In a method for the manufacture of a preform comprising a core of core glass and a jacket of jacket glass for drawing glass fibers by casting molten glass onto a pre-defined body of solid glass, the improvement comprising etching the surface of a solid rod of said core glass and only then surrounding said solid rod of said core glass with molten jacket glass by introducing said solid rod into a cylindrical hollow form having a larger cross-section and composed of material having good thermal conductivity, filling out a resulting annular cavity between said rod and the surrounding hollow form with said molten jacket glass, allowing said molten jacket glass to solidify in said hollow form and subsequently removing the solidified cast member from the hollow form.