Paint as a Evidence in a Forensic Investigation
Paint as evidence falls into two main categories just as any other type of evidence does. Paint can simply indicate the "class" from which it comes. This could mean differentiating between automobile paint, house paint, nail polish and other types of paint. This is usually done through very detailed chemical analysis of the various layers of paint and physical examination of paint samples. Although various manufacturers may use the same supplier of paint the layers and colors may vary from manufacturer to manufacturer (Saferstein, 2005).
Paint can also indicate individual characteristics. This is only truly possible when there is an exact match of the edges of two paint chips. However, when the physical properties such as the color and number of layers and the chemical and biological profiles show multiple points of similarity with reference sample the probability of a common source increases.
As with any evidence there is rarely an absolute match. The best the investigator can do is to establish so many points of similarity between two samples that the probability of the two samples having different points of origin becomes statistically minute.
Paint chips and paint residue is one of the most common types of trace evidence which can be found at a crime scene. The most common use of paint transfer is in the case of an automobile accident. However, there are millions of places where painted surfaces can be found. Anytime these painted surfaces come in contact with another solid surface some small amount of transfer occurs. Of course this is in the case of dried paint.
Paint smears from wet paint are another source of evidence. Smears from wet paint may transfer onto anything that touches it. Wet paint which has been disturbed is a good source of hair and fiber evidence. Wet paint may also have finger or palm prints impressed into it.
There are various types of paint. There is automobile paint, house paint( exterior and interior), paints artist's use, nail polish and paints used on ceramics just to name a few. As I started to write this article I began noticing all of the things around me which were painted. The list is mind bogglingly long and holds massive potential for use in forensic examinations.
The most common use of paint chips and transfer is in the case of automobile accidents. Paint transfer is also used in cases of burglary where the tools used to break into a home or vehicle has trace evidence in the form of paint on them (Saferstein, 2005). Any type of weapon you can imagine which has been used in a violent altercation where it impacted walls, vehicles and other painted surfaces may have trace evidence in the form of paint on it. Bullets which strike walls, vehicles and other painted objects may also have trace evidence in the form or paint on them. The source of the paint on any of these things can help an investigator reconstruct a crime.
Paint is a mixture of organic and inorganic substances. The organic component is usually the pigment contained in the paint which gives it its color. The "binder" holds the substances together. A "solvent" is added to paint to give its liquid form (Saferstein, 2005). Solvent evaporates in air and allows the paint to convert to a solid. Dried paint contains the binder which is usually inorganic and the pigments which are usually organic but can be either.
In the case of a car accident invariably there is paint transfer. The transfer can be from one vehicle to the other or from a vehicle to any object the vehicle may have struck. Dual transfer is when vehicle paint is transferred to the object it strikes and paint from the object is transferred to the vehicle. Vehicles are especially good for recovering paint transfer because the underlying metal of the vehicle bends on impact. This causes the overlying paint layer to separate from the metal and fall away as chips. Sometimes if the impact occurs on an angle some of the paint is transferred from one vehicle to another object or vehicle by the friction between the two objects. To see this just go through any drive through lane at a fast food restaurant and look at the pole next to the window. It is usually covered with paint transfer where vehicle have scrapped up against it.
Auto manufactures use different numbers of coatings and primers which can help identify the make and model of a vehicle. Most manufacturers use a minimum of fours coatings. The first coat is the Electro coat Primer which is bonded to the raw metal. This provides a surface to which the following layers of paint can bind. This first layer also creates a smooth surface which hides imperfections in the metal (Saferstein, 2005). This can also be useful in the case of damage caused by a hit and run or bullet hole which has been concealed. Even if the repainted areas of the vehicle are repainted with the same colored topcoat as used by the manufacturer it is unlikely the same electro coat formulation will be used.
The second coat is the Primer surfacer which also helps hide seams and other defects. This layer is pigmented to hide the color of the electro primer. This layer is usually matched to the top coat color. Grey is used for light colors. Red is used for dark colors. Even if there is little evidence of the topcoat color the Primer color can be used to indicate the shade of the vehicle (Saferstein, 2005).
The third layer of paint is the Basecoat. This layer is the layer which gives the vehicle its color. In older vehicle the pigmentation may be made from various heavy metals. Due to environmental and health concerns modern vehicles pigmentation is organic in nature. Some manufacturers may use metal oxides, aluminum or crushed glass to create a sparkly appearance (Saferstein, 2005).
The final layer of paint is the Clear coat. This layer is not pigmented. The clear coat protects the underlying layers from scratches and damage from the sun and rain as well as salt and abrasion from blowing sand (Saferstein, 2005).
When examining paint chips from a vehicle the number of layers used and the sequence in which they are applied can help determine the make and model of the vehicle. As well, when examining two samples such as a known and a questioned sample the number and sequence of the layers can help determine whether the sample could be from the same source.
Physical matches are rare but do occur on occasion. A physical match is when one paint chip from a known source is matched to a sample from a suspects' car, clothing or weapon etc. The match is made by comparing the edges of two samples with a stereomicroscope. This allows the two samples to be compared side by side. Just as the striations on a bullet are compared so are the edges of two paint samples. Those that fit together like a jigsaw puzzle can be considered exact matches (Saferstein, 2005).
Examining the Evidence
The very first thing an examiner will do is to review the method used to collect and store the sample and the chain of custody. It is important to determine that there was no possible contamination of the sample. If all appears to be correct then the examiner will consider the sample size, the number of samples and the environmental factors which may have affected the condition of the sample. The examiner will review the specifics of the case and determine what tests are required to answer the specific questions in the case. (U.S. Dept. of Justice (FBI) Forensic Science Communications, July 1999 Vol. 1 No#2, Scientific Working Group on Material Analysis).
Other methods of physical examination start with a visual comparison of the colors of the known and questioned samples. If the two samples appear to be of the same color and shade then the layers can be examined individually. If the colors and shades do not seem to match the investigator may choose to end the analysis and try a different sample. It should be noted that differences in the color, number of layers and the thickness of a same can occur in samples from the same source. To reduce the possibility of this occurring, the reference sample should be taken from as close to but not on the damaged area (Saferstein, 2005). Samples on solid objects are removed using a scalpel to peel away the paint. This can be done in the field as in the case of vehicles. When removable object such as clothing, weapons or tools are involved they can be packaged and shipped to the lab and processed there. Other sources of questioned samples can include tools, floors walls, buildings, fabrics skin and fingernails (U.S. Dept. of Justice (FBI) Forensic Science Communications, July 1999 Vol. 1 No#2, Scientific Working Group on Material Analysis). The sample should be taken before any processing with lift tape of fingerprinting powders are used since these things can change the chemical profile of the sample.
If microscopic observation of the layers and color of the sample does not excluded the sample the examiner may choose to proceed with further microscopic examination. Other methods may include the use of a Scanning Electron Microscope.
Another method to test the similarity of two samples is to apply variety chemical solvents. These chemicals are added to each layer of questioned sample and reference and the reaction is noted (Trace Evidence Procedures Manual, www.dfs.virgin.gov/services/traceevidence).
Chemical and Biological Analysis
The more common way to link one sample to another is through chemical analysis of the various components of the paint. The purpose of this type of analysis is to find as many points of similarity as possible in both the organic and inorganic components.
The most common type of analysis is pyrolysis gas chromatography. This procedure primarily tested the chemical composition of the binder in the paint rather than the organic pigments. In this process the paint is heated until it reaches a gaseous state it is then passed through a column which separates the various components. The components are passed through a detector and a graph, know as a pyrogram, and of the various components are created. This system can help distinguish paints of similar color but different chemical composition (Saferstein, 2005).
This test is not always definitive because the binders used by the manufacturer may vary due to cost and supply even if the color remains consistent (Saferstein, 2005).
Other tests include the use of a Scanning Electron Microscope. An Electron beam is passed over a sample producing backscatter, secondary electrons and x-ray emissions. The x-ray component of the scan indicates the specific elements present while the x-ray defraction can identify crystal forms of pigment (U.S. Dept. of Justice (FBI) Forensic Science Communications, July 1999 Vol. 1 No#2, Scientific Working Group on Material Analysis).
Infra red Spectrophotometry can be used to identify the binder composition of each layer by observing how the sample absorbs various wave lengths infrared light (Saferstein, 2005).
Using these methods among others the investigator compiles a physical, chemical and biological profile of the sample.
Most of the methods of chemical analysis are scientifically complicated and require sophisticated equipment. Here is an experiment you can do which demonstrates an easy way to distinguish one color from another.
1. Choose three or more water soluble markers (usually for kids). The three should all be the same color but different brands.
2. Fill three glasses of equal size with an inch of water.
3. Cut three 4 inch by 1 inch strips of coffee filter paper. Draw a line with pencil 1/2 inch from the top and bottom of the strip.
4. Place a dot of ink from each marker just under the line at the bottom of the strip. Above the top line pierce the paper with a pencil or skewer of some kind. Suspend the paper in the water so that the 1/4 inch is in the water. Make sure the ink dot is not in the water. Keep track of which brand of marker ink is in which glass. It is easiest to just place the pen you used next to the glass.
5. Wait about 15 minutes and remove the strips. Lay them on a counter top next to the marker it correlates to in order to let it dry. (Do not put them on an absorbent surface like paper towel.)
What you will see is that the organic pigment used to make the shade of marker you chose have separated into the various colors used. Each manufacturer uses a slightly different formulation. Say for instance that the color your chose was green. Some manufacturers use a mix of green, yellow and brown while others might use green, blue yellow.
6. Once the strips are dry measure the distance from the bottom line to the top of the first color. Then measure from the first color to the top of the next and so on.
What you get is something like this:
Blue = 4.5cm
Yellow= 2.5 cm
This is your pigment profile which can be used to create a graph representing each profile. If you imagine that each manufacturer tests their inks in this way for quality and keeps track of the results you could hypothetically contact the manufacturers in an attempt to find a brand which matches your profile. This is similar to how pigment is paints are matched to a particular brand used on various items.
Reference data basis for paint may come from several sources. Color chip books used by autobody shops may help match a color to a make and model. The same chip books may be available through paint dealers. Paint manufacturers may have records of the top and undercoat colors. These same manufacturers may have chemical formulations for particular colors. Pyrograms and Infrared Spectrograms can also be used.
In Canada the R.C.M.P. Forensic Laboratory's Scientific Working Group on Materials Analysis maintains a data base of paints and there formulations (Saferstein, 2005) (U.S. Dept. of Justice (FBI) Forensic Science Communications, July 1999 Vol. 1 No#2, Scientific Working Group on Material Analysis).
Finally paint can be used to establish the sequence of events of a crime. Imagine this scenario. A young woman is found dead from apparent blunt force trauma to the head. There is blood evidence of an attack in the living room, hallway and bedroom. It is not clear whether she was dealt the fatal blow in the living room or the bedroom. Next to her body the investigators find a hammer.
On the hammer which was packaged at the scene and sent to the lab was the following. On the claws of the hammer is white exterior paint matching the paint from the outside of the windows. On the strike end of the hammer are several types of evidence. The first layer is blood and hair. Under this layer is blue interior paint which matches the color in the living room. Under this layer is green interior paint matching the shade in the hallway. Tool marks in both the living room and the hallway seem to be hammer marks.
Of course all of the available microscopic, biological and chemical tests will be conducted to make sure the paint did originate at the woman's home. From the sequence of the paint evidence on the hammer the investigators could hypothesize that the perpetrator used the hammer to break in a window.
He or she them confronted the woman in the living room where the wall was struck with the hammer. She or she pursued the woman down the hallway once again striking the wall. The final layer was blood and hair which suggests the victim was killed in or near the bedroom.
This is a very hypothetical scenario but it illustrates how paint can be used to determine the possible sequence of events.