applications the glasses according to the invention can be utilized particularly advantageously as core glass materials for making optical stepped fibers such optical stepped fibers at the peripheral wall of the core glass comprise a completely adjoining cladding glass preferably having a numerical aperture of 0 3 to 0 75 herein as a cladding glass matched to the core glass a cladding glass may be utilized comprising in wt sio 2 60 to 72 b 2 o 3 lt 20 na 2 o lt 18 k 2 o lt 15 li 2 o lt 2 mgo lt 3 bao lt 3 sro lt 4 cao lt 6 zno lt 3 f u2212 lt 1 other oxides 0 to 3 wherein the glass may be molten by adding refining agents up to 1 wt preferably up to 0 5 wt again apart from any unintended contaminants no other oxides are added apart from refining agents in amounts of up to 1 wt preferably of up to 0 5 wt again apart from any unintended contaminants the cladding glass is free of cs 2 o according to a preferred development of the invention the sum of the components na 2 o li 2 o and k 2 o within the cladding glass is smaller than 3 wt the preferred sio 2 amount of the cladding glass is about 66 to 72 wt the invention will.