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Fabrics are composed of individual threads or yarns made of fibers Fibers are knitted, woven, bonded, crocheted, felted, or knotted together Most are either woven or knitted Individual qualities of fabrics include: Degree of stretch Absorbency Water repellence Softness Durability Yarn Continuous strand of fibers or filaments that may be twisted together Warp Lengthwise yarn in fabric Weft Crosswise yarn in fabric Blend Fabric made up of two or more different types of fibers Plain Simplest & most common pattern Warp & weft yarns pass under each other alternately Design resembles a checkerboard Satin Yarn interlacing isn’t uniform Interlacing weave passes over four or more yarns Creates long floats Twill Warp yarn is passed over one to three weft yarns before going under one. Makes diagonal weave pattern Denim is most common example Knitted fabrics are made by interlocking loops into a specific arrangement May be one continuous thread or a combination Yarn is formed into successive rows of loops & then drawn through another series of loops to make the fabric All fabrics are long chains of monomers called polymers Limited number of natural fibers but 1000s of artificial fibers Polymer properties vary Chemical structure of monomers vary Chemical bond of monomers vary Because of these differences, polymers can be distinguished form one another forensically Characteristics that can be used to identify & compare fibers as forensic evidence: Fiber cross section Chemical structure Microscopic observation Burn tests Thermal decomposition Chemical tests Density Refractive index Fluorescence Synthetic fibers are forced out of a nozzle when they are hot & then woven Holes of the nozzle are not always round so fiber filament may have a unique shape in the cross section Microscopic observation Compare color, diameter, twist, number of strands, etc Thermal Decomposition Use heat to break down the fiber to basic monomers Example: acetate fibers decompose to form acetic acid Burn Tests – how fibers burn Odor Color of flame Color of smoke Appearance of residue Chemical Tests Solubility & decomposition Separates waft & weft Density Mass of an object divided by its volume Especially useful for single fibers Nondestructive to fibers Refractive index Measurement of the bending of light Becke line outside fiber edges means liquid has higher refractive index than fiber as it passes from air into a solid or liquid Becke line is halo-like shadow appearing around an object immersed in a liquid of different refractive index If liquid & fiber have same refractive value, fiber becomes invisible because it bends light at the same angle as liquid If liquid has lower or higher refractive value, the fiber can be seen Fiber has “halo” of a dark boundary Becke line inside fiber image means liquid has lower refractive index than fiber Fluorescence Absorption & reemission of light when exposed to UV radiation Expose to both long & short wave radiation for comparison May be caused by chemical properties of fabric itself or dyes/optical brighteners added to fabric Used for comparing fibers as well as spotting fibers for collection Component of dyes can be separated & matched to unknown More than 7,000 different dye formulations Chromatography is used to separate dyes for comparative analysis The way a fabric accepts a particular dye can be used to identify & compare samples Put clothing items individually in paper bags Make sure that different items are not placed on the same surface before being bagged. Make tape lifts of exposed skin areas & any inanimate objects Removed fibers should be folded into a small sheet of paper & stored in a paper bag Fiber evidence in court cases can be used to connect the suspect to the victim or to the crime scene In the case of Wayne Williams, fibers weighed heavily on the outcome of the case Williams was convicted in 1982 based on carpet fibers that were found in his home, in his car, & on several murder victims