Abstract: A linear burner for the synthesis of silica by vapor-phase reaction of a silicon-containing feedstock in a flame comprises at least five slots opening to an exit face (52a) of the burner and extending side-by-side in the elongate direction of the linear burner, and includes means (P1-P10) to supply a separate gas flow to each of said slots. An assembly of separator plates (52) is disposed between opposed casing parts (50, 51), which assembly defines the slots. At least one of the casing parts (50, 51) defines at least part of a respective plenum chamber (54) for each gas flow, each plenum chamber (54) communicating with a different one of said slots in the separator assembly. The invention also extends to a method of operating such a linear burner.