How does esda detect indentations

The document to be processed may need to be humidified slightly if it has been kept in, or had as its source, a dry environment. This will help the electro-static charge develop. An interior page from a pad of paper is an example of such an environment. In more humid climates exposure to normal room air would serve this same purpose. Most documents exposed to normal room conditions where the air is naturally humid do not need preconditioning.

The page suspected of bearing indentations is covered with a mylar material which is then pulled into firm contact with the paper by a vacuum drawn through a sintered porous bronze plate. The mylar covering prevents damage to the original document. This results in a variably charged surface with the heavier static charge remaining within any impressions, even those that are microscopic in depth.

The toner is strongly attracted to static electricity and is retained on the mylar surface in accordance with the amount of residual static charge present at any given surface point.

The areas of the document containing the higher static electric charge retain greater portions of the black toner resulting in a deposit of toner at the indentations in the paper.

While these developed indentations may be photographed, they are normally preserved by means of placing an adhesive backed clear plastic sheet over the mylar while it is still being held in place by the vacuum of the ESDA. If the recovered indented writing is of a high enough quality, it may serve as a method to associate somebody to the questioned document using the handwriting in the indentations for comparison with known handwriting.

The examiner using electro-static detection equipment should exercise caution. Extreme voltages are part of the application and the quantities of ozone produced by the wand are far in excess of the safe exposure rates recommended by U.

Government agencies and should be considered a health hazard. Operating the ESDA in a negative pressure environment such as a chemical hood is desirable.

The advantages of electrostatic detection are twofold. ESDA is non-destructive. The indentations are revealed on the protective mylar surface, and are fixed by applying pressure-sensitive adhesive plastic over the cellophane. Secondly, the original document remains unharmed throughout the process. ESDA is extremely sensitive and indentations which are not revealed by any other method may be readily observed and recovered.

Documents suspected of containing latent indented writing must be very carefully packaged. Large manila envelopes can make good packaging material if the document is first encased in thin cardboard. This will keep the questioned document from acquiring additional stray indentations. This instrument is used to produce a permanent 'lift' from paper which provides a visual map of indented impressions in that paper.

It is extremely sensitive meaning that indentations found up to seven sheets below the page where the original writing was made may be visualised. This Mylar film is then electrostatically charged using a 'wand' containing a fine wire charged to 7KV. Where the paper is smooth the charging is generally uniform, however where the fibres of the paper have been disturbed by paper-paper contact caused by indentations, the electrostatic charge is different to the background.

This creates a latent image. In order to visualise this latent image, black toner similar to that used in laser printers is applied to the surface of the Mylar.