CHEMICAL PROCESS

CHEMICAL PROCESS: 

CHEMICAL PROCESS By WILFREDO R. BERALDE

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Basically, all the images recorded on the sensitized materials by the action of light are INVISIBLE. These latent images are still temporary and can be ruined when accidentally exposed to lights.

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To make the latent image visible and permanent, chemical processing is necessary. This processing is known as the DEVELOPMENT PROCESS of the films that makes the latent image visible and permanent. The developed image is the reverse of the original objects photographed and called NEGATIVE.

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There are several factors to be considered in the development process of the sensitized materials. DENSITY is the degree of darkness of the images developed. It can either be too dark or too light, depending upon the amount of the metallic silver formed. On the other hand, CONTRAST is the difference between one tone and another, which is so apparent when the density of the image is viewed, contrast affect the tonal value of the photograph

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During the development process of the sensitized materials, the silver halide where the latent is recorded will be developed and turned to metallic silver that formed the image on the negative. The details of the images developed will determine whether the sensitized materials are accurately processed or not.

a. CHEMICAL BASIS: 

a. CHEMICAL BASIS Light-sensitive materials that undergo physical and chemical changes in reaction to light provide the technological basis of all photography.

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A complex photo-technology has been created to use such changes to record images formed by light – in other words, to make photographs. Modern photo-technology includes a wide variety of materials and methods for improving the sensitivity and other qualities of light-sensitive materials, for making the images permanent, and for reproducing and modifying the images when they are produced.

b. FILM PROCESSING: 

b. FILM PROCESSING Behind nearly every successful photographic print is a properly exposed and correctly processed negative. The processing of any film, black and white, and color is a simple sequence which, IF FOLLOWED CORRECTLY will always lead to predictable, and consequently successful results. And yet, there is still considerable scope for varied and individual treatments.

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The fundamental approaches to processing black and white and color films are very similar. There are one or two extra stages in the process of colored films, and the timing, temperature control traditionally needed to be much more precise than for black and white. However, recent developments in the filed of color processing and printing have simplified matters considerably and it is now almost as simple to work with color materials as it always has been with black and white.

c. Black and White Film Processing: 

c. Black and White Film Processing There are four basic steps to produce a black and white negative from exposed film, they are: DEVELOPMENT STOP BATH FIXING WASHING AND DRYING

Development : 

Development The purpose of development is to convert the latent image in the exposed film to a real image. When the picture is taken, the silver halide crystals exposed to light change, but the change is invisible. Development converts these changed crystals to black silver, eventually bringing the dormant image to life. The way in which the image develops depends on the fine balance between the intensity of development (governed by developer strength and temperature) and the film.

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An amateur police photographer may ask the photo supplier of the best camera, film, and developer suitable for black and white development. The most common developer is the Universal developer whose instruction for using is shown in its container, like the mixing of the working solution and the ideal temperature for both film and printing process.

Stop Bath: 

Stop Bath For complete control ovr development time, many photographers use a “stop bath” after development to cut short the process abruptly. The stop bath neutralizes the developer and also helps prevent the contamination of the third bath, the fixer, helping to prolong its active life.

Fixing : 

Fixing Even development, the image is not actually visible. This is because the unexposed silver halides are still present in the emulsion, which now has an opaque milky appearance. The fixing baths loosens these unexposed halides and makes them soluble in water so that they can be washed away to leave the visible silver image.

Washing and Drying: 

Washing and Drying The final washing of the film removes any traces of unwanted halides and residual chemicals. Most photographers prefer to add a small quantity of wetting agent, at this stage to reduce drying marks on the film.

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Films are usually hung up to dry in a dust free place, sometimes using warm air to speed up the process. A weighted clip on the bottom end of the film keeps it straight during drying. Without a weight, the film will curl and this can cause problems at the printing stage.

d. Factors Affecting Development: 

d. Factors Affecting Development Concentration of strength of the working solution Temperature of the chemical involved in the development process Time element required Agitation of the sensitized materials during the development process.

e. Sequence of Developing Film: 

e. Sequence of Developing Film LOADING THE TANK. A stainless steel or plastic tank has a spiral reel where the film is being carefully rolled outward, if possible without touching each other. Winding the film on this kind of reel takes some practice. The investigator should try it out with a spoiled film in the light before making an actual winding of the film inside the darkroom. It takes time to practice and familiarize this kind of work, but a little patience will make one a good photographer someday.

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DEVELOPMENT. The temperature of the developing or working solution must be tested with the thermometer. The time of development is properly observed (if possible written and posted at the front of the photographer for him to be familiarized with) and the timer is readied but not started. When the developer is poured in at the hole of the developing tank, the clock is started as quickly as possible.

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The cap is put on the pouring hole and the tank is then turned upside down to wet the film. The tank is now brought back and slammed down sharply on the table. This is the only way to loosen the tiny bubbles that may be formed during the shaking of the tank. If these bubbles will not loosen, the portion of the film where said bubbles formed will not be affected by the developer and therefore will not be developed.

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AGITATION. The chemicals in the developer that are close to the film get used up quickly. It is necessary to mix the solution from time to time to bring fresh solution in contact. This is called agitation, but it is not done all the time as the word might suggest. It is done at even, regular intervals, usually every 30 seconds (where 5 seconds of this is reserved for tapping the top of the tank to let bubbles goes up) until the developing time ends.

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RINSING. About 10 or 15 seconds before the end of the correct time, the developer is poured off into a container (do not mix it with the unused developing solution) and the tank is filled with water (or stop bath also known as hardener) – if this is used it is allowed to act for the required time and then poured off and thrown away. The tank is rinsed once with pure water before pouring in the fixing solution and this must be done twice or thrice, then fixing solution is poured in.

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FIXING AND WASHING. The hypo is allowed to act for not more than five minutes. It is poured off and the tank can now be opened. The tank is put under faucet of running water and rinsed for five minutes.

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Hypo eliminator may be used and allowed to act for five minutes before rinsing the film thoroughly within 10 minutes. Where there is a scarcity of water, the tank is used by filling and emptying it five times while the film is still inside. The water is allowed to stand for two minutes. Before drying, a wetting agent may be applied so that drying is faster, no heating is required in drying the negatives and avoid dust at this stage.

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DRYING. The film should be allowed to dry naturally and thoroughly in a place free from dust.

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MARKING AND FILING. Keeping track of a lot of negatives can be a problem unless some system is used. Different police departments have policies about where to store films and how to mark them. Negatives and prints are not stored as one. They are stored separately and are given an assigned numbers for easier identification.

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Single sheets of cut films are numbered with the system adopted by a particular police station. If films are cut, each group of films taken at the same time on the same case get an “identification number.” All the negatives from one group or roll are placed in one filing envelope. Some police photographers get a proof of sheet by making a contact print on an 8 x 10 enlarging paper of all the films before storing it after making a general statement for evidential purpose.

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Others opt to use the Data Card. A data card is prepared for each photograph taken. All information should be recorded to allow proper interpretation of the photograph and dispel any questions of improper techniques or processing procedures. The completed data card should be filed with negative. Certain information should be furnished on the back of each print to identify and clarify it. A data outline may be stamped on the reverse side of the print and the appropriate information entered upon completion.

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Whatever scheme applied in filing and marking, care should be exercised in writing on the reverse side of the prints so that the emulsion of the print that was developed is not damaged.

f. Enlarging and Printing: 

f. Enlarging and Printing Black and white prints are essentially photographs made in a darkroom, and the process has many similarities to taking a photograph in a camera and processing the film. The negative is projected unto light sensitive printing paper by an enlarger. The image is then focused, exposure is calculated, and the paper is exposed. The exposed paper, bearing a latent image, is then developed to turn the exposed silver halides in the paper emulsion to black, forming a positive image.

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Any unexposed silver halides are dissolved in a fixing bath, leaving a permanent representation of the original subject in tones of gray ranging from black through white. Because the whole darkroom is normally the camera (though special enclosed enlarger do exists) the process must be carried out in semi-darkness.

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An ENLARGER is used for this purpose. It looks like a projector whose function is to project the miniature negative image onto a larger sheet of printing paper to produce a reasonable viewable image. This is done by placing a selected (film) negative to be enlarged in the negative holder and projects it on a clean white sheet paper until the desired size and contrast is achieved. Use some measuring instrument to mark the position of the paper used because this is the very place of the printing paper.

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Under a safelight, expose the negative using the enlarger and develop it using the same working solution as in developing the film. But the time elements for enlarging and printing is quite shorter than the required time for processing film. As a common practice most of the investigators and photographers make a series of sample ranging from 90 seconds, 2 minutes, or even higher than this, then compared the results of the photographs before the final copy of enlargement is processed.

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In this process, the police photographer will use developing trays instead of developing tank.

Things Needed for Darkroom: 

Things Needed for Darkroom A table or bench large enough for an enlarger and trays. Plastic bucket for mixing chemicals Photographic thermometer Developing tanks and reel Plastic bottles Large plastic funnel Safelight for printing and enlarging

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Clock or interval timer Developing trays Contact printer Graduated cylinder Film clips/plastic clothespin Photographic enlarger Miscellaneous supplies Water for washing