Magnetic Powders: Types, Principle, Fingerprint Development

Magnetic powders are fine ferromagnetic powders applied to the latent fingerprint surface with a magnetic wand. It is a type of metallic powder that was introduced by MacDonell in 1962 along with Magna Brush.

Note: Metallic powder is different from Magnetic Powders. All magnetic powders (eg: iron) are metallic powder, but all metallic powder isn’t magnetics (eg; fine lead, bronze, etc.). Additionally, some metallic powder is dust using a glass fiber brush instead of a magnetic wand.

Working Principle of Magnetic Powders

Working: When the magnetic powder comes under the influence of a magnetic field it forms a brush that is used to dust the prints over the surface.

And when a magnetic powder is applied over the latent fingermarks, it selectively adheres to the latent prints due to its affinity for oils and moisture, leaving the substrate “paint-free.”

History And Development of Magnetic Fingerprint Powders

1962: MacDonell introduced Magna Brush as the applicator for magnetic powders. It forms a brush-like structure at the tip because of the magnetic field.
1970s: Aluminum flake powder was introduced and became the primary powdering technique for many agencies and police.
1991: James et al. developed ‘Magneta Flake’, a magnetic flake powder that has both characteristics of magnetic flakes and granular powders.
1993: James shows how Magneta Flake’s platelike iron particles are more efficient than iron grit powders.
1996: Whitfield trials and field work state how magnetic powders can develop fingerprints where non-conventional fingerprint powders don’t work.
2004-07: Bandey, Gibson, and Hardy reported their studies based on the compressive comparative studies among metal flakes, granular, magnetic flakes, and magnetic granular powder on various surfaces.
2010: Nag et al develop dark iron magnetic flake powder that serves better contrast on lighter backgrounds.

How Magnetic Powders are Made?

Magnetic powders are made by the process of grinding and milling the iron grit particles to produce granular and flakes of magnetic powder respectively.

Components and Composition of Magnetic Powders

In any magnetic powder, there are three components: pigment and binders. They work together to provide adhesion to latent fingermarks without painting the whole surface.

Pigments: Non-magnetic flakes of copper or aluminum are used to add color and contrast to latent prints. The particle size ranges from 10–20 μm in diameter.
Carriers: Magnetic iron carrier for preferential adhesion to a latent print residue with particle size around 50 μm in diameter.
Surface Enhancers: Stearic acid promotes a smoother and more reflective powder surface and improves adhesion. They are added during the milling process. They are used for the production of flake powders.

Note: James et al. (1991a, 1991b) demonstrated how stearic acid coating on magnetic flake powder enhances the lipophilic interaction with fatty acid of fingermarks. The optimum coating thickness stated to be 70nm.

Processes In Manufacturing of Magnetic Fingerprint Powders

Grinding: For the production of granular particles of metal and their oxides of various sizes. They are made by smoothing the rough surface of coarse iron grit to form granular particles.
Milling: For the production of Flakes powders with surface enhancements like stearic acid. They are produced by passing the iron grit through a ball mill to produce iron flakes.

When Magnetic Powders Are Used? And on What Surfaces?

Magnetic powders are used for the development and recovery of latent prints on any non-porous and semi-porous surfaces. It is also proven that they can develop good fingermarks even on leather, plastics, walls, and human skin.

You can say, they can replace conventional non-magnetic powders. The major distinctive difference lies when it comes to texture surfaces such as oranges, non-polished woods, and leather, that have grooves or depressions over their surfaces.

With a textured surface, magnetic powders are more prone to stick to grooved (depressed) areas with a single stroke with minimal pressure. Conversely, the non-magnetic powders used relatively hair brushes that cause more resistance to grooved surfaces resulting in fingermarks disruption.

This is the same reason why fingerprints on highly polished surfaces are dusted with magnetic powders.

How to Choose Magnetic Powders Based on Surfaces?

Surfaces	Preferred Powder	Reason
Paper	Non-magnetic powder and chemical method	Paper soaked finger marks residue and can’t be destroyed with relatively heavy brush strokes
Texture Surfaces	Magnetic powders	Prints on grooves can easily be distributed with heavy brushstrokes of non-magnetic powder and
Smooth surface	Magnetic and non-magnetic powder	Best for practicing developing fingermarks
Highly Smooth Surfaces	Magnetic powders	Very less pressure and resistance are required with a magnetic applicator. And even a single heavy stroke of brush can remove the prints
Metal and metallic tapes	Non-magnetic powders	Magnetic field disruption and immovable magnetic applicator
Wood Surfaces	Magnetic powders	Grooves can easily be developed without much resistant
Dark surfaces	Gray/fluorescent magnetic or non-magnetic powders	Better contrast
Gloves	Magnetic powders	Comparatively less pressure and resistance to fingermarks.
Ceramic Cups	Magnetic powders	Bailey reported that magnetic powder outperformed non-magnetic powder (Source)
Vertical Surfaces	Non-magnetic powders	Magnetic powder tends to drop off with strokes.
Horizontal Surfaces	Magnetic and Non-magnetic powders	Magnetic powder is relatively easy to apply

Types of Fingerprint Powder For Latent Fingerprint Development

In general, there are four types of magnetic fingerprint powder for the development of finger marks on different surfaces. These are:

Magnetic Granular powders
Magnetic Flake powders
Magnetic Fluorescent powders
Thermoplastic Powders With Iron Base

1. Magnetic Granular Fingerprint Powders

Manufacturing Process and Composition: These powders have large magnetic carrier particles of metal (iron, 20–200 μm), small non-magnetic particles of metal oxides (Fe₃O₄, 3-12 μm), and pigment. Smaller non-magnetic pigmented particles adhere to fingermark ridges and develop fingermarks.

Surfaces: These magnetic granular latent fingerprint powders are most effective on textured and polyvinyl chloride (PVC) plastic surfaces.

The most common pigmented granular magnetic powders are:

Dark colored: Black and Jet black
Light colored: Gray, gray-white, white (rare) color

They are typically used over non-porous light-colored surfaces. However, for darker textured surfaces, white-gray powder can be used.

2. Magnetic Flake Powders

Rather than rounded particles, they have a flakes structure. These flat, platelike particles are more efficient for fingermark development than the iron grit normally employed in commercial magnetic powders

The first magnetic flake powder was developed by James et al, in 1991. They are usually available in lighter colors.

Manufacturing Process: Magnetic flake powders are produced by milling spherical carbonyl iron with 3 to 5% of stearic acid in a suitable solvent to produce smooth-edged flakes. Their sizes usually range from 10 to 60μm.

Surfaces: Mostly employed in the development of smooth and dark textured surfaces.

3. Magnetic Fluorescent Fingerprint Powders

These magnetic powder fluorescence in the spectrum ranges from 450 nm to 570 nm. Commercially, they are available in red and green colors. The particles have granular geometry rather than flakes.

Surfaces: For the development of fingermarks over multicolored plastic bags, glass, and non-porous smooth surfaces.

4. Thermoplastic Powders

Thermoplastic powders are essentially photocopy toners that are used to visualize and develop fingermarks on paper tissues. It was first discussed for forensic purposes in 1967 by Jones RG in his research paper “Fused Fingerprints” without any magnetic component.

But in 1982, Ball R. uses a modified mixture of iron-based magnetic photocopy toner with a magnetic brush for visualizing latent fingermarks.

Read More:

What’s Inside a Magnetic Fingerprint Powder Kit?

In a magnetic fingerprint powder kit, there are:

Black magnetic powder
Gray-white magnetic powder
Fluorescent magnetic fingerprint powder
Magnetic wand (Magna brush)
Fingerprint Lifters: Latent lift cards with black or white background, Gel lifting card, and casting materials.
Writing Implements include markers.
Observing light: UV light or alternative light. Both for visualizing normal prints and fluorescent magnetic powder development.

Please note: Magnetic Fingerprint Powder Kit is not much different than regular fingerprint development kit. Instead of only magnetic powders they also have non-magnetic powders and fingerprint brushes. Some fingerprint kits also have chemical developer liquids.

How To Develop Latent Fingerprints Using Magnetic Powders?

The following process should be followed for the enhancement and development of fingerprints using magnetic powders for forensic purposes:

Step 1: Identify the Latent Fingerprints For Development

Use alternative light to find the prints that need to be processed. Examiners should also add identifier tags on each item with fingermarks.

Step 2: Photography Under Oblique Lighting

Before beginning the development phase, photographs should be taken with oblique lighting (and alternate lighting). Each set of photographs should include at least two of the following:

Wide area photography of the impression with identification tags and scale.
Close-up photography of fingermarks
Location of the item
Condition of the item (as necessary if wet, contains possible blood, etc.)

Step 3: Application of Magna Brush on Magnetic Powder

Magna brush (magnetic wand) has a small magnetic with a de-magnetized rod, which on pull/push releases the attached magnetic powder. A magnetic brush should be brought to the magnetic powder container that will attract powder and form a ‘brush’.

Note: Magnetic powder on the brush should not be hung in the air as it tends to fall easily. Try to bring a magnetic container and Magna brush close to the impression.

Step 4: Applying Magnetic Powder on Fingerprint Surfaces

Magna brush with magnetic powder is applied to the surface, avoiding any direct contact of the magnetic applicator to the surface.

Magnetic powder is the only part that should physically touch the surface. This minimizes the possibility of damaging the finger marks by scratches to the surface and degrading ridge details.

Step 5: Re-Photography After Developing Finger Impressions

Once the development completes, the fingerprints should again be photographed. If the fluorescent magnetic powder is used, an alternative light source could be used for the development process. Photographs of ridge details contain:

Close-up photography of ridge details
Mid-range photography

Step 6: Lifting of Magnetic Powder Impressions

Lifting fingerprints should be avoided in case, the material can be transportable, or a semi-porous surface like paper (captures paper fibers too). Following are the lifting techniques that are used at the laboratory as well as crime scene searches.

Surface	Types	Lifters	Procedure
Flat-smooth	Plastic, polished wood	Card lifters	Scoop lifters from one side and to another.
Non-flat smooth	Doorknob	Tape lifters	Place on one side and first make a circle in between and then scoping towards edges.
Flat Textured	Unpolished wood	Gel tape lifters	Place gel tape on a textured surface and press it over the surface (gel fills the grooved surface)
Non-flat textured	Orange fruit	Casting Materials	Using a syringe and nozzle shouldn’t touch the impression surface.

Before removing the lifting, sweep your finger across the tape and press out any air bubbles.

Note: In case of lifting visible prints, avoid lifting them using lifters. This is because lifters work better to lift the powdered prints from the surface without visible fingerprint residue. If possible, after photographing, items should be packed for laboratory analysis.

Step 7: Final Photography

The wrapup photography should include the casted material over the surface along with prepared casted prints. In each step, take close-up photography of each element of the surface after removing the cast. Photographs of ridge details contain:

Close-up photography of ridge details
Mid-range photography
Identifier tags, (not common)
Case details, timing, date, etc. (should be).
Processing and development techniques used
Sketch and orientation if utilizing lift cards

Step 8: Packaging of Developed Fingermarks

Following are the surface packaging considerations.

Non-porous surfaces should always be processed prior to packaging.
Lift cards can be packaged together in envelopes.
Packaging lift cards should be organized: prints from one area should be packed in different envelopes. Eg: The crime scene has multiple prints on different cars. So, pack prints from each car in separate envelopes.
Avoiding wrapping evidence in plastic as it most probably destroys the latent prints.
In the case of cast fingerprint impressions from texture surfaces, the cast should be labeled with identifiers.
Try to use cardboard containers with segments for different pieces of evidence.
Lastly, label the packed containers with the investigator’s initials, casting time, and date.

Can I Use Magnetic Fingerprint Powder Without Magnetic Brush?

Yes, you can. If you don’t have a magnetic brush, such as a Magna Brush, you can definitely use a regular non-magnetic fingerprint brush. Animal-haired brushes with mop-style fiber mounts are the most frequent and best alternative for magnetic brushes with magnetic powder.

For a smooth surface, you should use camel-haired brushes over squirrel fibers because they are less rough and allows you to get more details over smooth surfaces. However, in any case, magnetic brushes work better with magnetic powder.

Advantages of Magnetic Fingerprint Powder

Because it does not require physical brushing (magnetic powder forms brush), very little pressure is applied in comparison to non-fingerprint brushing, making it easy to develop marks without damaging it.
Extensively used powder for development on horizontal surfaces.
By passing a magnetic wand over fingerprints, the excess powder can be removed easily and less wastage is seen.
Easy to clean up in case of a spill.
Easier to learn skills than with conventional non-magnetic powders.

Disadvantages of Magnetic Fingerprint Powder

Difficult to apply on vertical surfaces, as they tend to fall more profoundly than a regular brush that uses non-magnetic powders.
Limited color options.
Not applicable to magnetic surfaces (but you can use animal hair brushes with magnetic powder).

References

Fingerprints: Analysis and Understanding the Science [Book]
Advances in fingerprint detection [Link]
Possibility of using fingerprint powders for development of old fingerprints [PDF]
Magnetic fingerprint powder on firearms and metal cartridges [Link]
Friction Ridge Skin-Comparison and Identification of Fingerprints, James F. Cowger
Magnetic flake powders for fingerprint development [link]
Forensic Science: An Introduction to Scientific and Investigative Techniques, by Nordby.
Fundamentals of Forensic Science, Academic Press by Jay A. Siegel
Criminalistics An Introduction to Forensic Science by Richard Saferstein