Special Techniques of Document Analysis

Electrostatic Detection Apparatus

ESDA is designed principally for the examination of single page documents, which are placed carefully over a gentle vacuum and then covered with an aerosol and/or glass beads. An electrostatic charge is induced over the document, and the combination of the vacuum and the static charge on the aerosol and glass beads highlights indentations by filling the grooves with visible material.

Although a simple and effective technique ESDA is basically a manual system and requires a high degree of proficiency in the operator. This is best gained by experience and practice on a variety of documents.
 

Results of indented writing examination can vary with paper type and reflect differences in the basic raw material, chemical treatments and impregnations and surface finishing. Variations in the degree of background development and the strength or contrast of the images are to be expected. Some types of paper respond very poorly to the imaging processes and may not yield images at all or, at best, weak images that require prolonged development. The one major factor that does affect the performance of the instrument is the moisture content of the document. This determines the conductivity of the paper which governs the efficiency of the image formation process.
Pencil erasures can often be detected with ESDA but only by examining the reverse side of the document. Excessive surface damage of the paper caused by the erasure itself will normally mask the indented writing if examined on the obverse.