the physical pressure applied to the writing instrument as one might expect. Rather, Seward in 1998 and 1999 proposed an alternative theory explaining the detection capability of an EDD as being due to a surface charge effect created by paper-to-paper friction specifically in the area where a writing instrument is pressed down into the top-most sheet of paper. Seward's model was based upon "charge transport through the Mylar-paper-platen structure" and is appropriately called the 'charge transport model'. Subsequent testing, demonstrated that the charge transport model, while not perfect, is sound. This testing also clarified that areas of indentation are less negatively charged than surrounding areas. It is this relative difference in potential that causes the toner to be attracted to the areas of indentation, rather than other areas on the mylar surface. Seward's model also helps to explain two unusual phenomena sometimes observed when using an EDD: 'pure' indentations caused by impact printing, for example, may not produce good EDD results indirect secondary 'impressions' may appear that are caused by lateral relative motion between two sheets of paper when the source sheet bears significant embossing. Additionally, indentations can sometimes not develop very well even though they are clearly visible. This may occur, for example, with very heavy or deep impressions or with impressions in glossy heavy-weight paper such as that used for brochures. In such situations, it is possible to use digital scanning and image-processing to document such writing impressions. Practical use Despite the complexity of the underlying theoretical mechanism, the practical use of an EDD is straightforward. Most devices are similar in their operation. The following are the key steps in using an EDD to visualize indentations: Evaluation of material Assess adequacy of material for examination: an EDD works best when used on a single sheet of clean, smooth paper (that is, paper without wrinkles, creases or stains). The presence of any of these factors may preclude effective examination or, at the minimum, mean that the results will be limited. Heavy-weight, coated or treated papers often produce poor results. The purported age of the document is not generally a factor of concern. Latent indentations have been developed on documents older than fifty years. Examination using side (oblique) lighting: as a general rule, all items should be examined using side lighting. This is important because visual examination may detect deep indentations even though the EDD may fail to develop those