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Making Silver Halide Emulsions - Emulsification, Ripening, Ostwald Ripening, Washing, Digestion and Sensitization, Sulfur Sensitization, Reduction Sensitization, Gold Sensitization

compounds solution gelatin sensitivity

Over a period now spanning more than a century the process of making silver halide emulsions has been greatly refined. One of the major advances during this period was the phased addition of different halides during crystal growth, resulting in what is known as core-shell grains where the center is of a different halide constitution to the outer layers.

The major players in this industry have all developed their own ways of efficient and effective manufacture, many of the details of which still remain commercially confidential. However, the underlying science of silver halide emulsion preparation remains the same and the fundamental steps are listed below.

Emulsification

A solution containing silver in a soluble form (usually silver nitrate) is mixed with another containing the relevant halides. This mixing takes place in a gelatin solution. This stage, often referred to as emulsification, can produce significantly different results depending on the conditions employed. Knowledge of these and the relevant process control tolerances are part of the art and science of emulsion making. Silver nitrate and halide concentrations, temperature, rate of mixing, and gelatin type and concentration are all key parameters in this step.

When silver nitrate and a halide such as potassium bromide are mixed in solution a silver halide (in this case silver bromide) is produced. The relative amounts of the reactants are arranged to be such that the concentration of the silver bromide produced is greater than its solubility. As a result, small crystals of silver halide fall out of solution. These crystals grow as they act as seed nuclei for other silver halide to fall out of solution. This is a key stage in the production of silver halide emulsions and many of the final characteristics of the emulsion are determined at this stage.

Gelatin is important even at this early stage of emulsion manufacture. The gelatin is absorbed onto the surface of the individual grains, preventing them from clumping together to form aggregates and protecting them from damage during subsequent stages.

Ripening

The primitive emulsion is held under conditions where there is significant solubility of silver halides. This is done using a combination of elevated temperature and the presence of silver halide solvents, notably ammonium compounds. To facilitate this step further chemicals may be added to the mix. The ripening step can be considered as a combination of two processes, both of which modify the particle size distribution and hence the final characteristics of the final emulsion.

Ostwald Ripening

Smaller grains of silver halide are re-dissolved and the silver halide re-deposited on larger grains. The result is that the average size of the silver halide particles increases and size distribution changes. This is a process best described as grain coalescence. Here two or more crystals grow into each other forming a fused unit. Alternatively one or more crystals can grow from the edge or corner of an existing crystal. These mixed grains contain crystal lattice imperfections and defects that influence the final sensitivity of the emulsion. Both of these processes have a profound effect on the sensitometry of the final emulsion.

Washing

The emulsion at the start of this stage consists of silver halide grains dispersed in a gelatin solution that also contains a number of soluble by-products of emulsion manufacture. These dissolved chemicals must be removed at this stage as they may adversely affect the shelf-life or performance of the emulsion. The term “washing” for this process is an archaic description stemming from the time when emulsions were chilled causing them to set then shredded and washed in water. This process was time-consuming, involved much heating and cooling, and required copious quantities of pure washing water. As a result, modern processes evolved such as chemical flocculation and reverse osmosis.

Flocculation works because gelatin can become unstable in solution at some pH (acidity/alkalinity) levels. This results in the silver halide, which contains emulsion, to be precipitated out of solution and then collected and washed as above. Once cleaned it can be re-dispersed simply by moving the pH back into the soluble range.

Reverse osmosis takes advantage of the fact that neither the silver halide grains nor the long-chain gelatin molecules can pass through the very fine pores of a semi-permeable membrane. However, the soluble impurities we wish to remove are able to pass through these pores. If the impure solution is placed under pressure on one side of such a membrane then water and soluble impurities will be forced through these pores leaving the emulsion remaining purified and concentrated.

Digestion and Sensitization

The purpose of this step is to increase the sensitivity of the emulsion. This is done primarily through the addition of certain chemicals to the emulsion. Some of these chemicals influence the overall sensitivity of the emulsion to light or charged particles. Others, known as spectral sensitizers, change the balance of wavelengths to which the emulsion is sensitive.

There is a balance to be made in sensitization between the benefits of increased sensitivity against the disadvantage of increased fog. For some products such as high-speed camera or aerial films there is a need for high sensitivity at the expense of some increase in fog. However, in other products such as high-quality printing papers and autoradiography emulsions the balance is on the side of reduced fog.

Sensitizing chemicals can be conveniently described in a number of classes.

Sulfur Sensitization

It was stated previously that a gelatin binder produced a more sensitive emulsion than one made with collodion. In the 1920s it was realized that this was due to some natural sources of sulfur compounds in gelatins that were acting as a sensitizer. Most photographic-grade gelatins today have this sulfur removed at the purification stage so that precise amounts can be added during sensitization.

These sulfur compounds work by producing silver sulfide centers on the crystal surface that act to stabilize the latent image. This increase in stability is greater at low exposure intensities so sulfur sensitization tends to have the greatest effect on low-intensity reciprocity failure.

Reduction Sensitization

Small amounts of reducing agents such as aldehydes can also increase photographic sensitivity. These types of compounds were again found in early gelatins and work by producing silver (rather than silver sulfide) centers on the crystal surface, which again act to stabilize the latent image. Reduction sensitization is commonly used in conjunction with sulfur sensitization.

Gold Sensitization

Gold compounds have been widely used, particularly in highspeed emulsions. In this case the effect is greatest at high exposure intensities, so it is particularly effective in dealing with high-intensity reciprocity failure. The major drawback with gold sensitization is the propensity to increase fog levels.

Other Chemical Sensitizers

There have been a number of patents and technical papers on the subject of other organic compounds for use as sensitizers such as formate ions. Although there have been widespread claims for these compounds they have not become mainstream in silver halide technology. The separate issue of spectral sensitization is dealt with previously.

Coating Finals

A number of other chemicals are commonly added before the emulsion is coated. Anti-foggants are chemicals used to restrain fog when processed in developers of high activity. However, some such compounds reduce emulsion sensitivity on exposure. Development accelerators may be incorporated, particularly in products requiring very fast processing times such as some medical and aerial films. Some products actually have developers incorporated into the emulsion that only require activation with a pH change.

Hardening agents work by cross-linking the gelatin molecules. These reduce the swelling of the emulsion layer in processing solutions and generally make them more robust. They may also have a positive effect on the adhesion of the emulsion to the base material. However, as this cross-linking process influences the flow properties of the emulsion, these hardeners must be added immediately prior to coating.

Stabilizers are added to restrain the further slow ripening of the coated emulsion that may occur on keeping. This ripening can cause sensitivity changes and fog. Once again, some of these compounds can act to reduce emulsion sensitivity on exposure.

Wetting agents are widely used to allow the emulsion to uniformly wet the surface of the base during coating. Inadequate wetting produces streaks of non-uniformity or bubbles, which result in artifacts in the processed image.

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over 6 years ago

Brilliant for my own understanding about emulsions.