A carbon electrode used for an arc discharge for manufacturing a quartz glass crucible, wherein at least one of a concave pattern and a convex pattern is formed on a surface of the carbon electrode in at least a range of 50 mm to 130 mm in a longitudinal direction of the carbon electrode from an end portion where the arc discharge takes place. Consequently, a carbon electrode that can suppress agglomeration of silica fume on the carbon electrode while manufacturing a quartz glass crucible is provided.

A carbon electrode used for an arc discharge for manufacturing a quartz glass crucible, wherein at least one of a concave pattern and a convex pattern is formed on a surface of the carbon electrode in at least a range of 50 mm to 130 mm in a longitudinal direction of the carbon electrode from an end portion where the arc discharge takes place. Consequently, a carbon electrode that can suppress agglomeration of silica fume on the carbon electrode while manufacturing a quartz glass crucible is provided.

Provided is a method for producing a synthetic opaque quartz glass where flame processing can be performed in high purity with a simple way and even a large sized one can be produced, and the synthetic opaque quartz glass. A method for producing a synthetic opaque quartz glass which comprises the step of heating and burning a quartz glass porous body under a pressure of from 0.15 MPa to 1000 MPa at a temperature of from 1200 C. The quartz glass porous body is prepared by depositing quartz glass particles which are produced by hydrolyzing a silicon compound with an oxyhydrogen flame.

Provided is a method for producing a synthetic opaque quartz glass where flame processing can be performed in high purity with a simple way and even a large sized one can be produced, and the synthetic opaque quartz glass. A method for producing a synthetic opaque quartz glass which comprises the step of heating and burning a quartz glass porous body under a pressure of from 0.15 MPa to 1000 MPa at a temperature of from 1200 C. The quartz glass porous body is prepared by depositing quartz glass particles which are produced by hydrolyzing a silicon compound with an oxyhydrogen flame.

One aspect relates to a process for the preparation of a quartz glass body, including providing a silicon dioxide granulate, wherein the silicon dioxide granulate was made from pyrogenic silicon dioxide powder and the silicon dioxide granulate has a BET surface area in a range from 20 to 40 m.sup.2/g, making a glass melt out of silicon dioxide granulate in an oven and making a quartz glass body out of at least part of the glass melt. The oven has at least a first and a further chamber connected to one another via a passage. The temperature in the first chamber is lower than the temperature in the further chambers. On aspect relates to a quartz glass body which is obtainable by this process. One aspect relates to a light guide, an illuminant and a formed body, which are each obtainable by further processing of the quartz glass body.

One aspect relates to a process for the preparation of a quartz glass body, including providing a silicon dioxide granulate, wherein the silicon dioxide granulate was made from pyrogenic silicon dioxide powder and the silicon dioxide granulate has a BET surface area in a range from 20 to 40 m.sup.2/g, making a glass melt out of silicon dioxide granulate in an oven and making a quartz glass body out of at least part of the glass melt. The oven has at least a first and a further chamber connected to one another via a passage. The temperature in the first chamber is lower than the temperature in the further chambers. On aspect relates to a quartz glass body which is obtainable by this process. One aspect relates to a light guide, an illuminant and a formed body, which are each obtainable by further processing of the quartz glass body.

In an exemplary embodiment, a quartz glass crucible 1 includes: a high-aluminum-content layer 14B which is made of quartz glass having a relatively high average aluminum concentration and is provided to form an outer surface 10b of the quartz glass crucible 1; and a low-aluminum-content layer 14A which is made of quartz glass having a lower average aluminum concentration than that of the high-aluminum-content layer 14B and is provided on an inner side of the high-aluminum-content layer 14B, wherein the low-aluminum-content layer 14A includes an opaque layer 11 made of quartz glass containing a large number of minute bubbles, and the high-aluminum-content layer 14B is made of transparent or translucent quartz glass having a lower bubble content than that of the opaque layer 11. The quartz glass crucible is capable of withstanding a single crystal pull-up step undertaken for a very long period of time.

In an exemplary embodiment, a quartz glass crucible 1 includes: a high-aluminum-content layer 14B which is made of quartz glass having a relatively high average aluminum concentration and is provided to form an outer surface 10b of the quartz glass crucible 1; and a low-aluminum-content layer 14A which is made of quartz glass having a lower average aluminum concentration than that of the high-aluminum-content layer 14B and is provided on an inner side of the high-aluminum-content layer 14B, wherein the low-aluminum-content layer 14A includes an opaque layer 11 made of quartz glass containing a large number of minute bubbles, and the high-aluminum-content layer 14B is made of transparent or translucent quartz glass having a lower bubble content than that of the opaque layer 11. The quartz glass crucible is capable of withstanding a single crystal pull-up step undertaken for a very long period of time.

One aspect relates to a method for producing an optical blank from synthetic quartz glass by vitrifying and shaping a porous, cylindrical SiO.sub.2 soot body having a longitudinal axis, in a heating zone including a melt mold with bottom plate. The SiO.sub.2 soot body vitrified in the heating zone at a vitrification temperature so as to form a full cylindrical, completely vitrified, transparent quartz glass body. Subsequently, the vitrified quartz glass body is shaped by softening in the melt mold at a softening temperature so as to form a viscous quartz glass mass which partly fills the volume of the melt mold, and cooling the quartz glass mass and removal from the melt mold so as to form the optical blank. During shaping in the melt mold, the full cylindrical quartz glass body is brought into contact by way of controlled supply with a centering means of the bottom plate.

One aspect relates to a method for producing an optical blank from synthetic quartz glass by vitrifying and shaping a porous, cylindrical SiO.sub.2 soot body having a longitudinal axis, in a heating zone including a melt mold with bottom plate. The SiO.sub.2 soot body vitrified in the heating zone at a vitrification temperature so as to form a full cylindrical, completely vitrified, transparent quartz glass body. Subsequently, the vitrified quartz glass body is shaped by softening in the melt mold at a softening temperature so as to form a viscous quartz glass mass which partly fills the volume of the melt mold, and cooling the quartz glass mass and removal from the melt mold so as to form the optical blank. During shaping in the melt mold, the full cylindrical quartz glass body is brought into contact by way of controlled supply with a centering means of the bottom plate.