Silicon, in various forms such as ferro silica, is used as a deoxidizer in filler metal and during the manufacturing of steel. The two welding processes where one might see the by product of the deoxidation is with GMAW and GTAW. The silicon oxide appears as glass like islands on the surface of the weld. The oxide island shatter when struck with a slag pick just as one would expect if glass was shattered with a small hammer. These islands can result in inclusions if they are not removed during interpass cleaning.

Best regards - Al