Abstract: A method of vitrifying a porous cylindrical article made out of glass soot, especially for manufacturing a preliminary blank for optical fibers. The article is heat-treated in a furnace in a vacuum or in an atmosphere that contains helium. The porous article is placed in a horizontal graphite tube in the furnace, is sintered therein for 20 to 40 minutes in a vacuum or in a helium atmosphere with reduced pressure at 1250.degree. to 1400.degree. C., and is subsequently vitrified by heat-treating the sintered article at first in the sintering atmosphere, while slowly rotating it in the hot graphite tube for 20 to 40 minutes at approximately 1450.degree. to 1600.degree. C. and then while rotating it in the graphite tube more rapidly than in the first stage for 10 to 30 minutes at approximately 1650.degree. to 1750.degree. C.

Abstract: A method of manufacturing practically stria-free, bubble-free, and homogeneous quartz-glass plates of any desired configuration and with a surface area that exceeds the cross-section of the full circular quartz-glass cylinder that is employed as a starting material. The cylinder is continuously lowered into a furnace shell flooded with an inert gas, in which it is heated to a flowing temperature in the range of 1700.degree. to 1900.degree. C. until some of the quartz-glass flows off into a graphite crucible. The crucible is preferably clad with zirconium-oxide.

Abstract: A method of manufacturing practically stria-free, bubble-free, and homogeneous quartz-glass plates of any desired configuration and with a surface area that exceeds the cross-section of the full circular quartz-glass cylinder that is employed as a starting material. The cylinder is continuously lowered into a furnace shell flooded with an inert gas, in which it is heated to a flowing temperature in the range of 1700.degree. to 1900.degree. C. until some of the quartz-glass flows off into a graphite crucible. The crucible is preferably clad with zirconium-oxide.

Abstract: Flat, transparent slabs of vitreous silica of optical quality are produced by melting granular starting material in a vacuum by means of an electrical heating system. The starting material is melted while sustaining a heat flow from the top to the bottom of the crucible. The bottom of the crucible is made permeable to gases. The heat isolation power of the crucible wall increases from the bottom to the top of the crucible.

Abstract: Flat, transparent slabs of vitreous silica of optical quality are produced by melting granular starting material in a vacuum by means of an electrical heating system. The starting material is melted while sustaining a heat flow from the top to the bottom of the crucible. The bottom of the crucible is made permeable to gases. The heat isolation power of the crucible wall increases from the bottom to the top of the crucible.