Controlled method for the production of clad glass rod

In a method of producing clad glass rod of substantially uniform diameter including the steps of:

• supporting within the same container a glass melt of a molten layer of a first glass on a molten layer of a second glass, said two layers being provided by two quiescent pools of glass, one overlying the other within said same container,
• drawing from the free surface of the upper layer a clad glass rod comprising an outer cladding of said first glass on a core of said second glass which is entrained at the interface between said layers,
• stabilizing said drawn clad glass rod by cooling said drawn glass layer by a cooling jacket which surrounds said drawn glass so as to cool said drawn glass into solid form above the surface of said upper layer, and
• varying the rate of drawing of said rod to generally control the diameter of the rod,
• the improvement comprising the steps of:
• sensing the rate of drawing and the diameter of the drawn rod, and
• moving the cooling jacket axially towards and away from the glass melt container in response to the rate of drawing and diameter sensing to render the drawn clad glass rod more uniform in diameter, particularly as the level of glass melt within the container varies.