Forensics Lab 8.0: Revealing Latent Fingerprints – Introduction


This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.

Even someone who knows nothing else about forensics knows about fingerprints. The individuality of fingerprints had been generally accepted, as established by forensic scientists and courts, by the early 20th century, and the billions of fingerprint specimens taken since then have confirmed fingerprints as unique individual characteristics. Figure 8-1 shows a full fingerprint that we made by pressing one of our fingers to a stamp pad and then rolling it against a sheet of paper.

Figure 8-1

Figure 8-1. A typical full fingerprint taken under controlled conditions

Unfortunately, prints found at a crime scene are usually partial, broken, smeared, or otherwise inferior to the perfect specimens found on fingerprint cards, so it’s often impossible to do a full comparison. Fingerprint examiners use points of comparison, also called points of identification, to compare unknown fingerprints against known specimens. A point of comparison is a particular individual feature of a particular fingerprint, such as where a ridge ends or bifurcates or the shape and number of ridges present in a whorl. If sufficient points of comparison exist between two fingerprints, it’s reasonable to assume that those two prints were produced by the same finger, even if parts of the questioned print are missing, smeared, or otherwise obscured. Figure 8-2 shows a typical questioned partial fingerprint found on a questioned document. Although some ridge detail is present, it’s unlikely that this print could be identified against a known print.

Figure 8-2

Figure 8-2. A typical questioned fingerprint

Dennis Hilliard comments
There are three levels of comparison:

  • Level one – the overall pattern: loop, arch, whorl
  • Level two – the dots, islands, bifurcations, ridge endings, etc.
  • Level three – poroscopy, the presence and spatial relationship of the pores on the print ridges

With poor to moderate quality prints, no pores, smudging, or cross hatching, there needs to be more points of identification. With prints of excellent clarity and good poroscopy, identification can be made with less points of identification.

The FBI has become more conscious of verification of print identification after the Brandon Mayfield mishap. See:

There are two types of fingerprints:

Patent Fingerprints

Patent fingerprints are visible to the naked eye under ordinary light. Visible fingerprints are patent fingerprints made by fingers touching a surface after they have been in contact with ink, paint, grease, soot, blood, or some similar substance. Plastic fingerprints are patent fingerprints left on an impressionable material such as wet paint, modeling clay, tar, putty, wax, soap, and similar materials. Patent fingerprints of either type are ordinarily readily visible to crime scene investigators, and may sometimes be photographed or lifted directly. In some situations, patent fingerprints may be treated to increase their visibility or contrast against the background surface.

Latent Fingerprints

Latent fingerprints are invisible to the naked eye under ordinary light, but can be made visible by dusting, chemical development, or an alternate light source.

What’s Alternate?

In forensics, the term alternate light source (or ALS) is used generically to describe any bright light source that emits light at a single wavelength or a narrow band of wavelengths. An ALS may emit light at any wavelength from far ultraviolet through the visible spectrum and into the far infrared. For example, a standard “black light” fluorescent tube is considered an ALS, as is a sodium- or mercury-vapor lamp.

Beginning in the 1980s, lasers became widely used forensically as ALSs. Being expensive, bulky, and limited to a single wavelength made lasers less than idea as ALSs, so in the 1990s, lasers were gradually supplanted by portable ALSs that could be configured with filters or slits to emit narrow-band light over a wide variety of selectable wavelength ranges. Nowadays, lasers are seldom used in forensics labs.

After any processing needed to reveal or enhance the print, fingerprints of either type are preserved by photographing them or by lifting them, either by carefully applying transparent lift tape to the surface that contains the print, peeling the tape from the surface, and transferring it to a card, or by electrostatic lifting, which uses an electrostatically-charged sheet of clear plastic to attract powder applied to the fingerprints. It’s at this point that the forensic scientist’s job ends and the fingerprint examiner’s job begin.

Dennis Hilliard comments

Lifting is generally done to allow a print to be photographed. If the object is mobile, the print is not lifted but preserved in place with lifting tape. If the surface is irregular, a lift is made for photographic purposes. Although I have never seen a fingerprint lifted by “electrostatic lifting,” it is often used to lift footprints in dust from flooring. In forensic laboratories or police departments that have officers trained in fingerprint examinations, it is my experience that the prints are processed and then examined by the same analyst. In Rhode Island and in many states throughout the northeastern US, the evidence suspected of having latent prints is collected by law enforcement officers. Our examiners train these officers to partially process certain types of evidence, since time is often of the essence.

Various methods are used to reveal latent prints. Some methods are non-destructive, which means if they are tried and fail to reveal prints, other methods may be used subsequently. Other methods–notably silver nitrate development and physical developer-are destructive, either in the sense that using them precludes using alternative methods to raise the prints or that using them may preclude testing the object for other types of forensic evidence, such as blood or DNA. The particular method or methods used, and the order in which they are applied, also depends upon the nature (porous, semiporous, or nonporous) and condition (e.g., wet, dry, dirty, sticky, etc.) of the surface that contains the prints, as well as the residue that constitutes the prints, such as perspiration, blood, oil, or dust.

Visual examination is always the first step in revealing latent fingerprints. Some latent prints are patent under strong, oblique lighting. Moving small objects to different angles under a fixed light source may reveal numerous prints, as may moving the light source itself when examining larger or fixed objects. Any latent prints that are revealed under oblique lighting are photographed before any subsequent treatment is attempted. Some prints revealed by visual examination may be undetectable by any other method. Done properly, visual examination is completely non-destructive. Done improperly, the handling required for visual examination may smudge or destroy prints that are invisible visually but potentially visible using other methods.

After visual examination is complete, the usual next step is to examine the specimen by using inherent fluorescence. Various components of the fingerprint residue, including perspiration, fats, and other organic components, and foreign materials present on the fingertips when the impression was made, may fluoresce under laser, ultraviolet, or other alternate light sources. In a darkened room, the questioned surface is illuminated with the alternate light source and viewed through a filter of complementary color. For example, long-wave ultraviolet (black light) tubes emit some visible light in the deep violet part of the spectrum. Viewing a surface so illuminated through a deep yellow or orange filter blocks essentially all of the reflected incident violet light, making any inherent fluorescence emitted by the fingerprint residues in the yellow through red parts of the spectrum more clearly visible. The inherent fluorescence method is usable on any surface, including surfaces that cannot be treated with powders or chemical methods, and may reveal latent prints that are not revealed by any other method. Like visual examination, examination by inherent fluorescence is non-destructive.

After visual examination and inherent fluorescence examination are complete, other methods may be used to reveal additional latent fingerprints. Fingerprint powders, iodine fuming, and silver nitrate are considered the “classic” methods, because they have been used since the 19th century. Despite their age and the availability of newer methods, all three of these methods, with some minor improvements, remain in use today.

Fingerprint Powders

Fingerprint powders are used primarily for dusting nonporous surfaces such as glass and polished metal, most commonly to reveal latent fingerprints on immovable objects at crime scenes. Powders are often used in conjunction with super glue fuming to enable a lift to be made.

A very fine powder is applied to the area that contains the latent print. The powder adheres to the residues that make up the fingerprint. Excess powder is removed by gentle brushing or using puffs of air from a syringe. After the excess powder is removed, the fingerprint is revealed and can be photographed or lifted. Fingerprint powders are available in shades from white through black, which allows the fingerprint technician to choose a powder that contrasts with the background surface. Fluorescent fingerprint powders are useful for raising prints on printed or patterned surfaces, which might otherwise make it difficult to see the pattern of the print itself.

Magnetic fingerprint powders are used with magnetic brushes, which allow excess powder to be removed without actually touching the print. Magnetic powders are often used to raise latent fingerprints on paper surfaces, an exception to the general rule about powders being used only on non-porous surfaces.

Iodine Fuming

Iodine fuming is used to reveal prints on porous and semiporous surfaces such as paper, cardboard, and unfinished wood. The object to be treated is placed in an enclosed chamber that contains a few crystals of iodine. Gently heating the crystals causes them to sublime (go from solid phase to gas phase without passing through the liquid phase). The violet iodine vapor adheres selectively to fingerprint residues, turning them orange. These orange stains are fugitive, so they must be photographed immediately. After a period ranging from a few hours to a few days, the iodine stains disappear, leaving the specimen in its original state. The developed prints can be made semi-permanent by treating them with a starch solution, which turns the orange stains blue-black. These stains persist for weeks to months, depending on storage conditions.

Iodine Spray Reagent (ISR)

Iodine spray reagent (ISR) is a liquid analog to iodine fuming. Like iodine fuming, ISR is used to reveal prints on porous and semiporous surfaces such as paper, cardboard, and unfinished wood, but ISR can be used on specimens for which fuming is impractical. ISR is made up as two stock solutions that are combined to make the working solution. Solution A (iodine) is a 0.1% w/v solution of iodine crystals in cyclohexane. Solution B (fixer) is a 12.5% w/v solution of alpha-naphthoflavone in methylene chloride. The working solution is made up by combining A:B in a 100:2 ratio, mixing thoroughly, and filtering the working solution through a facial tissue or filter paper. The working solution is sprayed onto the questioned surface, using the finest mist possible. Latent prints develop immediately and should be photographed as soon as possible.

Roll Your Own ISR

We didn’t have any alpha-naphthoflavone on hand (or any cyclohexane, for that matter), so we decided to see what we could accomplish with what we did have on-hand. Iodine has such a high affinity for the fats present in fingerprints that we thought almost any iodine solution should work, at least after a fashion. As it turned out, we were right.

We transferred a gram or so of iodine to a small spray bottle and added a few mL of lighter fluid, which formed a beautiful violet solution. Not all of the iodine dissolved, and we’d used the last of our lighter fluid, so we topped off the bottle with 70% ethanol. Iodine in ethanolic solution is brown, so we weren’t surprised to see the solution turn a deep purple-brown color. One of us then pressed his fingers to a sheet of copy paper. We sprayed that area of the paper (in the sink; spraying iodine is very messy) and used a hair dryer to evaporate the solvent. Figure 8-3 shows the results. Not ideal, certainly, but much better than what we expected.

Figure 8-3

Figure 8-3. Fingerprints revealed by spraying with an iodine solution

Silver Nitrate

Silver nitrate is also used to reveal prints on paper and similar surfaces. The surface is treated with a dilute solution of silver nitrate by spraying or immersion. The soluble silver nitrate reacts with the sodium chloride (salt) present in sweat to produce insoluble silver chloride. The surface may or may not be rinsed with water after treatment to remove excess silver nitrate. In either case, the treated surface is exposed to sunlight or an ultraviolet light source, which reduces the silver chloride to metallic silver, revealing the prints as gray-black stains. Careful observation is required to make sure the prints are not overdeveloped, particularly if the surface was not rinsed after treatment. In extreme cases of over-development, the entire surface may turn black. Silver nitrate development is destructive, and so is used only after iodine fuming and other development methods. Three variants of silver nitrate solution are used, a 1% w/v aqueous solution, a 3% w/v aqueous solution, and a 3% w/v ethanolic solution. The alcohol solution is used on surfaces such as wax paper, coated cardboard, and polystyrene foam that repel water and so cause the aqueous solutions to bead.

Silver nitrate is used last, if it is used at all, because using it precludes subsequently using any other development method. Silver nitrate may succeed where other development methods fail, because silver nitrate reacts with the non-volatile sodium chloride present in fingerprint residues. Very old fingerprints may have lost all of their volatile residues, but the sodium chloride residue remains. Silver nitrate has been used successfully to develop latent prints that are years, decades, even centuries old.


Ninhydrin was introduced in 1954 as the first of the modern fingerprint development methods. In 1910, the English organic chemist Siegfried Ruhemann synthesized ninhydrin (triketohydrindene hydrate), and reported that it reacts with amino acids to form a violet dye that was subsequently named Ruhemann’s Purple (RP). Forensic scientists must have been asleep at the switch, because it wasn’t until 44 years later that S. Oden and B. von Hoffsten reported in the March 6, 1954 issue of Nature that ninhydrin could be used to develop latent fingerprints. Although amino acids are present in only tiny amounts in fingerprint residues, RP is so intensely colored that ninhydrin development produces stark visible images of latent prints.

Like iodine fuming and silver nitrate development, ninhydrin development is most useful for prints on porous surfaces. The questioned surface is simply sprayed with or dipped in a dilute solution of ninhydrin. After a period ranging from a few minutes to several hours, the prints self-develop as purple stains. In some cases, allowing development to continue for 24 to 48 hours reveals additional latent prints. Processing can be sped up by heating and humidifying the treated surface with a steam iron. After development with ninhydrin, prints may be sprayed with a 5% w/v solution of zinc chloride in a 25:1 mixture of MTBE (methyl tert-butyl ether) and anhydrous ethanol. This reagent causes a color shift from purple to yellow-orange and makes the developed prints fluoresce under an ALS.

Two variants of ninhydrin solution are used, depending on the surface to be treated. The standard formulation is a 0.5% w/v solution of ninhydrin in a 3:4:93 mixture of methanol:isopropanol:petroleum ether. The alternate formulation is a 0.6% w/v solution of ninhydrin in acetone.


DFO, also known by its chemical name of 1,8-diazafluoren-9-one, operates by the same mechanism as ninhydrin, reacting with amino acids in fingerprint residues to form visible stains. DFO stains are much fainter than those produced by ninhydrin, but DFO stains fluoresce directly, without after-treatment. DFO was popularized by UK police forces, and is still more widely used in British Commonwealth countries than elsewhere. DFO is somewhat controversial. Many experts maintain that DFO is more sensitive than ninhydrin and provides superior detail. Other experts have questioned the reliability of DFO, and prefer to use ninhydrin alone.

If DFO is used, it must be used before ninhydrin, PD, or silver nitrate is applied. DFO reagent is a 0.05% solution of DFO crystals in a solution made up of methanol:ethyl acetate:acetic acid:petroleum ether in a 20:20:4:164 ratio. DFO is applied by spraying or immersion, followed by drying, retreating the surface, drying again, heating the treated surface to 50 °C to 100 °C for 10 to 20 minutes, and finally by viewing under an alternate light source at 495 nm to 550 nm. Raised prints are photographed through an orange filter. Because DFO reagent is expensive and the required procedure is complex and time-consuming, DFO treatment is used less often than it might otherwise be.

Sudan Black

Sudan black is a dye that reacts with the sebaceous perspiration component of fingerprints to form a blue-black stain. Sudan black is used primarily for wet surfaces, including those contaminated with beverages, oil, grease, or foods, and is also useful for post-processing of cyanoacrylate-fumed prints, particularly those on the interior of latex or rubber gloves. The Sudan black reagent is simply a 1% w/v solution of Sudan black in 60% to 70% ethanol, although it is usually made up by dissolving the solid dye in 95% ethanol and then adding distilled water to reduce the ethanol concentration to 60% to 70%. In use, the questioned surface is immersed in the Sudan black solution for about two minutes and then rinsed gently with water. Raised prints are visible as blue-black stains.

Blood Reagents

Blood reagents are used to develop latent prints and enhance visible prints that include blood in the fingerprint residues. These agents are also commonly used to reveal latent bloodstained footprints, hand marks, and so on.

Amido black, the oldest of these reagents, is a dye that stains proteins in blood residues blue-black. Several variants of amido black reagent are used. The most common is a 0.2% w/v solution of amido black in a 9:1 mixture of methanol:acetic acid. The questioned surface is sprayed with or immersed in this solution and allowed to soak for 30 seconds to one minute, after which it is rinsed with a 9:1 methanol:acetic acid solution. The dye/rinse procedure can be repeated to increase contrast. After a final rinse with water, the specimen is dried and photographed. Surfaces that are likely to be damaged by methanol can be treated with an alternate formulation that contains 0.3% amido black w/v, 0.3% sodium carbonate w/v, and 2.0% 5-sulfosalicylic acid w/v in an 89:5:5:1.25 mixture of water:acetic acid:formic acid:Kodak Photo-Flo 600. Questioned surfaces are sprayed with or immersed in this solution for three to five minutes and then rinsed with water. Again, the treatment can be repeated to increase contrast. An alternate water-based amido black reagent can be made up that is 0.2% w/v with respect to amido black and 1.9% w/v with respect to citric acid in a 998:2 mixture of water:Kodak Photo-Flo 600. The questioned surface is sprayed with or immersed in this solution and allowed to soak for 30 seconds to one minute, after which it is rinsed with water. Repeated treatments may be used to increase contrast.

Leucocrystal violet (LCV) is an alternative to amido black, and provides similar results. LCV reagent is a solution in 3% hydrogen peroxide that is 0.2% w/v with respect to LCV, 2.0% w/v with respect to 5-sulfosalicylic acid, and 0.74% w/v with respect to sodium acetate. When this solution is sprayed on a questioned surface, the prints develop in about 30 seconds, after which the surface is blotted dry and photographed.

Coomassie brilliant blue is another alternative to amido black that provides similar results. Coomassie brilliant blue reagent is a 0.1% solution of Coomassie brilliant blue R dye in a 2:9:9 mixture of acetic acid:methanol:water. When this solution is sprayed on a questioned surface, the prints develop in 30 to 90 seconds, after which the surface is rinsed with a 2:9:9 solution of acetic acid:methanol:water. Repeated treatments may be used to increase contrast. After the final treatment, the surface is rinsed with distilled water, dried, and photographed.

Crowle’s double stain is still another alternative to amido black that also provides similar results. As you might expect, the developer solution uses two dyes. It contains 0.015% w/v Coomassie brilliant blue R and 0.25% crocein scarlet 7B in a 3:5:92 mixture of trichloroacetic acid:acetic acid:water. The questioned surface is sprayed with or immersed in this solution, allowed to soak for 30 to 90 seconds, and then rinsed with a 3:97 mixture of acetic acid:water. After the final treatment, the surface is rinsed with water, dried, and photographed.

DAB, also known by its chemical name of 3,3′-diaminobenzidine tetrahydrochloride, is the newest of the blood reagents. The DAB method is relatively complicated and expensive, but it sometimes provides usable results where no other method works. The DAB process requires four reagents. Solution A, the fixer, is a 2% w/v solution of 5-sulfosalicylic acid in distilled water. Solution B, the buffer, is a 1:8 mixture of 1 M pH 7.4 phosphate buffer solution in distilled water. Solution C is a 1% w/v solution of DAB in distilled water. Developer is made up by combining 180 parts by volume of Solution B with 20 parts Solution C and one part 30% hydrogen peroxide.

Prints may be developed by the DAB submersion method or the DAB tissue method, depending on which is better suited for the specimen. For the submersion method, the specimen is soaked for 3 to 5 minutes in a tray of Solution A to fix the prints, followed by rinsing for 30 seconds to one minute in a tray of distilled water. After the first rinse, the specimen is soaked for up to five minutes in a tray of developer, until maximum contrast is achieved. After a final rinse in distilled water, the specimen is air-dried or dried with a hair dryer, after which it is photographed. For the tissue method, the surface is covered with an unscented facial tissue or a thin paper towel, which is then sprayed with Solution A and allowed to soak for three to five minutes. The tissue is removed and the area is rinsed for 30 seconds to one minute with distilled water. A new tissue is placed over the subject area, saturated with developer, and allowed to soak for up to five minutes. When development is complete, the area is rinsed thoroughly with distilled water, dried, and photographed.

Small Particle Reagent (SPR)

Small particle reagent (SPR) is a liquid suspension of solid particles of dark gray molybdenum disulfide, applied to the questioned surface by spraying or dipping. SPR works in the same way as fingerprint powders-by physical adhesion of particles to fatty fingerprint residues-but unlike dry fingerprint powders, SPR can be used for processing wet surfaces, including surfaces soaked in liquid accelerants and other organic solvents. SPR is also used on glossy nonporous surfaces such as glass and plastic, coated glossy papers, and surfaces covered with glossy paint. Prints raised by SPR are extremely fragile, and should be photographed before any attempt is made to lift them. Modified versions of SPR are available in white, gray, and fluorescent forms, all of which are based on chemicals other than molybdenum disulfide.

Roll Your Own SPR

Although SPR is readily available from forensic suppliers, you may want to try making your own. To do so, add about 5 g of molybdenum disulfide (dry powder lubricant sold as Moly Lube and similar trade names) to about 100 mL of water to which you’ve added 1 mL of liquid dish washing detergent. Agitate this suspension thoroughly before use, and apply it by spraying or dipping. Allow the SPR to act for one minute, and then rinse the surface gently with water. We tried this ad hoc method, and it actually kind of worked.

Super Glue Fuming

Super Glue fuming, also called cyanoacrylate fuming from the primary component of Super Glue, was discovered by accident in 1976 when Masao Soba noticed white fingerprints on the surface of a super glue container. Frank Kendall improved the process and adapted it to latent fingerprint development, reporting his findings in a 1980 paper. Since that time, Super Glue fuming has become one of the most frequently-used latent print development processes.

Super Glue fuming is used to develop latent prints on nonporous glossy surfaces such as glass, plastic, and polished metal. Like dusting, cyanoacrylate fuming is a physical process. Cyanoacrylate vapor is selectively attracted to fingerprint residues, where it builds up as a crystalline white deposit. The developed latent prints may be photographed as is, or may be dusted or treated with various dyes that enhance the visibility and contrast of the prints. Although the exact mechanism remains unknown, it’s suspected that the super glue fumes are catalyzed by the tiny amount of moisture attracted by the sodium chloride residues in latent fingerprints.

The standard method for Super Glue fuming is to place the object to be fumed in an enclosed chamber (aquariums are often used) that contains a small electric heater. An aluminum weighing boat is placed on the heater, and the temperature set to high. When the boat is hot, a few mL of cyanoacrylate are added to the boat. Fuming commences immediately, and is ordinarily complete after 30 seconds to 10 or 15 minutes. Alternatively, a cotton ball can be soaked in 0.5 M sodium hydroxide and allowed to dry. Once dry, the cotton ball is placed in the chamber and moistened with a few drops of cyanoacrylate. Fuming begins within a few seconds and is allowed to continue until the latent prints become visible.

Super Glue fumed prints can be photographed directly, or treated with dyes to increase the visibility and contrast of the prints and make them easier to see against a patterned background surface. With the exception of Sudan black, most of these dyes are fluorescent, with absorption wavelengths that match to the emission wavelengths of commonly-used forensic alternate light sources. For example, rhodamine 6G may be excited with a light source from 495 nm (blue) to 540 nm (green-yellow), with maximum absorption at 525 nm (green). Rhodamine 6G fluoresces in the range of 555 nm (yellow-green) to 585 nm (orange), with maximum emission at 566 nm (yellow). By viewing the treated surface through a filter that blocks wavelengths below about 555 nm but passes longer wavelengths, the treated fingerprints are visible as a yellow glow against a dark background.

Various fluorescent dyes are used alone or in combination to post-process super glue fumed prints. Standard individual dyes include rhodamine 6G, Ardrox, 7-(p-methoxybenzylamino)-4-nitrobenz-2-oxa-1,3-diazole (MBD for short), basic yellow 40, safranin O, and thenoyl europium chelate. The most commonly used combination is RAM, a mixture of rhodamine 6G, Ardrox P133D, and MBD. Other mixtures are also used, including RAY (rhodamine 6G, Ardrox, and basic yellow 40), and MRM 10 (MBD, rhodamine 6G, and basic yellow 40).

Physical Developer (PD)

Physical developer (PD) is useful for developing latent fingerprints on most porous surfaces and some nonporous surfaces. It is particularly useful for revealing latent prints on paper currency, paper bags, and porous surfaces that have been wet. PD is a destructive process, and so is always used last if at all. PD is an alternative to the silver nitrate method. You can use one or the other, but not both. Whichever you use must be the last method you apply. PD is normally used after DFO and/or ninhydrin, and often reveals latent prints that neither of these methods revealed.

The “physical” part of the name is a misnomer. PD is not a physical process (like dusting), but a chemical one. It depends on a redox reaction that reduces silver ions to metallic silver, which stain the latent fingerprints a gray-black color. The PD working solution is unstable, in the sense that it must be used immediately after it is made up, but it is this very instability that allows PD to work as well as it does. PD is expensive, complex, finicky, destructive, and requires a great deal of experience to get good results. Despite these criticisms, PD is used because it often gets results when no other method works. For this reason, many forensics labs routinely use PD as the final step in processing latent prints.

The PD process requires four solutions, with a fifth solution optional. Solution A is a 2.5% w/v solution of maleic acid in distilled water. Solution B (redox solution) is an aqueous solution that is 3% w/v with respect to ferric nitrate, 8% w/v with respect to ferrous ammonium sulfate, and 2% w/v with respect to citric acid. Solution C (detergent) is an aqueous solution that is 0.3% w/v with respect to n-dodecylamine acetate and 0.4% w/v with respect to Synperonic-N. Solution D is a 20% w/v solution of silver nitrate. Solution E (bleach) is a 1:1 mixture of standard chlorine laundry bleach with water.

The specimen to be treated is first placed in a tray of solution A and agitated for five minutes or until any bubbling has ceased, whichever is longer. The specimen is then transferred to a second tray that contains the working PD redox solution, made by combining solutions B:C:D in a 100:4:5 ratio and mixing thoroughly. The specimen is soaked in the working PD redox solution for 5 to 15 minutes, with constant agitation, after which it is rinsed thoroughly with water, air-dried or dried with a hair dryer, and photographed.

Bleach solution may be applied at the operator’s discretion as a final step. This solution darkens the developed prints, lightens the background, and removes ninhydrin stains, but may also eliminate detail that is visible before bleaching. The specimen is simply dipped in the bleach solution for about 15 seconds and then rinsed, dried, and photographed. It’s important that the final rinse be thorough, because otherwise the specimen may degrade very quickly.

Adhesive Surface Techniques

The FBI uses the phrase adhesive surface techniques to describe four processing methods used to raise latent prints on sticky surfaces such as the adhesive side of sticky tapes, sticky labels, peel-and-stick plastics, and so on. Surprisingly, three of the four methods–alternate black powder, ash gray powder, and sticky-side powder–are powder based. (One might think that these powders would adhere equally well to the adhesive and the fingerprint residues, but they do not.) All three powders are applied in the same way–made into a thin paste, brushed onto the questioned surface, and rinsed off with cold water–and provide similar results. The primary differences among these three powders are their colors, chosen to provide contrast against different surface colors. The final method, a 0.1% w/v solution of gentian violet (also called crystal violet) in water. The questioned surface is sprayed with or immersed in the gentian violet solution for one to two minutes, after which it is removed and rinsed with cold water. The gentian violet stains the latent prints, which can then be viewed and photographed under ordinary light.

Vacuum Metal Deposition (VMD)

Vacuum metal deposition (VMD) is similar conceptually to cyanoacrylate fuming, but substitutes metal vapor for cyanoacrylate vapor. The questioned surface is placed in a chamber from which the air is evacuated. The chamber also contains small pieces of gold and zinc that can be heated electrically until they vaporize. The specimen is exposed first to gold vapor, and then to zinc vapor. The metal vapors adhere selectively to fingerprint residues, revealing latent prints as metal-plated traces on a pristine substrate. Because VMD requires expensive equipment and materials, its use is limited to well-equipped and well-funded forensics labs.

The FBI categorizes these processes as Standard (used routinely) or Optional (used only in special situations or as supplements to Standard processes), as follows:

Standard Processes

Adhesive surface techniques (alternate black powder, ash gray powder, gentian violet, and sticky-side powder), amido black (methanol base), amido black (water base – Fischer 98), DAB, DFO, fingerprint powders, iodine fuming, ISR, LCV, ninhydrin (petroleum ether base), PD, RAM, silver nitrate, Sudan black, Super Glue fuming, and VMD.

Optional Processes

Amido black (water base), Coomassie brilliant blue, Crowle’s double stain, fluorescent super glue dyes (Ardrox, MBD, MRM 10, rhodamine 6G, safranin O, and thenoyl europium chelate), Liqui-Drox, and ninhydrin (acetone base).

Obviously, from this plethora of techniques–and these are only the most popular of a larger group–we needed to choose a subset for the lab sessions in this section. We settled on iodine fuming, silver nitrate, ninhydrin, and Super Glue fuming, which happen to be the Big Four in real forensics labs and also have the advantage of being inexpensive and use materials that are relatively easy to acquire. We’ll also use gentian violet to develop prints on cellophane tape, and dusting to develop prints on glass. Finally, we’ll use two liquids found in most homes to reveal latent fingerprints on brass cartridge cases.

August 16, 2009

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