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Silver Dye-Bleach Photography - Basic Color Photographic Principles, History of Silver-Dye Bleach

materials dyes cibachrome material

MATTHIAS SCHELLENBERG, (ING. CHEM. ETHZ) RETIRED
Ilford Imaging Switzerland GmbH

ERNST RIOLO
Ilford Imaging Switzerland GmbH

HARTMUT BLAUE
Ilford Imaging Switzerland GmbH

Basic Color Photographic Principles

Color photographic systems are based in general on the subtractive principle, where the image dyes absorb the complementary color of the white light.

Almost all of the commercially available color films and papers are based on chromogenic processes. The image dyes are formed during processing were needed to reproduce the original picture as a negative or a positive image. The color developer is oxidized by exposed silver halide and reacts with color couplers, which are incorporated in the layers, under formation of dyes. The complex chemistry of chromogenic image formation is described in Chemistry of Developers and the Development Processes.

Silver dye-bleach materials use a chromolytic process to give a color reproduction of the original image. The photographic material contains the full concentration of the image dyes cyan, magenta, and yellow required to reproduce black. The dyes that are not needed for the image are bleached during processing. This procedure leads to a positive picture, because in areas with white exposure all dyes are bleached and the layer becomes transparent, areas with no exposure maintain the full amount of all original dyes and are consequently black.

History of Silver-Dye Bleach

The first suggestion of a silver-dye bleach process was made in 1905 by Schinzel. He destroyed the dyes by oxidation reactions using hydrogen peroxide. The image was distributed and silver metal catalyzed the reaction. The reductive silver-dye bleaching was invented and patented by Christensen in 1918. Strong reducing agents such as sodium dithionite or stannous chloride were used. This reductive bleaching was also catalyzed by silver metal formed in a preceding development. No commercial product resulted from these early inventions.

Bela Gaspar developed the first useful silver-dye bleach system in 1930. He received a number of patents from his process and produced positive print materials that were sold for some time under the name Gaspar Color. Gaspar color materials contained water-soluble dyes in three layers with appropriately sensitized silver halides. The dyes were bleached by a so-called catalytic action of silver when immersed in hydrobromic acid or an acid solution of thiourea. Effectively the developed silver metal was already the reducing agent for dye bleaching in this early process.

Kodak developed a silver-dye bleach material under the name Azochrome until 1941 but never commercialized it. Ciba started research work on silver-dye bleach after 1950, but it was not until 1963 that the first material under the name Cibachrome was commercialized. The silver dye-bleach systems continually improved to a top quality in color reproduction and image stability, but the market share remained limited due to the low photographic speeds and the high prices of the materials. In 1989 the brand name Cibachrome was changed to Ilfochrome and Ilfochrome II print materials were sold as professional products with a high reputation.

Not only did the Cibachrome materials improve from 1963 to its current highly sophisticated levels, but the processing was drastically simplified and shortened from initially almost an hour down to just a few minutes.

Basic Structure of Silver Dye-Bleach Materials

A gelatin layer containing a cyan dye and silver halide sensitized for red light is coated on a paper or film base. On top of the cyan layer is a magenta layer containing silver halide sensitized for green light. This is followed by a yellow layer with blue-sensitive silver halide. After the red, green, or blue exposure of this material, a silver image can be developed in the cyan, magenta, or yellow layer, respectively. In practice the effective structure of silver dye-bleach materials is sophisticated enough to correct for several unwanted interferences during exposure and processing.

Processing Fundamentals

In the first step, the exposed silver halide of a Silver dye-bleach material is developed to a metallic silver image by a conventional black and white developer. The second step is a bleaching bath, where the silver metal is used to bleach the dyes. The bleaching reaction takes place in the immediate vicinity of the silver grains. The chemistry of this step will be discussed below.

The third step requires the oxidation of any excess silver that has not been used for dye bleaching. The oxidation agent is generally copper sulfate in the presence of bromide, which transforms silver to silver bromide. The separate silver bleach is eliminated in all modern silver dye-bleach processes. Dye and silver bleaching have been combined in one bath that bleaches dyes and all excessive silver (see below).

The last step dissolves the silver halides in a fixing bath. Finally, a wash eliminates all silver compounds and the reaction products of dye bleaching. Every processing step is separated from the previous and subsequent steps by a wash. The result of this process is a positive reproduction of the original image.

Silver dye-bleach systems have clear advantages and disadvantages when compared to chromogenic imaging. The main advantage is a simple and stable process that leads to a direct positive image. Consequently, these pictures are very sharp. The high resolution is due to the absorption of scattered light by the image dyes (cyan absorbs red light, magenta absorbs green light, and yellow absorbs blue light). An excellent micro film is based on the silver dye-bleach principle: At low speed the absorption of the dyes in the sensitization range of the silver halide emulsions in the same layer reduces the photographic speed of silver dye-bleach materials (e.g., for red-light-sensitized silver halide emulsion is embedded together with the cyan dye in the same layer and the dye removes a part of the red light, which is no longer available to expose the emulsion grains).

The application of pure and stable azo dyes in the coated material results in a brilliant final picture of an excellent stability against light exposure and under archival conditions. Consequently, silver dye-bleach materials are ideally used for the printing of slides or colored originals. For classical camera applications, the photographic speed is too low.

Fastidious and delicate manufacturing problems of silver dye-bleach materials kept the production yields relatively modest, which resulted in high prices per square meter of coated material. Silver dye-bleach never conquered large market segments, but it filled specific market gaps in the professional field.

The advantage of light stability compared to chromogenic systems was a strong marketing argument. Silver dye-bleach pictures are best suited for large format applications that are exposed to sunlight. Silver dye-bleach materials have been sold mostly to professional photographers for artistic applications and exhibitions.

Manufacturing Aspects of Silver Dye-Bleach Materials

Conventional silver halide emulsions used for other photographic films and papers are used for silver dye-bleach materials. Therefore no special technique is used to produce them. Due to the absorption of the embedded dyes, high-speed emulsions are needed to get a reasonable speed for silver dye-bleach materials.

Dyes are applied as aqueous solutions. Concentrations of only up to 2 percent can be prepared due to the low solubility of the available dyes. This fact leads to a high water load of coating solutions, which limits the production speed.

Additives

In addition to the main components of the photographic system (silver halide emulsions, sensitizers, and dyes), stabilizers, wetting agents, moistener, and gelatin hardener complete the final coating solutions. These additives are mostly conventional components used in the photographic industry. Except the small quantity of sensitizers, which are dissolved in alcohol, all of these additives are applied as aqueous solutions.

Although manufactured with the same technique and installations as other photographic materials, silver dye-bleach material production needs some special requirements.

The speed of silver dye-bleach materials not only depends on the silver halide emulsions but also on the amount of dyes contained in it. An increase of dye by 10 percent lowers the speed of the corresponding layer by about 0.2 logE. Therefore, to achieve a good reproducibility of silver dye-bleach materials regarding speed and filtration, a sophisticated flow metering system for coating solutions is needed to get an accurate coating weight.

In chromogenic materials image dyes are built up during processing proportional to the preceding exposure. In silver dye-bleach materials dyes that are not needed for the image are bleached. That means that for silver dye-bleach materials coating disturbances and inhomogeneities are mostly detected in highlights of an image, the most critical area for the human eye for homogeneity and color variations. Therefore a much higher accuracy in coating is needed in silver dye-bleach production.

Further complications for silver dye-bleach manufacturing arise from the fact that coating solutions containing dyes are, depending on concentration, more or less non-Newtonian (thixotropic). Standard coating heads used in the photographic industry are designed for Newtonian fluids. When using such coating heads with silver dye-bleach coating solutions, non-acceptable cross profiles result. For the same reason as mentioned above, silver dye-bleach materials are very susceptible to color variation across the coated width; maximum variations of ±0.5 percent are acceptable. Therefore specially designed coating heads had to be developed. Development work to continuously improve the coating quality was as important as research work for improvement of silver dye-bleach materials and processing chemistry.

Marketing Arguments for Silver Dye-Bleach Systems

Compared to chromogenic color processes in which not only the color dyes are formed during the chemical treatment along with some disturbing impurities, the Cibachrome/Ilfochrome chromolytic process is free of such annoying side products. The dyes are incorporated into the sensitive layers of the Cibachrome materials during manufacturing. Only the most pure and light stable dyes are used for the production of the various silver dye-bleach products.

A processed sheet of Cibachrome/Ilfochrome material shows the following features:

Impressive color rendering and reproduction

Excellent sharpness

Superb color purity

Outstanding light stability

Cibachrome/Ilfochrome Classic transparent and print materials are coated on a polyester base, which makes the materials outstanding in their remarkable dimensional stability and the super glossy surface of the white opaque prints material. Since 2005 Ilfochrome Classic is the world’s only direct positive color material. This permits a unique way to print from original transparencies and the ultimate way to print from digital files.

Copy Materials

Cibacopy materials have a simple six-layer structure (as shown in Figure 45) and a fast processing speed. Cibacopy is a high-contrast material specially designed for the direct reproduction of artwork, drawings, reports, and other documents with repro cameras. Its high resolution even allows a brilliant and extremely sharp reproduction of screened pictures combined with small typefaces.

The Copy System was launched in 1978 at Photokina in Germany. Color copies, either in the form of prints on a white resin coated paper (CCO) or as transparencies (CTR) for use as overhead projection slides, were made on a daylight operating camera/processor with a very short processing cycle of 6 minutes.

Microfilm

The Color Micrographic films are available as master film (CMM) and print film (CMP) in 35 mm rolls and in selected sheet sizes. The micrographic films are processed in the silver dye-bleach process P-5.

The main features of the film materials are excellent resolution and light and archival stability.

Sophisticated Silver Dye-Bleach Systems for Highest Quality: Market Development

In 1967 a few highly specialized professional labs started the production of brilliant and light stable color enlargements on media coated on white opaque triacetate base processed in P-7A chemistry. The treatment lasted almost an hour. A transparent material (CCT) was introduced in 1969 for the production of long-lasting large display transparencies.

During the 1970s several significant improvements were made to both materials and the process chemistry. The print material was coated on white-pigmented polyester and the transparent material on clear polyester base. Both were processed in the improved four-bath process P-10 in 36 minutes.

A new shorter processing chemistry (15 minutes) was introduced later under the code P-18. It was the first three-bath process with combined dye and silver bleaching working at 30°C.

Cibachrome-A Print System for Hobbyists

A complete Cibachrome system for the hobbyist’s darkroom was introduced in America by Ilford Inc. in 1974. This system consisted of a pack of print material, a set of P-30 chemicals, color correction filters, a small developing drum, and user instructions. The processing time was 12 minutes. There were only three working strength solutions—developer, bleach, and fix—all of which were discarded after use. The working solutions were prepared from liquid concentrates (P-30) and later from easily soluble powders (P-30P).

Shortly after the successful introduction in North America the system was launched in Europe and other countries worldwide. Cibachrome-A gained widespread popularity during the late 1970s and this remarkably improved the degree of brand awareness of Cibachrome all over the world.

Cibachrome II Print Materials

In 1980 a new Cibachrome II material was introduced together with P-3, a new three-bath process requiring 18 minutes at 30°C. The major improvement was the introduction of the self-masking system described above. The product range included:

Cibachrome II print on white pigmented polyester base (CPS II)

Cibachrome II print on resin coated paper base (CRC II)

Cibachrome II transparent on clear polyester base (CTD II)

The introduction of self-masking meant that first development became more critical and, consequently, Cibachrome II materials were intended for use by laboratories using processors with automatic solution replenishment.

The net result of this new self-masking Cibachrome II system was a great improvement in the reproduction of most colors—especially blues, purples, yellows, browns, and greens. Flesh tone rendition also was better than in unmasked Cibachrome prints.

In 1987 an additional low-contrast print material (CF) was introduced. It was coated on white-pigmented polyester and adapted to high-speed automatic printing equipment.

A new fast professional three-bath process (P-3X) was introduced to provide better productivity to commercial labs. This replenished three-bath process has a processing time of 11 minutes, 30 seconds at 36°C and is compatible with all existing Cibachrome print materials. The replenishment rates are low and therefore waste production is reduced.

Private Label Silver Dye-Bleach Products—Photo-Me International

In 1976, Photo-Me, a company specialized in “while-you-wait” portrait kiosks, became interested in using Cibachrome photo material for their color prints. The Cibachrome process available at the time (P-10) was far too slow for the production of dry-to-touch color portraits in less than 4 minutes. Ilford R&D was able to develop entirely new solutions that reduced the processing time from 36 minutes down to 4. Photo-Me was then able to introduce their new generation of photo booths based on Cibachrome technology all over the world.

Short History of Cibachrome Market Introduction

Brand Product Names

Silver Halide Materials: General Emulsion Properties - Why Use Silver Halides?, Silver Halide Emulsions, The Silver Halide Grains, The Gelatin Medium, Emulsion Coating [next] [back] Silent Victory: The Kitty O'Neil Story

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