So you have your great great aunt's embroidery sampler, or maybe your son's athletic jersey, and you'd like to have it mounted and framed for posterity. You've picked out the frame, but with the glass options your framer suggests the more expensive acrylic over using regular glass. Why is this?
It has to do with humidity and temperature. Glass conducts humidity and temperature better than acrylic, allowing moisture and heat (or cold) to affect the fabric underneath. Unlike paper (which tends to be sturdier for the most part), this causes the fibers of the textile to expand and contract. You won't necessarily notice the effects on newer fabrics or synthetics, but over time, this causes fabric to get brittle. Silk in particular is susceptible to changes in temperature and humidity and will literally "powder", or crumble, over time.
Most acrylics have a UV coating on them, blocking the harsh effects of the sun, which can bleach out or chemically alter your cherished art, photography or heirlooms. Nowadays, there is even glare-free acrylic glazing!
So next time you come by the frame shop, you can impress your framer by insisting you would like a UV resistant acrylic glaze for your textile project!