indentations properly. Documents that have been subjected to high levels of humidity will not generally retain indentations. As a result, other forensic examinations involving the wetting of a document, such as fingerprint development using a ninhydrin solution, should not be done until after examination for indentations. Preparation Humidification: an EDD works best when the relative humidity is less than 60%. Some studies have also suggested that absolute humidity is important. To help ensure proper humidification, most manufacturers provide a 'humidification chamber' that can be used to prepare a document for examination. It is important to note that over-humidification is also possible so care must be taken to not actually wet the document. Fitness-for-use (FFU) test; AKA, control sample: since an EDD may fail to detect indentations when they are actually present (for a variety of reasons), it is important to check on the equipment's performance using a control sample (commonly called a FFU test) known to have indentations. Recommendations include using a FFU test sample at the beginning and end of a series involving the examination of multiple questioned documents, or placing a small FFU test sample adjacent to each questioned document on the platen. The FFU test can be prepared by hand using a similar writing instrument and similar paper. Alternatively, a special device call the 'Gradient®' can be used to produce control indentations that vary in depth in a known manner thereby permitting the examiner to determine not only that the EDD is working but also the degree to which it is sensitive. It is not essential to precisely match the questioned document in terms of paper type or pen type since the objective is to show that the device is functioning properly. Placement on platen: the goal is to place the questioned document flat on the platen surface with few wrinkles or distortions (ideally, none). The document should not extend beyond the platen edges and enough room should be present around the document to allow the charging film (see below) to seal against the platen surface. Type of document: as noted above, an EDD works well with a single sheet of paper up to a legal-sized sheet. However, if there are concerns about biological or chemical contamination, due to the condition of the questioned document, it is possible to use a second sheet of clean paper, e.g. copier paper, known to be free of indentations as a