The process of revealing serial numbers that have been filed, scraped, polished, or otherwise obliterated. Although most common in firearms cases when serial numbers on weapons have been removed, any metal object with a stamped serial number such as engine blocks, tools, and equipment can be treated in a similar manner. As shown in the figure, when a serial number is stamped into a metal object, the force of compression is transferred to the metal below the indentation, causing imperceptible damage and strain. The depth of this strain can be several times the depth of the original impression. This damage to the metal makes it more susceptible to attack by oxidizing agents such as strong acids.
When someone removes a serial number by filing or grinding, typically he or she stops when the number is no longer visible, leaving a portion of the surface in which the weakened metal still exists. When the surface is treated with an etching material (also called a chemical etchant), the strained metal will dissolve faster than the undamaged metal. If successful, such treatment will allow visualization of the original number. Similar techniques can be used when a serial number is altered by overstamping or other methods of chemical and physical obliteration.
The type of reagent used to restore the number will depend on the type of metal substrate. For example, aluminum may be treated with sodium hydroxide (NaOH), while steel calls for a mixture of hydrochloric acid (HC1) and copper (II) chloride (CuC^). Other metals may require reagents and acids such as chromic, sulfuric, or nitric. Regardless of the reagent used, the first step is always photographic documentation of the surface followed by cleaning and polishing. The reagents may be applied with a cotton swab, cotton ball, or by building a dam of clay around the area of interest and pooling the reagent on it. Careful monitoring is essential since the strained metal region may be very thin and may dissolve away quickly. A variation of the manual application of reagents is an electrolytic method in which a battery is attached to the metal (positive terminal), while the negative terminal is attached to the swab or cloth soaked in the reagent. The result is faster, requiring even closer scrutiny to avoid overprocessing.
For ferrous metals (those containing iron), another approach can be used that has the advantage of being nondestructive. The strained metal beneath the stamp will contain tiny cracks and imperfections that will lead to unevenness in the magnetic field produced when the metal is placed between the poles of a magnet. Tiny flakes of iron (iron filings) can be lightly dusted on the surface or applied using a thin oily suspension. If successful, the filings will congregate over the stamped area and reveal the original number. X-rays, heating, cooling, and ultrasonic methods have also been used.