A Chemistry 20PresentationBy Joelene Gavronsky
Made of what?Traditional Photographic film is made of one of three kinds of plastic: Polyester, Nitrocellulose, or Cellulose acetate.That plastic is then coated with an emulsion that contains silver halide salts and gelatin. Silver halide salts are the following chemicals: AgBr, AgCl, AgL, AgF, AgF2, and Ag2F
How does that make a picture?The various crystal sizes of the coating on the plastic determine the sensitivity, contrast and resolution of the film.When the coating is exposed to bright light the various chemicals form an invisible image on the plastic.You then need to use a few chemical processes to create the visible image: the photo.
What are the differences between black and white and colour film?In black and white film there is only one layer of silver salts. When developed, the silver salts change to metallic silver, which appear as the dark part of the negative film.In colour, there are at least three layers. Dyes in the layers make the crystals sensitive to specific colours. The blue layer is normally first, followed by the green layer, and then the red layer. Just like the black and white film, the silver salts are changed to metallic silver, but this time the combine with colour couplers to produce the colours. Newer films have as many as twelve layers with over twenty chemicals in each.
How are black and white negatives made?First the film is soaked in water(H2O) to swell the gelatin layer.Next the metallic silver is produced using a few different chemicals:Metol (C7H10No)2SO4Phenidone C9H10N2ODimezone C11H14N2OHydroquinone C6H4(OH)2These are mixed with an alkaline agent like:Sodium carbonate Na2CO3Borax Na2B4O7-10H2OSodium Hydroxide - NaOH
How are black and white negatives made?Throughout the whole process of developing, the temperature is monitored to be sure it goes well. The time it is in the chemicals is crucial too, so that is closely monitored as well.Once the chemicals have done their job, the next step in creating the negative is to use a stop bath. This stops the process from continuing.There are two main kinds of stop baths:The first halts all development. The negative is put in either acetic acid (C2H4O2) or citric acid (C6H8O7), which neutralizes the chemicals.The second is simply a rinse with water(H2O), that takes off the chemicals. However, this does not stop the developing completely. It will continue slightly for an indefinite amount of time.
How are black and white negatives made?The next step is the fixer, which stabilizes the image. This removes any remaining silver salts(but leaves the metallic silver), and it is necessary because if left in the silver salts would darken and fog the image.There are two common salts used for this process:Sodium thiosulfate Na2S2O3Ammonium thiosulfate (NH4)2S2O3Now the film is washed with clean water(H2O), taking out any remaining chemical from the fixer. If the fixer is allowed to stay in, it will stain, fade, and discolour the image. Sometimes, sodium sulfite(Na2SO3) is used to reduce time the washing takes and get it out more completely.
How are black and white negatives made?The film may now be rinsed in a surfactant(a wetting agent) to help it dry right. This is especially needed when hard water is used. With very hard water, the film may need to be rinsed in distilled water first.Now the film is dried, cut and put into protective sleeves.
What about a positive?The process is the same until the stop bath.After the stop bath, the film is bleached, removing the image from the film and leaving the silver salts. This step is also what changes it from a negative to a positive.Fogging is the next step, either by chemicals or light. This deteriorates the image.The silver salts are then developed.Then it is the exact same as the negative again, starting with the fixing.
And colour?Colour is very similar to black and white. The main difference is the film that is used, which was discussed earlier.First there is the developer, with one thing changed: dye couplers are added which bring out the colours.Then it is bleached, to convert the silver image to the silver halides.Then there's the fixer, followed by the cutting and drying.
What happens afterwards?With the fixer a lot of silver comes off of the film. Oftentimes, the silver will be collected to be sold from the fixer water. There are many processes to do this.The most common one is simple, called Metallic replacement, consisting of a bucket and steel wool. The iron in the steel wool reacts with the silver thiosulfate, causing the silver to sit at the bottom like a sludge.Galvanic Plating consists of a copper0clad iron screen. An electrical charge is sent through the solution and the silver becomes stuck to the screen.Electrolytic Plating has two electrodes in the fixer solution. Silver is deposited on the Cathode in almost pure form.The last main one is precipitation, which uses sodium borohydride (NaBH4), sodium dithionate (Na2S2O6), or similar chemicals.
And the film?The film, too, contains silver that can be extracted.The traditional method involves burning the film and taking the silver from the ashes through a process called pyrometallurgy.Another way is using an alkaline solution.A third begins by grinding the film into small pieces, adding an enzyme to decompose the gelatin, adding sodium borohydride (NaBH4) to separate the silver from silver halide, coagulating metallic silver so it settles into a sludge, and lastly roasting this sludge so the organics are removed.