Restoration of Erased Markings on Metal Surfaces


Manufacturers of valuable items, like automobiles, bicycles, firearms, watches, sewing machines, art objects, etc, often register serial numbers on their products as a means of identification or to establish ownership. Individuals in possession of inexpensive silver or stainless steel items frequently have their names engraved on them for identifying purposes. When stealing such items, criminals may try to change or eliminate the identity marks by filing the surface containing marks with a file, sandpaper, or a grinding wheel. It is impossible to remove such marks without leaving visible proof of the attempt. The forensic laboratory is increasingly called upon to restore such erased numbers or names to enable the police to identify the stolen property with the owner. It is often possible in the laboratory to restore the obliterated marks, partly or wholly.

Types of Marks

There are generally three types of marks that are commonly found on the articles;

  • Cast Marks

These types of marks are raised above the surface of the metal but sometimes cast marks may sink into the metal surface. Such marks form part of the mold and appear on the metal surface when it is cast. The main product is usually not identified by cast-marked numbers or names. It may only identify a component of the product. It is not possible to restore if the cast marks are erased. It is not of much help either.

  • Engraved Marks

There are two types of engraved marks: those made with a chisel and those made with an electric engraver. If chisel or electric engraver markings are obliterated, the marks can be recovered as long as the erasure is not too deep. However, restoring engraved numerals is significantly more difficult than restoring punched numbers or marks. This is because the engraved marks are made with much less pressure leaving very little alteration or crystal structure of the metal.

  • Punched Marks

The majority of articles include serial numbers, which are obtained by striking the metal surface with a dye that contains relief marks. The dye is forcibly hammered into the metal, which results in compression of the metal causing disturbance of the surface underlying it. These types of erased punched marks can often be restored, just like engraved marks, if the filing was not too deep.

Principle of Restoration

Serial numbers are frequently stamped with hard steel dyes on a metal body, frame, or plate. These dyes strike the metal surface with a force. This results in the crystalline microstructure of the metal getting compressed and strained. The strained surface extends a short distance beneath the indented number. When a suitable chemical reagent is applied, the strained and unstrained areas will dissolve the metal at a different rate. The unequal rate of dissolution, of strained and unstrained metal, permits bringing out the original numbers. But if the erasure is deep and has reached the zone of strain, then it is usually not possible to restore the number of marks.


The simplest and most effective approach for recovering erased numbers is chemical etching. It is simple to use and does not require the purchase of expensive equipment. It can be used on any object of any size or shape.

The procedures involved require a high level of competence as well as a lot of patience. The materials used are potentially harmful and should only be used following all relevant health and safety regulations. They should only be used in places that have been designated specifically for this purpose. If laboratory conditions are not available, use rubber gloves to protect your hands from corrosive acid and operate in the open air away from any source of naked flame when working with volatile liquids.

Preliminary Examination

Examine the metal surface after washing it with acetone to remove grease and debris. With a hand magnifier, examine the surface to determine whether there has been any erasure. Look for any irregularities in the background pattern. This pattern will appear as milled marks, which are formed by grinding the surface before stamping the serial numbers, or as cast marks, which are created during the production process. If it is disturbed, suspect erasure. Remove the paint from a large area surrounding the surface, even if no erasure is visible, to see if the piece containing the chassis number was taken out and replaced by welding or pasting a metal plate with a new number. After recording the original look, remove any pasted plates from the original surface with trifluoroethanol. Take out the welded plate as well. Look for any erased marks on the lower surface. Take note of how the mark was erased and whether it was fixed later. To observe the erased number, examine the surface with carefully adjusted illumination, preferably oblique lighting.

Use a single light to strike the surface bearing the serial number at a low angle for photographing. Take numerous exposures, moving the light in each one so that it strikes the surface from a different angle. Make use of process film. This approach helps in the visibility of faint serial numbers.

Determine the metal type from which the object is made. This is required to select the proper chemical etchants.

Preparation of Surface

(i) Remove any oil or paint from the surface by cleaning it with benzene or acetone. You can also use solvents like gasoline, professional paint remover, or a 50/50 mixture of acetone and chloroform. A gentle toothbrush should be used to help the solvent dislodge deposits from the stamped surface.

(ii) Using an emery cloth or other fine abrasive, mirror-like finish or a relatively smooth surface. After eliminating all scratches and other gross marks with fine emery, coarse emery paper is utilized. Remove only as much metal as is necessary. During the polishing process, keep an eye on the surface since digits can be revealed.

(iii) Use a solvent, such as acetone or another grease solvent, to clean the area once again. Fingerprints can interfere with the reaction, so don’t touch the region with your bare hands.

(iv) Heating- Use a blowlamp or a Bunsen burner to heat the area. It’s important to avoid overheating. When a metal is heated to red hot, the temperature is high enough to soften it, and when it cools, the metal becomes homogeneous and no longer distinguishable. Heating the metal surface until it is just too hot to touch is a useful guide. Allow the metal to cool before beginning the etching process.

(v) Take the photograph of the entire item, and record details of the obliterated area by close-up photography. Use a 35 mm camera with a macro lens.


  • Steel surfaces (chassis and engine of cars, guns, gas cylinders, etc.)

The etching reagents are in two solutions.

Solution 1 (Fry’s reagent)

  • Crystalline cupric chloride 90gms.
  • Concentrated hydrochloric acid 120ml.
  • Water 100ml.

Solution 2

  • 15 percent Nitric acid.

Swab the surface by applying the solution one for approximately 2-3 minutes with cotton wool dipped in the reagent. Look for any digits revealed and record these. Then, clean the surface with acetone and again examine.

Swabbing the surface with cotton wool dipped in solution two for about 1 minute. Look for any digits that have been revealed and write them.

Using Fry’s and 15% nitric acid etching reagents alternately, continue etching until the entire number is visible. It will take two or three hours. Because faint markings form and then vanish before the mark is entirely etched, a continuous check on the appearance of the mark is required.

  • Copper, Brass, German Silver and other Copper Alloys

These metals react to:

  • Ferric Chloride 19 gms.
  • Hydrochloric acid 6 ml.
  • Water 100 ml.

Using a swab, apply the reagent until the number appears. When the reagent takes a long time to develop the erased mark, it builds a plasticine wall around it. Use the solution as the bath. Allow 24 hours for the reagent to remain on the metal. 20 gms has been discovered by some workers. For brass and copper, a solution of ammonium persulphate and water (up to 100cc) produces the best results.

  • Stainless Steel

Swab with a dilute sulphuric acid solution or a 10% hydrochloric acid solution in alcohol. It can be used to restore numbers that have been obliterated.

  • Lead (Motor car batteries etc.)
  • Glacial acetic acid 3 parts
  • Hydrogen peroxide

This solution has produced excellent results. Time frame: 10–30 minutes Clean the metal with concentrated nitric acid once the erased number appears.

  • Zinc Alloys
  • Sodium hydroxide 10 percent solution in water.

Development is slow. So, use the plasticine bath method. Time 10-16 hours.

Alternate solution

  • Chromic acid 20gms.
  • Na2 SO4 1.5 gms
  • Water (or) 100 ml.
  • Nitric acid 25%

Cast Iron and Cast Steel

Apply a 10% sulphuric acid solution with potassium dichromate continuously. The speed of action will be slow. Apply the reagent regularly. It may be necessary to construct a plasticine wall around the number. Then it is filled hollow with the reagent. At regular intervals, remove the solution, examine the surface, and replace it with a fresh solution. The restoration is obliterated as a result of overdevelopment.

  • Aluminum Alloys (Engine surface of the motorbikes, auto-rickshaw engine, vehicle identification plates, etc.)

The best reagent is Villela’s solution.

  • Glycerin 30 ml.
  • Hydrofluoric acid 20ml.
  • Nitric acid 10 ml.

Using a swab, apply the reagent until the number appears. Use the reagent very carefully because it is very harsh on the fingertips. It will take 5 to 15 minutes.

Visualization and Photography of the Restored Marks

As far as possible, restored marks should be distinct enough to be visualized and photographed. When the etched surface is slightly moistened with etchants, it is typically easier to visualize and photograph. Because it’s difficult to see the restored number, especially on the engine and chassis of the vehicle, there should be enough lighting to see it. Portable beam lighting can be set up to provide low-angle viewing illumination across a large surface area.

However, a powerful light source calibrated appropriately for capturing their photographs on the camera screen should be present for photography. Flexible, powerful fiber optic illuminators are ideal for this application. For time exposure, a 35 mm camera mounted on a tripod will be quite useful. A macro lens with extension rings would allow the camera to remain near to the number area while capturing a magnified image on film.

Restoration on Wood

Many wooden articles like butts of firearms, artwork, furniture, etc. are stamped with identifying markings which may be erased by cutting, filing, or by using abrasive, like sandpaper. If the area beneath the erasure has been disturbed by the original punching, it is often possible to restore the markings by allowing a jet of steam to play on the suspected area. This process softens the pressed wood fibers, which may shoot up and reveal the markings.

Restoration on Plastics

Modern products which are often made of plastic or molded plastic have their serial numbers similarly punched as on metal objects. If erasures are attempted on these surfaces, it is sometimes possible to restore them by treatment with suitable solvents, since the compressed plastic material under the punched marks will be less rapidly soluble than the rest of the surface.

Another method, which has been found to give good results, is to gradually heat the surface and cool. The rearrangement of the material of the plastic takes place through this process, which results in showing up the original number.

Restoration on Leather

Sometimes leather surfaces may also have punched marks. The attempt may be made by the culprit to obliterate these marks to avoid identification. Application of dilute alkali such as potassium hydroxide can make the erased marks reappear on the same analogy as that of wooden surfaces.

Case Study

A truck was transporting sugar from one state to another with the owner, cleaner, and the driver in the vehicle. The driver and cleaner both connived to kill the owner in the course of their journey. This they did and absconded with the truck. The truck was later discovered but was found to have a different chassis and engine number. Alteration of the number was evident, and hence the truck was taken to the laboratory for examination. On examination, only the last five numbers were found to be tampered with and altered. These numbers were successfully restored. Based on the identification of the vehicle the driver and the cleaner were charged for the murder of the owner and theft of the vehicle.

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