he began to look for a way to transfer existing images to the lithographic stone without redrawing them. He recalled Senebier's early work with resins exposed to light.

• So he coated a stone with a thin coat of Bitumen of Judea, a form of asphalt. He then covered the stone with a picture drawn on thin paper. He exposed this sandwich to sunlight for what was probably several days. Then he washed the stone with a solvent (oil of lavender.) One could see a faint image of the original picture in the stone. When the stone was inked and a piece of paper was applied a copy of the original drawing was produced. (Not a very good image.)

• This was the earliest example of what we now call photolithography.
  • Although the materials have been refined, this technique it still used today.
  • Most newspapers are produced by a similar effect called photo-offset printing .
    • the underlying surface is a zinc metal sheet
      • a chemical mask is produced by light exposure and development
      • the mask is used to control the etching of the zinc.
      • the printing process involves ink that is picked up by the etched zinc areas.
• Still, this is a far cry from photography with a a camera.
• Niepice returned to the other side of the family business, silver work.

• It was long known that a fine frosty texture could be produced on silver by exposing it to iodine vapors. After exposure to light this textured area became dark and quite distinctive.
  • Some silver was treated by this manner, using wax to protect the areas what were not the be colored. Again, the process required an artist to produce the mask by drawing it into the wax layer.
  • Niepice made a natural jump-- he exposed a flat sheet of silver to iodine, producing a light sensitive surface. Patterns could be produced by exposing this to light through a stencil or a drawing on thin paper.
    • The big step was when he put this into a camera, aimed it out a window and recorded the image of the surrounding scenery. This photograph, taken in 1826, is the oldest example of photography in the modern sense. (The picture required many hours of exposure and you can actually observe shadows on both sides of some of the building features.)

• There were still several important steps before this became a usable technique. Niepce
  • 1. It was necessary to fix the image-- these photographs would vanish in a few days
  • 2. It was important to decrease the exposure time from hours to at least minutes.
  • 3. For many photographs it was important that a positive image was obtained-- dark objects should appear dark in the photograph.

• Until well into the 1930's it was generally accepted that the photograph was the same size as the film. A portable camera would produce a small picture. If you wanted a large photograph, you needed a very large camera.

• Niepice needed money to develop his inventions and he joined with an artist named Daguerre.
  • Niepce died within a few years and the final commercial form of the process occurred after his death.
  • The name of the product became, not surprisingly, a Daguerreotype.

• 1. It was discovered (1837) that soaking the finished photograph in a strong salt solution fixed the image.
  • This formed a soluble silver salt with the unexposed silver iodide, allowing it to be rinsed from the plate. The image was silver metal and it remained on the plate. Today we refer to this process as fixing.
  • 2. It was discovered (two years after Niepce's death) that exposing a weak image to mercury vapor caused the image to become much more intense. In fact, shorter exposures (which did not produce a visible images) could be used. Treating these plates with mercury vapor produced a very strong image. Today we refer to this process as developing.
• Actually the Daguerreotype had an interesting appearance. The underlying surface was a polished silver sheet. (Actually it was silver plate on brass to keep the cost down.)
  • The developed silver image was somewhat rough, black, and it scattered light better than the surrounding silver sheet.
• If you view a daguerreotype straight on, it is clearly a negative image of low contrast (gray on black)
• If you tilt the plate slightly and surround it with a black background, you see the color values reverse because of reflection. In this position, a positive image is observed. Daguerreotype were always sold in black velvet cases to create the effect of a positive image.

Getting your picture taken was still an ordeal. Exposure times in bright sunlight were 20-30 minutes or longer and movement blurred the image. There are few pictures of children from this period since they seldom stayed still long enough. Adults put up with clamps behind the head and other techniques to keep their pose. Landscapes and architectural details were much more cooperative.

Fox-Talbot and the Calotype

• About the same time an Englishman named Fox-Talbot approached the problem in a different manner.
• He coated thin paper with silver salts.
  • he soaked the paper in salt water
  • then treated the paper with silver nitrate
  • this formed silver chloride crystals in the paper fibers.
• Initially he covered these papers with opaque objects like fine lace or fern leaves
  • He then exposed these to light (for several hours)
  • He got nice images of the objects.
• He independently learned the art of fixing the image-- he used hyposulfite of sodium...
• He tried other silver salts and discovered that adding gallic acid to the mixture produced images much faster and he was able to expose images inside a camera.
  • Inspired by stories of Neipce/Daguerre he began exposures through a camera and made true photographs around 1835
• He soon discovered that treating the exposed picture with gallic acid solution had the same effect--clearly discovering the role of a chemical developer.
• Since his images were on thin paper, he could use them as samples and copy them as often as he wished by the same process. These new copies reversed the blacks / whites so these were positives.

• The Callo type--as he called it-- had several advantages over Daguerreotypes.
  • a. he could make multiple copies
  • b. these were clear positive images (when viewed at any angle)
  • c. paper was a cheaper material so larger images were not much more expensive (although they required bigger, more awkward cameras)
  • d. the images had better contrast, although they had some loss of fine detail due to the texture of the paper.

The Collodion Process

• Before Fox-Talbot's process could be a commercial success, Archer (1851) developed a technique for coating a glass plate with collodion (cellulose nitrate or gun cotton)

• This material was already in commercial use as the plastic "cellophane"
• This collodion layer was soaked in salt water and then in silver nitrate to produce a silver chloride light sensitive layer.
  • (Exposure to ammonia vapors made it even more sensitive, although the reasons were unknown)
• These plates were dried and then exposed in the camera.
  • The plates needed to be freshly prepared-- they lost sensitivity in a few hours.
  • The plates could be developed later in the day; gallic acid was still the developer of choice.
• The collodion plates had most of the advantages of the Fox Talbot process, but
  • they had better detail (no paper texture was involved)
  • the negatives were rigid and handled better
• Within a few years the collodion process surpassed the other technologies.
  • The Callo-type was never a commercial success and the Daguereotype was largely obsolete.
  • Most of the Mathew Brady photos of the Civil War were Collodion prints

The tin type

• If a collodion negative is prepared on a black enameled metal sheet, an effect similar to the Daguerreotype appeared. The shiny black area looks black. The dark silver image, being finely textures, scatters light and appears gray. Ultimately, the result is a positive image.
• These are one shot photos-- (you can't print through a metal plate)
• Around the time of the civil war these were the cheapest photos and were a popular family investment.
• Side light. One very important role of photographs was to produce images the dead, especially infants and youngsters. Remember, families generally had no images of their young children.