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Laboratory Preparation .. Chemical Solutions for Color Processing .. Selection and Use of Materials .. Book of Photography

Selection and Use of Materials
Laboratory Preparation

- Chemical Solutions for Color Processing:

The processing of colored emulsions generally follows the same method as black and white emulsions, but it is complicated by the necessity of forming an image composed of colorants while the common silver image is absent. The processing times, temperatures required, and consideration of the formulas are more precise
than black and white.

- Development Solutions for Color Negatives:

The developer is usually derived from paraphenylenediamine. It converts the silver halogen to the imprinted one while the secondary developing materials act to form a color image - yellow-red-cyan - in each of the three layers of film. All color negatives today are masked, meaning that yellow and pink pigments are formed at the same time as the color development in the unimprinted parts of the red-red and cyan layers. This principle greatly improves the clarity of the colors recorded by the film.

- Developing solutions for reversible color films:

The first development forms a classic black and white negative. The silver halogens are then blocked - chemically or by exposure to light - and a second color development gives a combined silver negative with a positive image composed of the colorants. The silver image is then bleached and fixed to remove the unhelpful silver halogens. The treatment is usually finished with a solidifying bath.

- Medium baths, hardeners and fixatives:

The light-sensitive mixture in color films is softer than the mixture in black and white films, although it is hardened (solidified) during processing. Sometimes, depending on the method, a starting bath or hardening medium is used to prevent excessive swelling of the light-sensitive mixture, especially when the treatment is made at high temperatures. Hardening or solidifying sometimes uses formaldehyde (a toxic substance) or its derivatives and alum, the usual hardening agent. Treatment with gormol delays desaturation and solidification of colors: it is usually used at the end of the treatment.

- Bleaching baths:

Unwanted silver images must be removed to leave room for images formed solely from the pigments. Potassium ferric cyanide is the most popular bleaching bath formant with bromine ions allowing the formation of silver halogens which will be removed for fixation. The modern equivalent is ferric secestrin which is more expensive and less odorous.

- Color fixatives:

They are very similar to the fixatives used for black and white but their pH must be controlled.

- Bleaching - Fixing:

To reduce the number of treatment solutions and to save time, the bleaching and fixing solutions are often mixed in one solution. This requires additional chemicals to counteract the formation of deposits. This combined solution or (plex) is more expensive than the two separate solutions.

- Mixing and using chemicals:

Treatment baths can be prepared from individual compounds or from pre-prepared powders or concentrated liquids.

- Equipment needed to prepare baths:

The presence of hot water is indispensable. A graduated basin must be present and plastic is less susceptible to damage than glass. A capacity of 600 ml is sufficient but a capacity of 1000 - 2000 ml is sometimes required.

A 25 ml tube is also required for concentrated liquids. Never prepare chemical solutions in a vessel made of aluminum, copper, tin or galvanized iron. Glass, ceramic (porcelain) or plastic are best. Stainless steel can be used for all baths except the bleach bath.

A good thermometer is also indispensable but do not use it to move the baths as it can break. Use a stirrer made of glass or plastic instead. A plastic funnel is also necessary.

- Preparing the dips from the separated compounds:

When you prepare a reagent from individual compounds, you must dissolve these compounds in a very precise proportion. For example, genol does not dissolve in a solution of sodium sulphate: therefore, it must be dissolved in water before adding the sulphate. Start by dissolving the materials in three-quarters of the required volume of solution water, for example 750 milliliters. Make one liter of solution. Most materials dissolve more easily in warm water at a temperature that the hand can tolerate. There is an exception for soda and caustic potash, which release a large amount of heat when they dissolve, so it is preferable to dissolve them in cold water.

If the reagent is made from prepared and ready-made materials, carefully read the instructions given by the manufacturer. There are usually two envelopes of materials because the easily crushed manifestations and basic materials cannot be preserved if they are in contact. The contents of the smaller envelope must usually be completely dissolved before dissolving the contents of the second envelope. When all the materials have dissolved, add the amount of cold water needed to make the required volume.

Solutions must be placed in tightly closed and secured vials. Opaque vials provide better preservation of solutions unless they are kept out of the light. If the vial is filled to the brim, the reagent is better protected from atmospheric oxidation. Glass or plastic vials can be used, but with a plastic stopper, as cork and metal are not suitable.

If you are thinking of placing the vials in a water bath (to give them the necessary heat), label them high on the vial and label them with insoluble ink. On the label, the nature of the solution and the date of preparation must be stated: If the solution has been reused, indicate each time the dates of use, taking into account that this solution will not be exhausted with each use. For example, 600 ml of reagent D76 can produce four or five films, but the treatment time must be increased by 10% after each film. For pre-prepared baths, it is best to follow the manufacturer's instructions carefully.

- Preparing baths from concentrated solutions:

The method is different when working with concentrated solutions, as the bath is usually only used once and is thrown away after each development.

First measure the amount of water needed after it has been heated to the required temperature for treatment, then add the specified amount of concentrated developer. This method is much faster and avoids the risks associated with reusing solutions, but it is more expensive.

Rapid fixers are usually sold in concentrated liquid form, but (unlike developer) the bath can be reused as a regular fixer. There is not much difference in price between powder fixer and liquid quick fixer.

- General instructions:

Extreme care is required to prepare color development solutions: proportions, durations and temperatures must be strictly observed. Black and white reagents also require careful preparation. For other solutions, a slight error can be accepted, but carelessness cannot be accepted.

Only prepare one solution at a time and clean the tools carefully after each preparation. The developer oxidizes quickly in air and gives brown spots. Most of the materials used in photography are harmless. Toxic and hazardous materials are referred to according to the applicable law. In any case, it is better to be careful.

- Strengthening:

Strengthening is a chemical process that allows the density of the negative image to increase. It is not necessary at all for the negative image to be displayed and displayed well. Do not attempt to chemically treat negatives if you can draw this image on more contrasting paper. In general, do the best possible drawing before attempting to strengthen.

- Strengthening with chromium:

Solution A: Potassium bichromate 50 grams and enough water to make one liter.

Solution B: Hydrochloric acid 100 ml, water enough to make one litre.

Take one part of A diluted in six parts of water. Add 1/4 to 2 times the amount of A (before dilution).

For example, take 100 ml of A and add water to it to make 700 ml, then add to this solution 25 to 200 ml of B. The weaker the concentration of the acid, the more important the strengthening is, but the action remains slower. When you bleach the negative completely, the image becomes light yellow with no traces of black. Check this by looking at the negative from the substrate side.

Wash until the yellowish tint disappears, then expose the film to diffused daylight - do not put it in the sun - or under strong artificial light before you dip it in a developer to blacken the image. Take a developer that contains a little sulfate, i.e. a paper developer, not a developer with small particles. Wash the reinforced negative film and dry it according to the usual method.

There are other formulas for strengthening, some of which are ready to use.

- Multipliers:

The multiplier reduces the amount of metallic silver that makes up the negative or positive image. The surface multiplier is used to reproduce graphs or other handwritten documents. The local multiplier, which allows working on a limited area of ​​the image on paper, is also important. It is not necessary to use the multiplier to reduce the contrast of a negative image: it is sufficient to draw on smoother paper.

- Farmer's multiplier:

Sodium hyposulfite solution A: 100 grams and enough water to make one litre.

Potassium ferric cyanide solution B: 20 ​​grams and enough water to make one litre. Take 100 milliliters of or 20 milliliters of B and 200 milliliters of water. This solution cannot be stored. Avoid contact with iron which causes Prussian blue to form, especially if you use (for applying the thinner to a local part) a small brush with a metal collar. After thinning, fix and wash with water.

- Iodine-Thiori thinner:

It is useful for local thinning of images on paper.

Solution A, 10 ml water.

Potassium iodide: 6 grams
Iodine: 10 grams dissolved in 100 ml methyl alcohol or isopropanol.

Solution B, Thiori 10 grams
Water, 100 ml
Mix two parts B with one part Fix. Without washing in water.

-Paper morphing:

Images with morphing should appear completely. The most vivid morphing degrees are obtained with iron metallic cyanide but these colors cannot match the colors usually obtained on colored paper.

We give some suggestions on pages 202-203.

- Color processing formulas:

Processing color films is more complicated than processing black and white films. The safest way to succeed in special processing is to use the processing kits suggested by manufacturers or factories specialized in this field. However, most of the individual compounds are not available in the market and on the other hand there are some compounds that are dangerous to work with in concentrated form.