Examination of documents for indented impressions
If text is written onto one page of a document while resting on another page of that document or indeed a separate document, indented impressions of the writing may be transferred and a permanent record of these impressions may be detectable.
A 6 page agreement with a signature page at the back is signed with the back page folded over and lying on top of the first page. Impressions of the signature may then be visible on the top page or pages of the agreement, depending upon the pressure applied by the writer. If the signature page has been substituted, the impressions of a different signature might be present on the top page(s).
An anonymous letter is written in a lined notepad. Impressions on the letter may reveal clues about the author based on previous entries in the notepad.
The ESDA (Electrostatic Detection Apparatus) or 'Docustat' employs a sensitive electrostatic process to reveal these faint impressions of writing. When present, these impressions are preserved in the form of black toner particles affixed to a clear plastic sheet.
The clarity of an ESDA trace depends upon many factors including the type of writing instrument, type of paper, amount of handling the document has received and how it has been stored. In favourable circumstances ESDA traces can reveal impressions on several sheets below the one bearing the original handwriting.
ESDA does not generally work well with very thick or thin papers, some coated papers, and documents that have been wet or crumpled.
Below is an example of an example of the effectiveness of ESDA: the photograph on the left showing the source paper and the photograph on the right illustrating the indented impressions that are present as identified through the ESDA process.
The ESDA process is non-destructive and will not alter or harm the original document.