https://www.avient.com/sites/default/files/2021-06/fl.datasheet-fiberglass.pdf
Composition
Large furnaces are used to melt silica sand, limestone, and various other
minerals to liquid form.
The liquid is then extruded through bushings
and are coated (sized) to help bundle the filaments and prepare the
fibers for composite resin interface.
They also add important characteristics, such as
SWELLCOAT® water-blocking, water repellence, adhe-
sion, color, and wear and UV-resistance to these and
many other applications.