Photography

Chemistry 128

November 22, 2002

• We will examine a number of ways we use light to create images and objects
  • The most obvious is conventional photography
    • snap a picture, have the film processed, have prints made from the negatives
    • (We'll covert half of the lab into a darkoom-- you can use an enlarger and make a print.)
  • We will look at the granddaddy of office copiers
    • Cyanotype or blue prints were the standard copies from 1840's to the early 1960's
    • (You'll get a chance to make your own blueprint)
  • There are a number of important processes based on photoresists
    • photoresists are light sensitive materials that can selectively mask a surface
    • photoresists are big business- uses include
      • offset printing, silk screen processes, printed circuit boards, making microprocessors and other integrated circuits, producing electric shaver heads, and sandblasting decorations on glass surfaces.
    • (In lab you will use photoresists to make a printed circuit board, to make a delicate metal object by electroforming and to etch a pattern into glass.)

• We'll discuss and explore some other imaging and printing techniques, based more on physics than chemistry.
  • (Digital cameras, scanners, office copiers, Ink Jet and Laser printers.)
• We will demonstrate a process that makes solid 3 Dimensional objects
  • A laser playing on the surface of a liquid selectively forms solid polymer (plastic)
• We'll also look into some other aspects of photography. These are important processes but they are relatively difficult to set up in a lab like this.
  • (Color photography, Polaroid Instant film, making microfilms and holograms.)

1 . Cyanotype or Blue Print

• The key chemistry is that Fe(III) or Fe³⁺ ion can be reduced to Fe(II) or Fe²⁺ by light
• The Fe(II) will then react with Fe(CN)₆4- to form an insoluble blue salt
  • coating a paper with ferric ammonium citrate and ferricyanide sensitizes it to light
  • when exposed to light the paper slowly turns blue (blue green)
  • a negative or a printed page controls where this exposure occurs
  • the exposed paper is then treated with water to remove unreacted material

• Cyanotype, Experimental Details
  • We have prepared a sensitizing solution
  • You can brush it on the surface of heavy paper and the coated paper to dry (in the dark.)
    • (Since it takes 1-3 hr. to dry you can't use the paper you prepare)
    • (We will have dried sensitized paper available for your use.)

• Take a sample of the treated paper.
  • (Avoid any unnecessary exposure to room lights)
• Select an image: a negative or a drawing or text on translucent paper.
• Place the negative on the Ultraviolet contact printer
• Cover the negative with the treated paper, yellow side facing the light
• Close the printer and turn on the light; 5 minutes is a typical exposure
• Remove and briefly inspect the paper (you can see the image)
• Place the paper in a tray of running water and observe the colors
  • You can intensity the color by placing the print in dilute hydrogen peroxide

2. Black and White Photography

• The light sensitive material in almost all photographic materials is silver halide
  • Silver chloride, bromide or iodide (we will mostly refer to the chloride)
• Since silver halides are insoluble, we are discussing small crystals of AgCl.
  • They are usually mixed with gelatin and coated on clear plastic sheets ( for film) or on paper (for making prints.)
• The key reaction is the reduction of Ag⁺ ion by light
  • the AgCl crystal is white; AgI is slightly yellow
  • the deposited silver atoms appear black

• One of the earliest forms of photography was the Callotype--
  • Silver chloride crystals are deposited on paper to form a light sensitive material
• We will provide you with a piece of paper that was treated with 0.3 M silver nitrate and allowed to dry.
  • Perform the next steps in reduced light and shield the paper as well as you can
  • Handle with forceps or rubber gloves -- silver salts will stain your fingers
  • Immerse the treated paper in a tray of 1M Sodium Chloride solution for about 15 seconds
    • the reaction forms AgCl and some of the precipitate will adhere to the paper
    • let the paper drain, place it on a paper towel and allow it to dry in the dark
• (We may give you a dry senstized sheet at this stage)
• Place a suitable negative or transparency on the UV contact printer
  • Cover with the treated paper, sensitive side down
  • Expose for 5 minutes and inspect
    • you should see a brown or black image
• • You need to treat this print material to remove the remaining AgCl
    • Otherwise it will continue to turn dark as the print is exposed to room lights.
  • You could immerse the paper in 6M NH₄OH
    • You saw in the inorg. qual scheme that AgCl dissolves in ammonia (as Ag(NH₃)₄+)
    • Photographers favor a similar treatment with thiosulfate ion (a fixer)
• • Place your print in the fixer bath for about 2 minutes, then rinse it in gently flowing water for a minute. (Proper washing takes 30 minutes but we won't be worried if this print turns yellow in a few years.)

Exposure in a Camera

Sensitized paper could also be place inside a camera and used to photograph a still life scene. This isn't practical for photographing people or animals, since exposure of several hours of sunlight is required for even a faint image.

Practical photographs required another step-- a chemical developer. We can kick up the response of silver halide based film by using a developer. Exposure is then typically 1/100 th of as second. Such film needs to be handled (processed) in total darkness. Our laboratory is not a good enough darkroom to attempt developing film The paper used for making prints responds only to blue light and is much less light sensitive than the films. We won't get great results in our mediocre darkroom, but the process will work reasonably well.

3. Making an Enlargement using a Chemical Developer

Experimental:

• Select a 35 mm negative
• Go to the hood containing the enlarger and load the negative
• View the image, enlarge as desired, focus and turn off the enlarger lamp
• Take one sheet of paper from the box (and close the box)
• Place the paper in the easel shiny side up and turn on the enlarger
  • Exposure times vary (we'll give you advice)

• Development: Slide the sheet into the developer bath in the second hood
  • A red safelight lets you observe the process
  • the image will emerge and develop in 1-2 minutes
  • After the picture emerges, remove the print from the developer and place it in the stop bath. This is dilute acetic acid to neutralize bases and stop development.
  • The safelight is deceptive, let the picture get a bit darker than you think appropriate.
  • After about 30 second, transfer the print to the fixer tray for 5 minutes Then remove and rinse in running water for about 5 minutes.
  • blot the picture dry and set it somewhere to finish drying

• The developer is chemical reducing agent
  • it chemically converts Ag⁺ (in AgCl) to Ag metal
• A good developer would be a disaster. If it converts all AgCl to Ag, the picture is totally black
  • We select a mediocre (slow acting ) developer
    • one that won't darken the silver salts much in an hour.
• The addition of a catalyst can speed up the reaction
  • A great catalyst is silver metal
  • crystals that were exposed to light carry their own catalyst
  • These darkened in 2-4 minutes
• So developing a film or print transforms latent image to a real image.

4. Chemically Reversing a Negative or a Print

Experimental

• Prepare an enlargement as in part 3 but DO NOT USE THE FIXER
• Take the print (from the stop bath) and place it in a bleach (Ferricyanide)
  • the bleach oxidizes the Ag-metal and forms silver ferricyanide
  • the remaining AgCl is not affected by the bleach
• Remove and rinse the print and take it to a bright light, exposing it for about a minute
  • this begins to darken the remaining silver chloride
• Put the print back into the developer
  • this changes the last of the AgCl to black silver
• Put the print into the fixer now
  • this dissolves the silver ferricyanide
• Rinse as in part 3
• This print will be a negative with the same bright and dark spots as the original negative
• (See the web notes for some historical applications)

Color Photography (and why we won't try it here)

• A typical color film has three layers
  • each layer is a light sensitive layer of silver chloride
  • each layer is treated in a way that makes it sensitive only to selected light
    • one region is red sensitive, one is green sensitive and one is blue sensitive
    • (your eye also has three color sensors covering the same colors)
  • each layer is developed normally
    • where the layer received light of the right wavelength, it darkened
    • the developer causes exposed areas to darken
  • the focus now shifts to the remaining silver halide
    • a color developer reduces the remaining AgCl to Ag
    • but-- the oxidized developer reacts with another compound in that layer
    • this reaction produces a colored product
      • the color is different in each of the three layers
      • (in color slides, red light forms red dye after exposure to red light)
      • finally, the last step is to oxidize and then dissolve all the silver that was produced.

• Why don't we make color prints in lab?
  • we need total darkness since color paper must also be red sensitive
  • we need several additional solutions and the process might take 15 minutes
  • the temperature and timing need to be well controlled to keep all three colors developing at the same rate
  • a color enlarger is more complex since we need to be to modify color as well as exposure time.

5. Photoresists and Printed Circuit Boards

• We will use a positive acting photoresist.
• This is a thin coating that is light sensitive
  • the coating is a mixture of two small organic molecules
  • if they are exposed to a weak base, they react to form a tough plastic coating
• The coating is sensitive to light
  • one of the two compounds is destroyed by UV light
  • if this happens, no protective film forms
• The coating is exposed through a transparent image
  • where light passes through the transparency, no surface film forms
  • where the transparency is dark, the light is blocked and we get a tough coating
• This allows us to selectively expose or protect a surface.
  • the protective coating is often called a mask
• A Printed Circuit board begins as a thin plastic sheet (electrically insulating)
• A thin layer of copper foil is glued to one side of the board
  • most modern boards are two sided or have extra layers
• The copper layer is covered with the photoresist
  • you can buy and apply your own photoresists; we buy boards already coated
• A transparency is used to form a protective mask on the copper surface
• A strong oxidizing bath is used to dissolve the unprotected copper.
• The remaining copper pattern provides the connectors, soldering locations and the copper connections between the various electronic devices on the board.

• Obtain a small (1" x 1") sample of PC board stock
  • Select a suitable pattern (high contrast)
  • Exposure will be made using a set of mercury lamps (UV)
  • place the PC board stock on the tray
    • cover it with the pattern
    • cove this with a sheet of glass
    • expose for 5 minutes (share exposure time with others)

• now, remove and inspect (you can often see a faint image)
  • peel off the thin protective plastic coating
  • develop in a tray of dilute sodium carbonate for 1.5-2 minutes
  • complete the development with a spray of tap water

• The board is immersed in warm etching solution
  • ammonium persulfate or ferric chloride
  • regular stirring of air bubbles are needed
  • in about 15-230 minutes, the exposed copper is etched away.

6. Photoresists and Sandblasting

• Here we will use a negative photoresist
• This is a coating of film of a polymer (plastic film)
  • this is a gelatin-like polymer, easily softened by warm water
• Ultraviolet light causes the molecules to cross-link
  • form bonds between molecules
• The cross linked material is much tougher
  • we rely on the tough material to protect a glass surface
  • we will sandblast the exposed regions to form a pattern in the glass

• Obtain a sample of the light sensitive film (avoid serious light exposure)
• Select a negative of comparable size
• place the negative over the shiny side of the film
  • place in a suitable UV light source and expose for 3-5 minutes
• Remove the negative and inspect the film
  • a faint image of the original image may be visible
• Take the film to a sink and wash it in a vigorous stream of warm water
• The final product is basically a stencil carried on a thin plastic sheet

• Allow the film to dry
• Using a water soluble glue, glue it to a glass surface
  • Shiny side up; When dry, peel off the thin plastic sheet used to carry the stencil.
• Take the coated glass to the sandblasting cabinet (see caution below)
  • a compressed air gun drives abrasive particles
  • the particles slowly etch or frost the exposed glass
  • the photochemical mask determines where this occurs
    • afterwards, the mask can be removed with hot water

• You will be given detailed directions and safety equipment
• Sandblasting equipment is dangerous unless used properly
• The dust is a health hazard if inhaled
• The sandblast stream will quickly damage unprotected skin

7. Photoresist Masks and Electroforming

• Using methods similar to those above, we will treat a stainless steel sheet
  • a photoresist coating is exposed to light and a mask pattern forms
• In this case, we then electroplate the product with copper metal
  • We place the sheet in a solution of copper sulfate
  • We connect it to the negative terminal of a battery or power supply
  • The add a second electrode of pure copper, connected to the positive terminal
• At the negative terminal (cathode) copper ion is reduced and copper metal plates out
  • of course, copper only forms where the mask exposes the metal surface
  • at the positive terminal some of the copper metal is oxidized
• It will take about 30 minutes to deposit a film thick enough for our purposes
• ...
• We can gently lift the copper from the surface (using a razor blade and gently pressure)
  • why? :
  • Stainless steel quickly develops a thin oxide layer
    • This oxide layer is like a thin, tough glass coating
    • that's why the iron in stainless steel doesn't rust
      • The layer is thin enough for the electrical current to pass
    • The oxide layer keeps copper away from the underlying metal
    • Therefore the copper is easily removed

• We say that the copper object has been electroformed
  • Electroforming is an excellent way to manufacture thin, intricate metal objects

• We will provide stainless steel sheets with photoresist patterns
• Connect the sheet to the negative terminal of the power supply
  • Immerse this in a copper sulfate solution
• Insert a copper wire connected to the positive terminal
  • Adjust to an electrical current of about 250 ma for about an hour
• Remove and rinse
• Gently slide the blade of a single edge razor blade under the edge of the pattern
• Lift the electroform copper object.

Other Examples we will discuss or demonstrate, mostly in the show and tell mode, not as experiments

• Xerography (name means dry + printing)
  • the operating principle of almost all office copiers and all laser printers
  • rely on a polymer that becomes electrically conducting when exposed to light
• Ink Jet and Bubble Jet Printers
  • literally squirt tiny droplets of ink at the paper
  • the print cartridges have tiny intricate patterns (often made using photoresists)
  • a microscope can reveal the actual dots and the resolution
• Instant Photography (Polaroid- TM)
  • the trick of building a complete darkroom inside the film package
  • this is really standard silver chloride film with developers, fixers and some special steps
  • Polaroid and color film under the microscope (can we see the separate layers?)
• Scanners and Digital Cameras
  • sensors are similar those we used in our diode array spectrometers
  • is photographic film a dying form? (pros and cons of film vs. digital imaging)
• Photolithography and Integrated Circuits
  • take a good look with a microscope
  • how do you make such tiny things reliably?
• Photolithography and the Art of Printing
• Historical Photographic Processes (1840-1890)
  • Daguerreotype
  • Civil War vintage photographs (colloidin and albumen plates)
    • The oldest form of color photography
      • Several methods were developed based on three photographs with filters
      • Autochrome plates, developed by the Lumiere brothers, 1904
      • revisited by Polaroid in the 1970's for "instant slides and 8 mm movies"