Electrostatic Detection Analysis
ESDA stands for Electrostatic Detection Apparatus. 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.
How it works
ESDA works by stretching a Mylar film (like Clingfilm) over the document being examined. 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. The toner preferentially sticks where the electrostatic charge congregates - i.e. in the areas of the indentations. The result is an image with a grey toner background and darker toner traces in the areas where there are indented impressions. The image is made permanent as a 'lift' by placing clear 'sticky backed plastic' over the toner, producing a fixed transparent image.