Sandblasting before slumping
Sandblasting before slumping is probably the most common (and easiest) way to achieve a matte surface.
Other options include acid etch (with or without sandblasting first) and coldworking to about a 400 or 600 grit finish. A 600 grit finish has a soft gloss and can be really beautiful when done well.
Helios Kiln Glass Studio