Selvage denim (alternative spelling: selvedge denim) is a type of denim which forms a clean natural edge that does not unravel. It is commonly presented in the unwashed or raw state. Typically, the selvage edges will be located along the out-seam of the trousers, making it visible when cuffs are worn.
The word "selvage" comes from the phrase "self-edge", the natural edge of a roll of fabric. As applied to denim, it means that which is made on old-style shuttle looms. These looms weave fabric with one continuous cross thread (the weft) that is passed back and forth all the way down the length of the bolt. As the weft loops back into the edge of the denim it creates this “self-edge” or selvage. Selvage is desirable because the edge cannot fray like denim made on a projectile loom that has separate wefts, which leave an open edge that must be stitched. This advantage is only realized on one edge of the fabric, however, as the fabric has to be cut to shape and anywhere it is cut the self-edge is lost.
Shuttle looms weave a narrower piece of fabric, and thus a longer piece of fabric is required to make a pair of jeans (approximately 3 yards). To maximize yield, traditional jean makers use the fabric all the way to the selvedge edge. When the cuff is turned up, the two selvedge edges (where the denim is sewn together) can be seen. The selvage edge is usually woven with a colored stripe: green, white, brown, yellow, and (most commonly) red. Fabric mills used these colors to differentiate between fabrics. Contrary to popular belief, the stripe is not sewn in finishing the product, but woven into the fabric itself.
Most selvage jeans today are dyed with synthetic indigo, but natural indigo dye is available in some denim labels. Though they are supposed to have the same chemical makeup, there are more impurities in the natural indigo dye. Loop dying machines feed a rope of cotton yarn through vats of indigo dye and then back out. The dye is allowed to oxidize before the next dip. Multiple dips create a deep dark indigo blue.
In response to increased demand for jeans in the 1950s, American denim manufacturers replaced the old shuttle style looms with modern projectile looms. The new looms produced fabric faster and wider (60 inches or wider). Synthetic dying techniques along with post-dye treatments were introduced to control shrink and twist.
Read more about this topic: Denim