SERIAL NUMBERS RESTORATION
Many consumer products and commercial items are identified by a unique serial number. In some cases, this is required by law; firearms and certain auto parts are examples. If the object is metal, the serial number is often stamped into the surface of the metal using a set of alphanumeric dies. Like footwear and tire treads, serial numbers of this type are three-dimensional impressions. When serial numbers are at issue in a crime, it is often because someone attempted to obliterate them from the object, making it more difficult to identify its owner or source. The issue then is whether or not a forensic scientist can restore and read the serial number. Most people, when faced with the task of wiping out a stamped serial number on metal will use a file or grinder. They will consider the job to be a success if they can no longer read any of the numbers or letters. They fail to realize that, even though the serial number may not be visible or readable, it is not really destroyed. It is possible and quite common for a forensic chemist to restore an obliterated serial number impression. In order to understand how this is done, it is necessary to learn a bit about how the chemistry and physics of a piece of metal changes when subjected to the stress of having a serial number stamped into its surface. Metals, like most solids, have a definite crystal structure and therefore an ordered arrangement of chemical bonds between atoms (called metal–metal bonds). When a serial number is stamped into a metal, two things happen to the metal under the num ber. First, it is compressed, making it denser than the surrounding metal. Second, the metal–metal bonds in the stamped area are disrupted and the metal structure becomes weakened. When someone tries to remove a serial number impression by abrasion with a file or grinder, the metal surrounding the stamped number is removed. Once the metal surface surrounding the numbers becomes level with the stamped numbers, then they cannot be seen anymore. However, the compressed, deformed metal under the numbers is still there unless the perpetrator continues the grinding process beyond where the numbers disappear. To restore the serial number, the metal surface that has been abraded is polished with a fine abrasive and then slowly treated with a corrosive acid. The acid slowly dissolves the metal. However, the metal that is under the serial numbers behaves differently toward the acid than the surrounding metal, which had not been disturbed by the stamping process. There are two possible ways that the stamped metal can behave differently. First, remember that it has become denser when compressed. Thus, this metal would be expected to dissolve more slowly than the less dense metal surrounding it. As the metal dissolves, the serial number would be seen to be raised above the faster dissolving, surrounding metal surface. Second, remember that the metal–metal bonds of the stamped metal have been disrupted by the stamping process, and thus weakened. This would be expected to cause the stamped metal to dissolve more quickly than the surrounding metal. The serial number would thus appear to be pressed into the metal once again. How do we know which mechanism dominates in the restoration of serial numbers? The best way is to observe the restoration process using a low-power stereo microscope. It can be easily seen that the serial numbers are lower than the surrounding metal surface as the numbers are restored. This means that the weakened bond theory must be most responsible for the dissolving process of the acid. The actual process of dissolving the metal to restore an obliterated serial number must be done carefully. Once a serial number is restored, it will eventually disappear and then will be gone forever. It is good practice to have a camera ready to take pictures of each number as it is restored so there will be a permanent, visual record of the restoration. The acid is generally applied with a cotton swab. When a number appears, the acid is washed off quickly to minimize further dissolving while the operator views the restored number and photographs it. There are a variety of metals that can have serial numbers stamped into them. Each type of metal requires different acids and conditions. Acidified, aqueous copper chloride solutions are used to restore serial numbers in iron and steel. The copper chloride acts to oxidize the iron so it will dissolve. The copper ions are reduced to metallic copper that will deposit on the metal surface. If the metal surface is stainless steel, a more powerful acidic solution is needed. Acids are too strong for aluminum sur faces, which would dissolve almost immediately. In such cases, mild alkaline solutions are used. Figure 22.13(a) and (b) shows a serial number that was stamped into the door of a farm tractor. The number had been ground off and then restored in the laboratory. Today, serial numbers are stamped into many objects other than metals. Is it possible to restore obliterated numbers in other surfaces? Plastics present difficult problems. They are generally polymers. These are very stable substances that are generally insoluble in most solvents so it would be extremely difficult to restore serial numbers in most plastics. Not all serial numbers are stamped into surfaces. Some are applied with decals or etched. Others are embossed and are raised above the surface of the object. These types of serial numbers cannot be restored using the same methods as are used with stamped numbers.
FIGURE 1 A comparison of a three-dimensional bite mark impression with an impression taken from a suspect (in red).
FIGURE 2 A restored serial number on the inside of the metal door of a tractor, where (a) shows the serial number has been obliterated by grinding and (b) shows how the number has been restored.
