What is Selvage Denim?
All woven fabric has a selvage of some type. Modern looms make a fringe selvage or a tucked selvage. The package of weft yarn does not travel back and forth across the loom, but rather a measured length of yarn is pulled or pushed across the loom. In both cases the yarn is cut after each insertion of the weft or 'pick'. A jet of air is a common way of inserting the weft yarn today. Fringe selvages do not withstand multiple washings very well. Tucked selvages are usually too wide or to thick to be used in a garment.
The selvage formed on a shuttle loom will not unravel because the yarn is not cut on each pick. It is quite suitable to be used as part of the garment without over edge sewing.
Wide versus Narrow
Today's shuttle-less weaving machines produce 68" wide fabrics. Narrow unfinished fabric at about 28" wide provided the right ratio of edge to square yards required.
Shuttle loom Weaving
In the original shuttle weaving process, a small bobbin of yarn is carried inside a shuttle that travels back and forth across the loom. Since the weft yarn is not cut after each weft insertion the tightly bound edge cannot unravel (sometimes called a fast selvage). If used as part of a garment, it will maintain its integrity throughout the life of the garment. Cone Denim's White Oak plant's original Hopewell looms were installed in 1905 and produced 28" unfinished fabric. As the shuttle loom technology evolved, new wider shuttle looms were installed. By the 1950's the widths had increased to 42" to 45" on most fabrics, although a significant number of looms remained at 28".
As the looms became wider, fewer garment producers utilized this selvage. Garment parts inside vintage jackets, shirts, overalls, and jeans may show one or more garment parts where the selvage was used.
Because of the high weight of the shuttle, the 28" shuttle looms were very slow by today's standards. They produced lots of defects and uneveness in the weave. The uneveness in yarn tensions and the lack of precision in controlling weft yarn spacing creates a different appearance in denim as compared to modern looms. In the late 1960's and early 1970's the transition to shuttle - less looms were much faster and produced a more consistent fabric, but the selvage was no longer usable in garments because it would not withstand multiple washings.