How is Wine Made?

Growing Grapes

Harvest

Initial Processing of the Grape Juice

Operations in a Winery

Fermentation - Turning Grape Juice into Alcohol

 

Malo-Lactic Fermentation

The winemaker may choose to allow a white wine to undergo a second fermentation which occurs due to malic acid in the grape juice. When malic acid is allowed to break down into carbon dioxide and lactic acid (thanks to bacteria in the wine), it is known as "malo-lactic fermentation," which imparts additional flavor to the wine. A "buttery" flavor in some whites is due to this process. This process is used for sparkling wines.

 

First Racking

Winery Aging

Stabilization and Filtration

 

Pasteurization

Bottling Wine

Cellaring Wine

Storing Wine

References

1. CHEMICAL PROCESS INDUSTRIES – R. Norris Shreve, Joseph A. Brink, IV Edition


2. INTERNET.- www.op.net/cgi-bin/doctxt/FAQs/wine

As you can see it is a rigorous and costly process that must be followed to get a new drug to your medicine cabinet.  It is estimated on average a company may spend $300 to $400 million dollars to get just one drug to your medicine cabinet.  It is not hard to see why some new medicines on the market are so expensive.  Let's take a look at the steps involved in developing a new drug.{parse block="google_articles"}

Preclinical Testing

This is the initial phase of the testing that begins in the laboratory.  A pharmaceutical company will conduct studies in the lab and on animals to show the biological activity of the compound against a targeted disease. The compound is then evaluated for safety.  These tests take about 3 1/2 to 4 years to complete.   The chemical engineer would be heavily involved in this phase of  drug development.

Investigational New Drug Application (IND)

After the above preclinical testing is completed the company then files an IND with the FDA to begin testing the drug in people.  The IND will become effective if the FDA does not disprove it within 30 days. The IND will show results of previous experiments, how, where and by whom the new studies will be conducted.   The IND also looks at the chemical structure of the compound, how it works in the body, and any toxic effects found in the animal studies. The IND will also look at how the compound is manufactured.   The IND must be reviewed and approved by the Institutional Review Board where the study will be conducted, and progress reports on clinical trials must be submitted at least annually to the FDA.

Clinical Trials, Phase I, II, & III

After the IND has not been disapproved within 30 days, then the next phase of testing begins, which is the clinical trials.

Phase I  

This phase of the testing takes about a year and involves about 20 to 80 normal, healthy volunteers. The tests study a drug's safety profile, including the safe dosage range. The studies also determine how a drug is absorbed, distributed, metabolized and excreted, and the duration of its action. 

Phase II 

In this phase, controlled studies of approximately 100 to 300 volunteer patients (people with the disease) assess the drug's effectiveness.  This phase normally takes about 2 years.

Phase III 

This phase involves 1,000 to 3,000 patients in clinics and hospitals. Physicians monitor patients closely to determine the efficacy and identify adverse reactions. This phase lasts about three years. 

New Drug Application (NDA)

After all of the clinical trials mentioned above are completed, then the company analyzes the data and files an NDA with the FDA if the data successfully demonstrates safety and effectiveness. The NDA is usually about 100,000 pages or more and contains all of the scientific information that the company has gathered. By law, the FDA is allowed six months to review an NDA.  In most cases the time from first submission of an NDA and final FDA approval usually exceeds six months. The average NDA review time for new molecular entities approved in 1992 was 29.9 months.{parse block="google_articles"}

Approval

Once the FDA approves the NDA, the new medicine becomes available for physicians to prescribe.  The company must continue to submit periodic reports to the FDA, including any cases of adverse reactions and appropriate quality control records.  The FDA requires some medicines to have additional studies (Phase IV) to evaluate the long-term effects of the drugs.

After reading the above steps in the development of a new drug, it is not hard to see why drugs cost so much when they finally do get to our medicine cabinets.  Many people in the pharmaceutical industry   are looking for ways to expedite the development of drugs and to decrease some of the money put into this development.  One resource that is being utilized in the development of new drugs is bioinformatics.  The Bioinformatics industry is still in its infancy stages, but it has started to change the way drug development has emerged since 1998.  Bioinformatics helps to take some of the fragmentation out of the development of new drugs.  Most drug development has been more or less by trial and error.  Bioinformatics uses information technology to be able to develop large databases and algorithms which help in the development of new drugs.

An example of this is the European Bioinformatics Industry (EBI).  The EBI serves researchers in molecular biology, genetics, medicine and agriculture from academia.  The EBI also serves the agricultural, biotechnology, chemical and pharmaceutical industries.  The EBI is able to serve these researchers and industries by building, maintaining and making available databases and information services that relate to molecular biology.  The EBI also carries out research in bioinformatics and computational molecular biology.  More information about EBI can be obtained from going to their website which is as follows: http://www.ebi.ac.uk/Information/index.html  

For those of you who are interested in learning more about the pharmaceutical pipeline (an industry term used for the research and development process of creating new drugs), Searle's Research and Development Department has a web site for you.  This web site actually takes you step by step in the development of a new drug.  The web site is as follows: http://www.searlehealthnet.com/pipeline.html   On this web site you can select from a few diseases such as cancer, cardiovascular related diseases, or arthritis as your primary target to be able to develop a new drug for. 

If the virtual formulation of a new drug is not enough, then all of you are invited to visit the following site from John Hopkins University, in which you can actually be the participant in a research study.  Most of these studies involve the development or refinement of drugs but some don't involve any drugs at all.   There is also a monetary incentive involved with some studies.  The web site is as follows:  http://www.jhbmc.jhu.edu/studies/index.html   Due to geographic limitations, it may be difficult to be able to attend the clinic visits that are required.  To be able to find out research studies in your local area, check with the nearest teaching/university based hospital and I am sure they would be more than happy to get you involved in a research study.

In summation, it is easy to see the vital role that chemical engineers play in the development of new drugs.  It takes the knowledge and skill of many disciplines to formulate a new drug.  When all of these discipline work together, it helps to expedite the formation of new drugs to help alleviate the effects of diseases for people around the world. 

The idea of heat pipes was first suggested by R.S.Gaugler in 1942. However, it was not until 1962, when G.M.Grover invented it, that its remarkable properties were appreciated and serious development began.{parse block="google_articles"}

It consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. A heat pipe is similar to a thermosyphon. It differs from a thermosyphon by virtue of its ability to transport heat against gravity by an evaporation-condensation cycle with the help of porous capillaries that form the wick. The wick provides the capillary driving force to return the condensate to the evaporator. The quality and type of wick usually determines the performance of the heat pipe, for this is the heart of the product. Different types of wicks are used depending on the application for which the heat pipe is being used.

Design Considerations

The three basic components of a heat pipe are:

1. the container
2. the working fluid
3. the wick or capillary structure

Container

The function of the container is to isolate the working fluid from the outside environment. It has to therefore be leak-proof, maintain the pressure differential across its walls, and enable transfer of heat to take place from and into the working fluid.

Selection of the container material depends on many factors. These are as follows:

• Compatibility (both with working fluid and external environment)
• Strength to weight ratio
• Thermal conductivity
• Ease of fabrication, including welding, machineability and ductility
• Porosity
• Wettability

Most of the above are self-explanatory. A high strength to weight ratio is more important in spacecraft applications. The material should be non-porous to prevent the diffusion of vapor. A high thermal conductivity ensures minimum temperature drop between the heat source and the wick.

Working Fluid

A first consideration in the identification of a suitable working fluid is the operating vapour temperature range. Within the approximate temperature band, several possible working fluids may exist, and a variety of characteristics must be examined in order to determine the most acceptable of these fluids for the application considered. The prime requirements are:

• compatibility with wick and wall materials
• good thermal stability
• wettability of wick and wall materials
• vapor pressure not too high or low over the operating temperature range
• high latent heat
• high thermal conductivity
• low liquid and vapor viscosities
• high surface tension
• acceptable freezing or pour point

The selection of the working fluid must also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe like, viscous, sonic, capillary, entrainment and nucleate boiling levels.

In heat pipe design, a high value of surface tension is desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force. In addition to high surface tension, it is necessary for the working fluid to wet the wick and the container material i.e. contact angle should be zero or very small. The vapor pressure over the operating temperature range must be sufficiently great to avoid high vapor velocities, which tend to setup large temperature gradient and cause flow instabilities.

A high latent heat of vaporization is desirable in order to transfer large amounts of heat with minimum fluid flow, and hence to maintain low pressure drops within the heat pipe. The thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling at the wick or wall surface. The resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid viscosities. Tabulated below are a few mediums with their useful ranges of temperature.

Wick or Capillary Structure

It is a porous structure made of materials like steel, alumunium, nickel or copper in various ranges of pore sizes. They are fabricated using metal foams, and more particularly felts, the latter being more frequently used. By varying the pressure on the felt during assembly, various pore sizes can be produced. By incorporating removable metal mandrels, an arterial structure can also be molded in the felt.

Fibrous materials, like ceramics, have also been used widely. They generally have smaller pores. The main disadvantage of ceramic fibres is that, they have little stiffness and usually require a continuos support by a metal mesh. Thus while the fibre itself may be chemically compatible with the working fluids, the supporting materials may cause problems. More recently, interest has turned to carbon fibres as a wick material. Carbon fibre filaments have many fine longitudinal grooves on their surface, have high capillary pressures and are chemically stable. A number of heat pipes that have been successfully constructed using carbon fibre wicks seem to show a greater heat transport capability.

The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. Often these two functions require wicks of different forms. The selection of the wick for a heat pipe depends on many factors, several of which are closely linked to the properties of the working fluid.

The maximum capillary head generated by a wick increases with decrease in pore size. The wick permeability increases with increasing pore size. Another feature of the wick, which must be optimized, is its thickness. The heat transport capability of the heat pipe is raised by increasing the wick thickness. The overall thermal resistance at the evaporator also depends on the conductivity of the working fluid in the wick. Other necessary properties of the wick are compatibility with the working fluid and wettability.

The most common types of wicks that are used are as follows:

Sintered Powder

This process will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. The photograph shows a complex sintered wick with several vapor channels and small arteries to increase the liquid flow rate. Very tight bends in the heat pipe can be achieved with this type of structure.

Grooved Tube

The small capillary driving force generated by the axial grooves is adequate for low power heat pipes when operated horizontally, or with gravity assistance. The tube can be readily bent. When used in conjunction with screen mesh the performance can be considerably enhanced.

Screen Mesh

This type of wick is used in the majority of the products and provides readily variable characteristics in terms of power transport and orientation sensitivity, according to the number of layers and mesh counts used.

Working Principle and Applications

Inside the container is a liquid under its own pressure, that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe.

Figure 1: Heat Flowing Through a Heat Pipe

Heat pipes have an effective thermal conductivity many thousands of times that of copper. The heat transfer or transport capacity of a heat pipe is specified by its " Axial Power Rating (APC)". It is the energy moving axially along the pipe. The larger the heat pipe diameter, greater is the APR. Similarly, longer the heat pipe lesser is the APR. Heat pipes can be built in almost any size and shape.{parse block="google_articles"}

Heat pipe has been, and is currently being, studied for a variety of applications, covering almost the entire spectrum of temperatures encountered in heat transfer processes. Heat pipes are used in a wide range of products like air-conditioners, refrigerators, heat exchangers, transistors, capacitors, etc. Heat pipes are also used in laptops to reduce the working temperature for better efficiency. Their application in the field of cryogenics is very significant, especially in the development of space technology. We shall now discuss a brief account of the various applications of heat pipe technology.

Space Technology

The use of heat pipes has been mainly limited to this field of science until recently, due to cost effectiveness and complex wick construction of heat pipes. There are several applications of heat pipes in this field like

• Spacecraft temperature equalization
• Component cooling, temperature control and radiator design in satellites.
• Other applications include moderator cooling, removal of heat from the reactor at emitter temperature and elimination of troublesome thermal gradients along the emitter and collector in spacecrafts.

Dehumidification and Air Conditioning

In an air conditioning system, the colder the air as it passes over the cooling coil (evaporator), the more the moisture is condensed out. The heat pipe is designed to have one section in the warm incoming stream and the other in the cold outgoing stream. By transferring heat from the warm return air to the cold supply air, the heat pipes create the double effect of pre-cooling the air before it goes to the evaporator and then re-heating it immediately.

Activated by temperature difference and therefore consuming no energy, the heat pipe, due to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature, increasing the moisture removal capability of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can be achieved at higher thermostat settings, which results in net energy savings. Generally, for each 1° F rise in thermostat setting, there is a 7% savings in electricity cost. In addition, the pre-cooling effect of the heat pipe allows the use of a smaller compressor.

Laptop Cooling

Heat pipe technology originally used for space applications has been applied it to laptop computer cooling. It is an ideal, cost effective solution. Its light weight (generally less than 40 grams), small, compact profile, and its passive operation, allow it to meet the demanding requirements of laptops.

For an 8 watt CPU with an environmental temperature no greater than 40°C it provides a 6.25°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90°C or less.

Figure 2: Heat Sink Inside a Laptop

One end of the heat pipe is attached to the processor with a thin, clip-on mounting plate. The other is attached to the heat sink, in this case, a specially designed keyboard RF shield. This approach uses existing parts to minimize weight and complexity. The heat pipe could also be attached to other physical components suitable as a heat sink to dissipate heat. (See photo of inside of laptop computer).

Because there are no moving parts, there is no maintenance and nothing to break. Some are concerned about the possibility of the fluid leaking from the heat pipe into the electronics. The amount of fluid in a heat pipe of this diameter is less than 1cc. In a properly designed heat pipe, the water is totally contained within the capillary wick structure and is at less than 1 atmosphere of pressure. If the integrity of the heat pipe vessel were ever compromised, air would leak into the heat pipe instead of the water leaking out. Then the fluid would slowly vaporize as it reaches its atmospheric boiling point. A heat pipe’s MTTF is estimated to be over 100,000 hours of use.

Laptop Thermal Control

Heat pipes have proven to be the excepted means of providing thermal control in notebook and Mobil PCs systems. Heat pipes can move and dissipate CPU generate heat selectively throughout the system without affecting temperature sensitive components. Low wattage heat pipes (under 20 watts) have standardized input plates to the heat pipe. The connection to the heat exchanger via the heat pipe can have any number of configurations to accommodate component placement, multiple power ranges and fan options.{parse block="google_articles"}

The heat pipe solutions for thermal control at this level is a component and overall systems requirement. Not only do the heat pipes take on a different configuration with multiple heat pipes and cooling fins, but also airflow becomes the critical design factor. Heat pipes designed to move 75 watts are usually flat with fin stacks from three to six inches, in many cases with fins mounted on each side of the CPU input pad. Input pads are standard using stand-offs, transition sockets, and bolster plates on the bottom of the PC board. The spring clips used on the fan/heat sink combination won’t work here. Airflow management is important in the overall efficiency of the heat pipe and should be calculated along with the intended heat pipe design.

Figure 3: Heat Pipe on a 500 MHz System

Thermal solutions are normally designed with multiple heat pipes, dedicated airflow and maximum input area. Fins stacks typically extend over both sides of the CPU. Input attachment to the CPU is with stand-offs, transition sockets or bolster plates.

The 500MHz operating system shown in Figure 3 uses two thermal products, heat pipes to transfer the CPU heat (100 to 300 watts) and a second internal or external cooling source. Input power is generated from multiple CPUs and components with single or multiple heat pipes. Cooling temperatures on the output range from -0° C to - 40° C. This system requires thermal isolation because of dewpoint considerations.

Flexible Solutions

Heat pipes are manufactured in a multitude of sizes and shapes. Unusual application geometry can be easily accommodated by the heat pipe’s versatility to be shaped as a heat transport device. If some range of motion is required, heat pipes can even be made of flexible material.

Two of the most common are:

Constant Temperature: The heat pipe maintains a constant temperature or temperature range.

Diode: The heat pipe will allow heat transfer in only one direction.

Figure 4: Flexible Style Heat Pipes

Mega Flats

Flat heat pipes are typically used for cooling printed circuit boards or for heat leveling to produce an isothermal plane. Mega flats are several flat heat pipes sandwiched together.

Figure 5: Flat Style Heat Pipe Materials

Some of the flat heat pipes manufactured are:

XY Mega Flats: Surface maintained within .01° F isothermal with concentrated load centers. 6" X 6" Mega Flat: Dissipated 850 watts from a printed circuit board.

Weight Reduction Mega Flats:

Standard - aluminum construction.

Lightweight - ½ the weight of aluminum.

Very light weight - 1/3 the weight of aluminum.

SEM C and SEM E Mega Flats in stock. Low and light weight coefficient of thermal expansion (CTE) Mega Flats - any CTE from 2 to 10. Alloy H: 70% more conductive than, or 40% less weight than copper clad invar.

Cost Effectiveness of Heat Pipes

The cost of heat pipes designed for laptop use is very competitive compared to other alternatives. Cost is partially offset and justified by improved system reliability and the increased life of cooler running electronics. Heat pipes, in quantity, cost a few dollars each while an entire cooling system will cost between $5 - $10 in production quantities, depending on the final design. Standard design products are available to reduce cost even further. Heat pipe manufacture has been a difficult area to compete in. Simple in concept, but difficult to apply commercially, the heat pipe is a very elusive technology & holds the key to the future of heat transfer and its allied applications.

While science is slowly putting together the pieces to explain the functioning of our vision system, the basic nuts and bolts of classical photography have been known for years, although certain details remain the subject of some discussion. Just as in the human eye, classical photographic systems are composed of two separate, but interrelated processes – the basic black and white image structure and the finer points of color reproduction.

This first installment on the chemistry of photography is intended to introduce, in a simplified way, the basic concepts of silver halide photography. It will not delve into the physics of optics, the functioning of cameras and lenses, photographic techniques, non-silver processes, or the artistic aspects of photography. Nor will it go beyond a cursory mention of color photographic processes, which will be left for the future. Anyone interested in more detail is referred to the selected bibliographic material cited at the end.

A Brief History of Black and White Silver Halide Photography

Perhaps the earliest reference to the concept of silver-based black and white photography is that of J. H. Schulze who observed in 1727 that a mixture of silver nitrate and chalk darkened on exposure to light. The first semi-permanent images were obtained in 1824 by Nicéphore Niepce, a French physicist, using glass plates coated with a dispersion of silver salts in bitumen (a coal derivative). In the early 1830’s, Niepce's partner, Louis Daguerre, discovered by accident that mercury vapor was capable of developing an image on a silver-plated copper sheet that had been previously sensitized by iodine vapor. The image, which was called a daguerreotype, could be made permanent by washing the plate with hot concentrated salt solution. In 1839 Daguerre demonstrated his photographic process to the Academy of Sciences in Paris. The process was later improved by using sodium thiosulfate to wash off the unexposed silver salts.

In 1841, an Englishman, William Henry Fox Talbot introduced a new system, the calotype process. The Talbot process involved a paper than had been sensitized to light by a coating of silver iodide. A negative image was produced on the exposed light-sensitive paper by bathing it in a solution of gallic acid in a development process essentially the same as that used today. If the paper base employed was semitransparent, the original negative image could be laid over another piece of sensitized paper which, when exposed and developed, yielded a "positive," or direct copy of the original. The process would be equivalent to what is termed "contact printing" today. Although the calotype process required less time than that of Daguerre, the Talbot images were not particularly sharp because of the fluidity of the medium employed to suspend the silver iodide crystals.

Originally, the silver salts were held on glass using egg white as a binder. This provided relatively sharp images although they were easily damaged. By 1871, the problem had been solved by Dr. R. L. Maddox, an amateur photographer and physician, who discovered a way to prepare gelatin dispersions of silver salts on glass plates. In 1887 George Eastman introduced the Kodak system in which a silver halide-in-gelatin dispersion was coated on a cellulose nitrate base and loaded into a camera. The camera could take 100 pictures and when all were exposed, camera and film were returned to Rochester, New York, for processing. With those innovations the age of modern photography had arrived.

Photochemistry of Silver Salts

To understand the fundamental chemistry of silver-based photography, we must look at the photochemistry of silver salts. A typical photographic film contains tiny crystals of very slightly soluble silver halide salts such as silver bromide (AgBr) commonly referred to as "grains." The grains are suspended in a gelatin matrix and the resulting gelatin dispersion, incorrectly (from a physical chemistry standpoint), but traditionally referred to as an "emulsion," is melted and applied as a thin coating on a polymer base or, as in older applications, on a glass plate.

Figure 1 shows a schematic representation of the silver halide process. When light or radiation of appropriate wavelength strikes one of the silver halide crystals, a series of reactions begins that produces a small amount of free silver in the grain. Initially, a free bromine atom is produced when the bromide ion absorbs the photon of light:

 Ag⁺Br^- (crystal) + hv (radiation) --> Ag⁺ + Br + e^-

The silver ion can then combine with the electron to produce a silver atom.

Ag⁺ + e^{- -->} Ag⁰

Figure 1: A Simplified Schematic Representation of the Silver Halide Process

Association within the grains produces species such as Ag₂⁺, Ag₂⁰, Ag₃⁺, Ag₃⁰, Ag₄⁺, and Ag₄⁰. The free silver produced in the exposed silver halide grains constitutes what is referred to as the "latent image," which is later amplified by the development process.

 The grains containing the free silver in the form of Ag₄º are readily reduced by chemicals referred to as "developers" forming relatively large amounts of free silver; that deposit of free silver produces a dark area in that section of the film. The developer under the same conditions does not significantly affect the unexposed grains.

The radiation or light sensitivity of a silver halide film (referred to in the trade as its "speed" and denoted on commercial film as its ASA in the United States or DIN in Europe) is related to the size of the grain and to the specific halide composition employed. In general, as the grain size in the emulsion increases, the effective light sensitivity of the film increases - up to a point. An optimum value of grain size for a given sensitivity is found to exist because the same number of silver atoms are needed to initiate reduction of the entire grain by the developer despite the grain size, so that producing larger grains reaches a point of diminishing returns and no further benefit is obtained.

All photographic emulsions contain crystals of varying sizes, but within a given emulsion the range is from less than 0.1 micron in slow emulsions (e.g., for paper prints) to a few microns in "fast" negative emulsions.

An interesting modern innovation in photographic emulsion technology is related to the basic concept of silver halide grain geometry. In a classical silver halide crystal, typically a cubic crystal lattice, the structure will be relatively symmetrical in that the orientation of the crystal in the coated film will always present the same approximate surface area to be exposed. Extensive research efforts led to the development of grain precipitation processes that produced flatter "tablet" grains in which the crystals possessed a more asymmetric geometry, and in which a larger surface area was presented for exposure for the same given weight of silver halide (Fig. 2). That development resulted in significant improvements in film sensitivity and reductions in the amount of silver needed to obtain a given sensitivity – and a potentially important reduction in the cost of the film.

The Latent Image and Image Development

The silver halide process is by far the most important of all of the radiation-sensitive photographic systems in use today. The principal reason for this superiority is the high sensitivity of the system - the amount of radiant energy required to produce a useful image – and the extreme flexibility of the system in terms of adjusting sensitivity, contrast, tonal range and other such aspects of the product. The impact of a single photon on a silver halide grain, for example, produces a nucleus of at least four silver atoms, and that effect can be amplified as much as a billion times by the action of a properly chosen reducing agent or "developer."{parse block="google_articles"}

The silver halides employed are silver bromide, silver chloride and silver iodide. The first two may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride.

Figure 2: A Schematic Representation of Cubic and "Tablet" Silver Halide Grains

As already noted, the silver halides used in photography are dispersions of microscopic crystals in a colloidal binder that is usually bone gelatin. Although such dispersions are referred to as emulsions or photographic emulsions, they are really dispersions.

When an exposed film is placed in a developer solution, the grains that contain silver nuclei are reduced much faster than those that do not. The more nuclei present in a given grain (i.e., the greater the exposure of that grain), the faster the reaction with developer and the darker the image at that site in the film. Factors such as temperature, concentration of the developer, pH, and the total number of nuclei in each grain determine the extent of development and the intensity of free silver (blackness) deposited in the film emulsion in a given time.

Not only must the developer be capable of reducing silver ions to free silver, but it must be selective enough not to reduce the unexposed grains, a process known as "fogging," within the time frame of the development process. Some substances that are commonly used as silver halide developers are listed in Table 1.

The developer is oxidized in the process. If not properly protected it can also be oxidized by air, a process that, if not prevented, will result in the loss of developer activity. To help prevent such effects, commercial developer solutions commonly contain preservatives such as sodium sulfite.

Gelatin - The Film Matrix

Gelatin is a protein extracted from animal hides, bone, and sinew that belongs to the class of substances known as hydrophilic ("water loving") polymers, which also includes other proteins, gums, starches, and a wide variety of synthetic polymers. Photographic grade gelatin is usually produced by an alkaline extraction process using bovine bones, although some acidic processes have been developed. Some special gelatins are also used that are derived from pigskins. The origin and quality of the raw materials used in the gelatin process, the conditions employed (pH, temperature, time, etc.), and the presence or absence of certain possible contaminants (e.g., mercury or other heavy metals, lipids, sulfur compounds, etc.) are of vital importance to the production of material suitable for use in modern photographic systems. In fact, photographic gelatin is much purer that that used in food applications.

Table 1: Common Silver Halide Developers

Gelatin solutions have the useful property of behaving as a liquid when warm and setting to a relatively hard gel when cool. When coated on a substrate and dried it is flexible and reasonably resistant to physical damage, but it readily absorbs water (as in the development process). Generally, the coated gelatin dispersion will contain some material such as formaldehyde as a "hardener" that serves to produce a limited number of cross links among protein chains that improve the physical characteristics of the coated material. The gelatin swells rapidly, absorbing water and dissolved development chemicals, but it does not dissolve or disintegrate at normal temperatures. These properties are essential in the preparation of a photographic emulsion, in coating it on film base or paper, and in the processes of development, fixation and washing.

Gelatin is more than a means of holding the sensitive silver halide salts in place on a on a film base, however. It is an integral part of the process and can significantly affect the properties of the final photographic product. Controlling the characteristics of the gelatin is absolutely essential to the photographic emulsion preparation, the process of coating the emulsion on the film base, the adhesion of the coated material to the film base, the wetting characteristics of the coated material in the development process, the physical characteristics of the dried developed product, and the long-term stability of the developed image. In a way, one could safely say that the gelatin is almost as important as silver to the overall photographic system.

In the preparation of a photographic emulsion the gelatin also acts as an anti-coagulant or stabilizing colloid. The silver halide formed in fluid gelatin does not precipitate out of solution but remains uniformly distributed throughout the preparation, ripening (see below), and coating processes. The gelatin is an important factor in determining the dispersity or range of grain sizes of the silver halide. By suitable regulation of the concentration of the gel, the temperature, and the rate of addition of the components, the grain size distribution can be controlled to meet specific requirements.

In the silver halide dispersion, gelatin molecules adsorb at the surface of the silver halide grain, surrounding the grain and forming a barrier that stabilizes the dispersion. The adsorbed layer also, in all likelihood, affects the radiation sensitivity of the grain and makes reduction by developers, more controllable. This is important in the development process and makes it possible to obtain desired results from a given system based on easily controlled parameters such as developer chemistry, development time, temperature, etc.

Photographic Emulsion Preparations

The exact methods used in preparing commercial photographic emulsions are closely guarded trade secrets, but the basic procedures are well known. There are two general classes of emulsions, the characteristics of which are determined by the end use. They are "negative" emulsions that are used for exposure in cameras and produce a reversed or negative image, and print images that produce the final photograph that we show off to our friends and relatives.{parse block="google_articles"}

Negative emulsions generally must exhibit a relatively wide flexibility in terms of sensitivity since they are used under conditions that are generally beyond the control of the casual photographer. It would be rather impractical if we couldn’t take our vacation pictures in bright sunlight and shade with the same camera and film. The professional photographer has the option of having several thousands of dollars of cameras dangling around her neck to suit the conditions. Most of us do not.

Print or paper materials, on the other hand, are used under highly controlled conditions of exposure and can therefore have a much more limited "range" of sensitivity and provide the user with more direct control over the final results of the development process.

The preparation of a negative emulsion involves four distinct, but interrelated, steps: (1) the formation of silver halide crystals in gelatin through a precipitation process, (2) the recrystallization of the silver halide grains by physical or Ostwald ripening, (3) a washing process that removes excess soluble salts from the emulsion, and (4) a digestion or chemical sensitizing process involving the heating of the emulsion to increase its sensitivity to incident light.

After crystallization, the emulsion is stored for several hours at a moderate temperature during which the average crystal size increases via Ostwald ripening in which the smaller crystals tend to dissolve while the larger crystals grow as crystallization nuclei.

Following this ripening process the by-products are removed and the amount of free halide is reduced. Historically this was accomplished by chilling the emulsion to a gel and forcing it through a perforated screen to form "noodles" which were then washed in running water. Other methods, which are generally trade secrets, but include membrane filtration techniques, are now used in some cases.

After washing, additional gelatin is added to bring the emulsion to its final gelatin composition. Quite often, the added gelatin is rich in sulfur containing amino acids. The final emulsion is then heated to a temperature between 50º and 80⁰C for about an hour to facilitate the interaction of the sulfur in the added gelatin and the silver crystals. The sulfur-silver halide interaction increases the number and size of the silver sulfide sensitivity centers and improves the characteristics of the grains. An alkaline gold thiocyanate may also be added at this stage to increase the sensitivity of the emulsion.

Other additives to the final emulsion may include:

1. stabilizers to retard changes in the size and size distribution of the grains;
2. antifogging substances to retard the development of unexposed grains of silver halide when the image is developed;
3. a gelatin hardener to prevent the gelatin from excessive swelling in processing;
4. surfactants and other components to control the wetting and other fluid characteristics of the emulsion during the coating operation;
5. surfactants, lubricants, and anti-static agents to control the surface properties of the dried emulsion;
6. color-sensitizing dyes that expand the range of light sensitivity of the emulsion.

Because the printing of images on paper is carried out in the darkroom under closely controlled conditions, the light sensitivity of the emulsion is not as critical as other visual aspects of the final print such as the tone and contrast of the image.

The emulsions used for developing papers differ from negative emulsions in a number of important respects. In preparing the paper photographic emulsion, the silver may be added to the gelatin solution containing the soluble halide, as in the preparation of a negative emulsion, the halide may be added to the gelatin silver solution, or the silver and soluble halide may be added simultaneously. The rate of addition and the concentrations involved are all designed to produce fine, uniformly sized crystals. In some processes, precipitation takes place in a slightly acid solution to inhibit recrystallization and growth of the crystal size. An excess of soluble halide is avoided for the same reason.

Paper emulsions are generally not heat ripened, as are negative systems, since that would result in larger average crystal sizes and tend to increase the sensitivity. Nor are they generally washed, because the concentration of salts is low and their presence tends to reduce further ripening in storage and changes in sensitivity. In addition, when the emulsion is coated on the paper, a significant amount of the soluble salts is absorbed by the paper stock and thus removed from the system. Those ions that remain in the emulsion may have a desirable influence on the color or contrast of the final image.

The speed or sensitivity of an emulsion can be adjusted in many cases by the use of color-sensitizing dyes (see below). Other additives may be antiseptics, such as phenol or thymol to inhibit the growth of microorganisms; hardeners, such as alum, formaldehyde and glyoxal to improve the physical characteristics of the emulsion during storage and development; and wetting agents, such as saponin or other surfactants to reduce surface tension and facilitate the coating of the emulsion.

Special papers with wider ranges of applicability in terms of contrast, spectral sensitivity, tonal qualities, etc., may be produced in one of two ways: (1) by the admixture of two emulsions of different contrast and color sensitivity, and (2) by sensitizing in such a way that the result varies with the wavelength of the exposing light.

The silver halide grains in a paper emulsion seldom exceed 0.01to 0.02 microns as compared with from 1.0 to 2.0 microns in a negative emulsion and the amount of silver halide in the coated paper per unit area is about one-fifth that of a negative material.

Color Sensitizing

The radiation sensitivity of silver halides ends for all practical purposes at about 525 mm . In Figure 3 Curve A illustrates the spectral sensitivity of a typical silver bromo-iodide emulsion and B illustrates the average human visual response curve. As the curves show, the maximum response of the eye is in the yellow-green near 550 mm which lies beyond the sensitivity range of the emulsion, which is much more sensitive to the violet and blue than the eye.

Figure 3: Curves Approximating the Light Sensitivity of Typical Silver Halide Crystals (blue) and the Human Eye (yellow)

The sensitivity of the silver halides may be extended to radiation of longer wavelengths by the addition of dyes or "color sensitizers." Although referred to as dyes, color sensitizers are not ordinary dyes in that they are not used to color cloth or other materials.{parse block="google_articles"}

Emulsion sensitizing results from the absorption of radiant energy by the dye at a wavelength that would not affect the silver halide, and the transfer of that "exposure" energy to the silver halide to form a latent image and make the affected grain developable. If a dye is a sensitizer, then its action depends upon the absorption characteristics of the silver halide-adsorbed color sensitizer complex, which may be quite different from the absorption of the dye itself. Since the sensitivity of such dyes varies greatly, it is often necessary to use a combination of materials to obtain a specific result. Some combinations, however, do not work well together, so that the system balance must be carefully studied before the final emulsion composition is determined. There are substances that may or may not be sensitizers themselves but greatly increase the sensitizing action of other dyes. These are known as super-sensitizers and are of considerable importance in facilitating the use of conventional sensitizing dyes. Whatever the dye used, the quantity required is always quite small.

Development

The rate of development of the individual grains in an emulsion is affected by so many factors, such as the rate of diffusion of the solution through the gelatin matrix, the adsorption of the developing agent, the solution of the silver halide, oxidation products of the developing agent and the accumulation of restraining by-products, that exact analysis of it is difficult. The time of appearance of a visible image is, within limits, a reliable indication of the rate of development. It varies with different emulsions and is quite different with different developing agents, but the variation with temperature, dilution and pH is almost directly related to the variation in the rate of development.

The rate of development, as determined from the change in the optical density of the developed image, is complicated by the fact that density increases in two different ways: (1) by the increase in the amount of silver as the grains develop and (2) by an increase in the number of grains in the process of development. Density grows rapidly at first and then slows down until development is complete and no further growth in density takes place. Prolonged development would, of course, increase overall density through the development of unexposed grains (fog).

Halting Development - The Stop Bath

Once the exposed image has been developed to the desired degree, it is necessary to halt the chemical process quickly to prevent over development and the production of fog. The solution used to that end is referred to as the "stop" bath. Since developers function a relatively high pH’s, the typical stop bath is simply a solution of acetic or some other weak acid. The action of the acid is so rapid it usually requires only seconds for the process to be effectively halted.

The Fixing Process

Once the developed image is obtained, a large amount of unexposed and undeveloped silver halide remains in the emulsion. If that silver halide is not removed before the image is exposed to radiation capable of producing a latent image, the image will continue to darken. The process of removing the residual silver halide from the image is called "fixing."

The silver halides are only slightly soluble in water; therefore, to remove the material remaining after development it is necessary to convert it to soluble complexes which can he removed by washing. Sodium thiosulfate, commonly termed "hypo," has been used for this purpose since 1839.

The reactions in fixing can be written as follows:

AgBr + S₂O₃^-2 --> AgS₂O₃^- + Br^- (adsorption complex)

which is followed by

AgS₂O₃- + S₂O₃^-2 --> Ag(S₂0₃)₂-³ (desorbed)

and by

Ag(S₂0₃)₂^-3 <--> AgS₂O₃^- + S₂O₃ ^-2; AgS₂O₃ <--> Ag⁺ + S₂O₃^-2

Within limits, the rate of fixation is indicated by the clearing time, i.e., the time required to remove all visible traces of silver halide from the image. This time depends on the concentration of thiosulfate, the temperature, the agitation of the solution, but more particularly on the emulsion and the extent to which the fixing bath has been used. Fine-grain emulsions fix in less time than those of larger grains, and paper emulsions of silver chloride fix faster than bromo-iodide negative emulsions. Thickly coated films, other things being equal, fix more slowly than those with a thin emulsion coating. The fixing time increases appreciably as the solution becomes depleted. With continued use the halide-ion concentration rises in proportion to the amount of silver halide dissolved. When the product of the silver-ion and the halide-ion activities reaches the solubility product of the least soluble silver halide present, the solution will dissolve no more of that silver halide and fixation will necessarily be incomplete

It is usually desirable to harden the gelatin after development, and while this may be accomplished by a hardening stop bath prior to fixing, the usual practice is to combine hardening with fixing. The conventional fixing and hardening bath contains in addition to the fixing agent:

1. An organic acid, usually acetic, to provide the necessary acidity to stop development and create the proper pH for effective hardening.

2. Sodium sulfite, which prevents the decomposition of the thiosulfate by the acid and forms colorless oxidation products of the developer thus preventing staining.

3. Alum as a hardening agent.

The hardening produced by alum is due to the reaction of the aluminum ions, Al⁺³, and the carboxyl groups of the gelatin with the formation of cross-linkages between chain molecules. The degree of hardening, other things such as temperature, alkalinity of the film when placed in the fixing bath, etc., being equal, depends on the pH of the solution which in turn depends on the relative proportions of acid, sulfite and alum.

Since the addition of developer tends to increase the pH of the fixing bath, the solution should be buffered against an increase in pH. For this reason weak organic acids, such as acetic acid, are used in preference to a stronger acid, such as sulfuric. The addition of boric acid increases the useful hardening life of potassium alum baths and reduces the tendency of the bath to form a sludge.

Reference and Further Reading

1. Mees, C.E.K, and James, T.H., The Theory of the Photographic Process, 3rd ed., The Macmillan Co., New York, 1966.
2. Neblette, C.B., Fundamentals of Photography, Van Norstrand Reinhold Co., Princeton, N.J., 1970.
3. Croome, R.J., Photographic Gelatin, Focal Press, New York, 1965.