Here is some interesting text from Lincoln discussing examples of the Silicon/Glass deposits in GMAW.
They attribute it to silicon added to the electrode wires as a deoxidzer in varying amounts as determined by the surface of the carbon steel to be welded. ER70S-6 being higher in silicon and ER70S-3 Being lower.
http://www.lincolnelectric.com/knowledge/articles/content/selectingmig.aspAnd it's no surprise Ed Craig has about 10,000 words dedicated to the phenomina you are describing.. He goes into great depth about, shield gas selection, mill scale, and solid wire electrodes, describing how each have an effect and how the effects vary in combination. Very good stuff.
http://www.weldreality.com/MIG_welding_gases.htmAs SMAW electrodes go, most core wires for carbon steel applications are the same. The differences are introduced in the fluxes. Sodium and potassium silicates used as binders while Silica is employed as a slagging ingredant. (lifted from Welding Principles and Practices Sacks and Bohnart)