A method for heat treating a synthetic quartz glass having a hydroxyl concentration with a maximum/minimum difference (ΔOH) of less than 350 ppm involves the steps of first heat treatment of holding at 1,150-1,060°C for a time of 0.5-10 hours, cooling down to a second heat treatment temperature at a rate of -7°C/hr to -30°C/hr, second heat treatment of holding at 1,030-950°C for a time of 5-20 hours, and annealing at a rate of -25°C/hr to -85°C/hr. Two stages of heat treatment ensures that the glass has a low birefringence. A method for heat treating a synthetic quartz glass having a hydroxyl concentration with a maximum/minimum difference (ΔOH) of at least 350 ppm involves first and second heat treatments and annealing, wherein the duration of the second heat treatment is from 10 to 15 hours

A conveyor device with accumulation buffer, comprising a structure (12) and a pair of chains (13) arranged on opposite sides of the structure (12) that wind up in a closed loop onto end pulleys (14) to define an upper conveying branch and a bottom return branch, comprises a plurality of shuttles (20), hinged at opposite sides to the chains (13) and fastened in a tilting manner to the structure (12), as well as a plurality of piece-carrying trays (30), in which each of the piece-carrying trays can be stably but removably fastened to each of the shuttles (20), the piece-carrying trays (30) also being able to be stacked on top of one another to form buffer columns of trays (17, 18), the conveyor device (10) also comprising lifting and lowering means (40) respectively of a tray (30) at the base of a first buffer column (17) and of a tray (30) at the base of a second buffer column (18), said buffer columns (17, 18) being arranged in succession on the upper conveying branch in the direction of movement of the shuttles (20), as well as a horizontal transfer slide (50) of a top tray from the first column (17) to the second column (18). A relative furnace for continuous heat treatment of glass containers comprising the conveyor device is also part of the invention.

A production method of synthetic silica glass according to the present invention comprises a first step of ejecting a silicon compound and a combustion gas containing oxygen and hydrogen from a burner to effect hydrolysis of the silicon compound in oxyhydrogen flame to produce fine particles of silica glass, and thereafter depositing and vitrifying the fine particles of silica glass on a target opposed to the burner to obtain a synthetic silica glass ingot; a second step of heating the synthetic silica glass ingot or the like obtained in the first step up to a first retention temperature of not less than 900°C, retaining the ingot or the like at the first retention temperature, and cooling the ingot or the like at a temperature decrease rate of not more than 10°C/h down to a temperature of not more than 500°C; and a third step of heating the synthetic silica glass ingot or the like obtained in the second step up to a second retention temperature of not less than 500°C nor more than 1100°C, retaining the ingot or the like thereat, and thereafter cooling the ingot or the like at a temperature decrease rate of not less than 50°C/h down to a temperature 100°C lower than the second retention temperature.

[Problem] To provide a process for producing a synthetic quartz glass in which an appropriate annealing rate can be selected according to the size of the synthetic quartz glass to be subjected to annealing and annealing can be performed at the appropriated annealing rate.

[Means for resolution] A process to producing a synthetic quartz glass, including an annealing treatment which includes: a step for putting a synthetic quartz glass as a parent material into a heat treatment furnace; a step for elevating a temperature in the heat treatment furnace to a prescribed keeping temperature that is equal to or higher than the annealing point; a step for keeping the keeping temperature; a step for annealing the synthetic quartz glass; and a step for taking the synthetic quartz glass out of the heat treatment furnace, wherein the process includes a step for determining an annealing rate v [°C/h] of the annealing step based on a value of S/V [mm²/mm³] wherein S [mm²] is the surface area of the synthetic quartz glass as a parent material and V [mm³] is the volume thereof and a target birefringence Re [nm/cm] for the synthetic quartz glass after the annealing, and the annealing step is performed at the determined annealing rate v.