A tube glass of the present invention includes an alkali silicate glass, in which a glass composition is substantially free of B.sub.2O.sub.3 and Al.sub.2O.sub.3, and a loss in mass ? (mg/dm.sup.2) in an alkali resistance test in accordance with ISO 695 (199105-15) is classified as Class A1.

The optical fiber offered is capable of not only restraining the attenuation due to glass defects, but also reducing the increase of manufacturing cost. The optical fiber is made of silica glass and includes a core and a cladding. The cladding encloses the core and has a refractive index smaller than that of the core. When the core is divided into inner core and outer core at half of the radius of the core, the average chlorine concentration of the inner core is larger than that of the outer core. The core includes any of the alkali metal group.

A method is provided for producing a glass ceramic element with a patterned coating is provided. The method includes: providing a glass ceramic element with a coating which is at least partially light-blocking and preferably opaque in the visible spectral range; irradiating the glass ceramic element with a pulsed laser beam on the face provided with the coating so that the coating is removed by ablation; during irradiating the laser beam is directed over the surface of the glass ceramic element so that a portion of the coating is removed which has a greater lateral extent than the diameter of the laser beam; and once the coating has been removed, irradiating the glass ceramic with the laser in the region where the coating has been removed, thereby optically modifying the glass ceramic in the irradiated region.

Composites comprising glass compositions and, in particular embodiments, glass fibers. Embodiments of the present invention relate to composite comprising a recycled material and a glass fiber. Additional embodiments of the present invention relate to methods for improving the properties of composites.

A quartz glass crucible (1) includes: a crucible body (10) made of silica glass; and a crystallization-accelerator-containing layer (13) formed on an outer surface of the crucible body (10). A concentration of a crystallization accelerator contained in the crystallization-accelerator-containing layer (13) is 1.0?10.sup.13 atoms/cm.sup.2 or more and 4.8?10.sup.15 atoms/cm.sup.2 or less. The quarts glass crucible is intended to be capable of not only enduring a single crystal pulling-up process that takes a very long time, such as multi-pulling, but also stably controlling the oxygen concentration and crystal diameter of a silicon single crystal by eliminating a gap between the carbon susceptor and the crucible as much as possible.

An optical fiber includes a core, and a cladding. When a refractive index of silica glass is set as n.sub.0, a refractive index of the core is set as n.sub.1, and a refractive index of the cladding is set as n.sub.2, a relative refractive index difference ? defined by Expression (1):
?[%]=100?(n.sub.1.sup.2?n.sub.2.sup.2)/2n.sub.0.sup.2(1)
is 0.2% or higher. A ratio of a maximum value of a concentration of the dopant composed of the alkali metal element or the alkaline-earth metal element in the cladding to a maximum value of a concentration of the dopant composed of the alkali metal element or the alkaline-earth metal element in the core is 0.06 or higher and 0.25 or lower.

One aspect is a process for the preparation of a quartz glass body including providing a silicon dioxide granulate wherein the provision includes providing silicon dioxide powder, and processing the silicon dioxide powder to obtain a silicon dioxide granulate. The silicon dioxide granulate has a larger particle diameter than the silicon dioxide powder. The processing includes processing the silicon dioxide powder to obtain a silicon dioxide granulate I, wherein the silicon dioxide granulate I has a first carbon content wC(1), treating the silicon dioxide granulate I with a reactant to obtain a silicon dioxide granulate II with a further carbon content wC(2), wherein the further carbon content wC(2) is less than the first carbon content wC(1), making a glass melt out of the silicon dioxide granulate and making a quartz glass body out of at least part of the glass melt.

One aspect relates to a process for the provision of a quartz glass body, including providing a silicon dioxide granulate, making a glass melt from the silicon dioxide granulate in an oven and making a quartz glass body from at least part of the glass melt. The oven includes a hanging metal sheet crucible. One aspect also relates to a quartz glass body which is obtainable by this process. One aspect further relates to a light guide, an illuminant and a formed body which are obtainable by processing the quartz glass body further.

The invention relates to a process for the preparation of a quartz glass body comprising the process steps i.) 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, ii.) Making a glass melt out of silicon dioxide granulate in an oven and iii.) Making a quartz glass body out of at least part of the glass melt, wherein the oven has at least a first and a further chamber connected to one another via a passage, wherein the temperature in the first chamber is lower than the temperature in the further chambers. The invention further relates to a quartz glass body which is obtainable by this process. The invention further relates to a light guide, an illuminant and a formed body, which are each obtainable by further processing of the quartz glass body.

An optical fiber preform includes a silica-glass core portion, and a cladding portion surrounding the core portion, the cladding portion being composed of a fluorine-containing silica glass having a lower refractive index than the core portion, the core portion including a first region that does not include the central axis thereof, the first region containing a first dopant selected from sodium, potassium, and compounds thereof, and a second region that includes the central axis, the second region containing a second dopant that reduces the viscosity of the silica glass, the second dopant having a diffusion coefficient of 110.sup.12 cm.sup.2/s or more and less than the first dopant at 2,000 C. to 2,300 C., in which the entire core portion has an average first dopant concentration of 10 atomic ppm or more and 2,000 atomic ppm or less and an average second dopant concentration of 10 atomic ppm or more.