A window includes a base region and a compressive stress region disposed on the base region. The compressive stress region includes Li.sup.+, Na.sup.+, and K.sup.+ ions. The compressive stress region includes a first compressive stress portion in which a concentration of the K.sup.+ ions decreases, a concentration of Na.sup.+ ions increases, and a concentration of the Li.sup.+ ions increases, from a surface of the window toward the base region. A second compressive stress portion is adjacent to the first compressive stress portion. In the second compressive stress portion, the concentration of the Na.sup.+ ion decreases and the concentration of the Li.sup.+ ion increases, from the first compressive stress portion toward the base region. The window thereby has a high surface compressive stress value and impact resistance.

A potassium nitrate level detection sensing module M in a strengthening furnace according to the present invention includes a level detection means, which is installed inside a glass strengthening furnace, installed at a height equal to a level line of strengthening liquid to be filled in the strengthening furnace F, and generates an electrical signal having a set threshold or more or an electrical signal having the set threshold or less when it comes into contact with the strengthening liquid and detects that the strengthening liquid filled in the glass strengthening furnace F has reached a set strengthening liquid level line. The potassium nitrate level detection sensing module in the strengthening furnace according to the present invention having such a configuration can automatically identify that potassium nitrate in a liquid state, which is melted in the strengthening furnace, has reached the set level line in the strengthening furnace.

A potassium nitrate level detection sensing module M in a strengthening furnace according to the present invention includes a level detection means, which is installed inside a glass strengthening furnace, installed at a height equal to a level line of strengthening liquid to be filled in the strengthening furnace F, and generates an electrical signal having a set threshold or more or an electrical signal having the set threshold or less when it comes into contact with the strengthening liquid and detects that the strengthening liquid filled in the glass strengthening furnace F has reached a set strengthening liquid level line. The potassium nitrate level detection sensing module in the strengthening furnace according to the present invention having such a configuration can automatically identify that potassium nitrate in a liquid state, which is melted in the strengthening furnace, has reached the set level line in the strengthening furnace.

A glass article includes lithium aluminosilicate, includes a first surface, a second surface opposed to the first surface, a first compressive region extending from the first surface to a first compression depth, a second compressive region extending from the second surface to a second compression depth, and, a tensile region disposed between the first compression depth and the second compression depth, where a stress profile of the first compressive region has a first local minimum point at which the stress profile is convex downward and a first local maximum point at which the stress profile is convex upward, where a depth of the first local maximum point is greater than a depth of the first local minimum point, and where a stress of the first local maximum point is greater than a compressive stress of the first local minimum point.

A glass article includes lithium aluminosilicate, includes a first surface, a second surface opposed to the first surface, a first compressive region extending from the first surface to a first compression depth, a second compressive region extending from the second surface to a second compression depth, and, a tensile region disposed between the first compression depth and the second compression depth, where a stress profile of the first compressive region has a first local minimum point at which the stress profile is convex downward and a first local maximum point at which the stress profile is convex upward, where a depth of the first local maximum point is greater than a depth of the first local minimum point, and where a stress of the first local maximum point is greater than a compressive stress of the first local minimum point.

Glass-based articles having defined stress profiles and methods for manufacturing such glass-based articles are provided. A non-limiting glass-based article comprises an outer region extending from the surface to a depth of compression, wherein the outer region is under a neutral stress or a first compressive stress, a core region under a second compressive stress, the second compressive stress defining a compression peak having a maximum compression value and a maximum width at zero stress in a range of from about 1 micrometer to about 200 micrometers, and an intermediate region disposed between the surface and the core region, wherein the intermediate region is under a tensile stress.

Glass-based articles having defined stress profiles and methods for manufacturing such glass-based articles are provided. A non-limiting glass-based article comprises an outer region extending from the surface to a depth of compression, wherein the outer region is under a neutral stress or a first compressive stress, a core region under a second compressive stress, the second compressive stress defining a compression peak having a maximum compression value and a maximum width at zero stress in a range of from about 1 micrometer to about 200 micrometers, and an intermediate region disposed between the surface and the core region, wherein the intermediate region is under a tensile stress.

A potassium nitrate level detection sensing module M in a strengthening furnace according to the present invention includes a level detection means, which is installed inside a glass strengthening furnace, installed at a height equal to a level line of strengthening liquid to be filled in the strengthening furnace F, and generates an electrical signal having a set threshold or more or an electrical signal having the set threshold or less when it comes into contact with the strengthening liquid and detects that the strengthening liquid filled in the glass strengthening furnace F has reached a set strengthening liquid level line. The potassium nitrate level detection sensing module in the strengthening furnace according to the present invention having such a configuration can automatically identify that potassium nitrate in a liquid state, which is melted in the strengthening furnace, has reached the set level line in the strengthening furnace.

A potassium nitrate level detection sensing module M in a strengthening furnace according to the present invention includes a level detection means, which is installed inside a glass strengthening furnace, installed at a height equal to a level line of strengthening liquid to be filled in the strengthening furnace F, and generates an electrical signal having a set threshold or more or an electrical signal having the set threshold or less when it comes into contact with the strengthening liquid and detects that the strengthening liquid filled in the glass strengthening furnace F has reached a set strengthening liquid level line. The potassium nitrate level detection sensing module in the strengthening furnace according to the present invention having such a configuration can automatically identify that potassium nitrate in a liquid state, which is melted in the strengthening furnace, has reached the set level line in the strengthening furnace.

An apparatus for thermally strengthening a glass sheet includes a first heat sink surface, a second heat sink surface separated from said first heat sink surface by a gap between the heat sink surfaces of distance g, and a liquid feed structure positioned to be able to feed a liquid to the gap, wherein the distance g is sufficiently small relative to a thickness t of a glass sheet to be processed such that when a sheet of thickness t is positioned within the gap of distance g, thermal transfer from a first surface of the sheet facing the first heat sink surface is more than 20%, 30%, 40% or 50% or more by conduction from the first surface of the sheet through the liquid to the first heat sink surface.