Editing Welding (section)

===Glass welding===
{{main|Glassblowing}}

Glass welding is a common practice during glassblowing. It is used very often in the construction of lighting, [[neon sign]]s, [[flashtube]]s, scientific equipment, and the manufacture of dishes and other glassware. It is also used during [[glass casting]] for joining the halves of glass molds, making items such as bottles and jars. Welding glass is accomplished by heating the glass through the glass transition, turning it into a thick, formable, liquid mass. Heating is usually done with a gas or oxy-gas torch, or a furnace, because the temperatures for melting glass are often quite high. This temperature may vary, depending on the type of glass. For example, [[lead glass]] becomes a weldable liquid at around {{convert|1600|F|C}}, and can be welded with a simple propane torch. On the other hand, quartz glass ([[fused silica]]) must be heated to over {{convert|3000|F|C}}, but quickly loses its viscosity and formability if overheated, so an [[oxyhydrogen]] torch must be used. Sometimes a tube may be attached to the glass, allowing it to be blown into various shapes, such as bulbs, bottles, or tubes. When two pieces of liquid glass are pressed together, they will usually weld very readily. Welding a handle onto a pitcher can usually be done with relative ease. However, when welding a tube to another tube, a combination of blowing and suction, and pressing and pulling is used to ensure a good seal, to shape the glass, and to keep the surface tension from closing the tube in on itself. Sometimes a filler rod may be used, but usually not.

Because glass is very brittle in its solid state, it is often prone to cracking upon heating and cooling, especially if the heating and cooling are uneven. This is because the brittleness of glass does not allow for uneven [[thermal expansion]]. Glass that has been welded will usually need to be cooled very slowly and evenly through the glass transition, in a process called [[annealing (glass)|annealing]], to relieve any internal stresses created by a [[temperature gradient]].

There are many types of glass, and it is most common to weld using the same types. Different glasses often have different rates of thermal expansion, which can cause them to crack upon cooling when they contract differently. For instance, quartz has very low thermal expansion, while [[soda-lime glass]] has very high thermal expansion. When welding different glasses to each other, it is usually important to closely match their coefficients of thermal expansion, to ensure that cracking does not occur. Also, some glasses will simply not mix with others, so welding between certain types may not be possible.

Glass can also be welded to metals and ceramics, although with metals the process is usually more adhesion to the surface of the metal rather than a commingling of the two materials. However, certain glasses will typically bond only to certain metals. For example, lead glass bonds readily to [[copper]] or [[molybdenum]], but not to aluminum. [[Tungsten]] electrodes are often used in lighting but will not bond to quartz glass, so the tungsten is often wetted with molten [[borosilicate glass]], which bonds to both tungsten and quartz. However, care must be taken to ensure that all materials have similar coefficients of thermal expansion to prevent cracking both when the object cools and when it is heated again. Special [[alloy]]s are often used for this purpose, ensuring that the coefficients of expansion match, and sometimes thin, metallic coatings may be applied to a metal to create a good bond with the glass.<ref>Freek Bos, Christian Louter, Fred Veer (2008) ''Challenging Glass: Conference on Architectural and Structural Applications''. JOS Press. p. 194. {{ISBN|1-58603-866-4}}</ref><ref>{{Cite book|last=Bolas|first=Bernard D.|url=http://archive.org/details/handbookoflabora02bolarich|title=A Handbook of Laboratory Glass-Blowing|date=1921| place=London| publisher= G. Routledge & Sons}}</ref>{{failed verification|date=July 2023|reason= searched for pretty much every key word in this paragraph in an attempt to track down a page number, and the closest thing found was a bit about the coefficient of expansion of liquid when forming a thermometer. maybe this citation is intended to support a different paragraph? }}