<P> 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 . </P> <P> 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 alloys 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 . </P> <P> Plastics are generally divided into two categories, which are "thermosets" and "thermoplastics ." A thermoset is a plastic in which a chemical reaction sets the molecular bonds after first forming the plastic, and then the bonds cannot be broken again without degrading the plastic . Thermosets cannot be melted, therefore, once a thermoset has set it is impossible to weld it . Examples of thermosets include epoxies, silicone, vulcanized rubber, polyester, and polyurethane . </P> <P> Thermoplastics, by contrast, form long molecular chains, which are often coiled or intertwined, forming an amorphous structure without any long - range, crystalline order . Some thermoplastics may be fully amorphous, while others have a partially crystalline / partially amorphous structure . Both amorphous and semicrystalline thermoplastics have a glass transition, above which welding can occur, but semicrystallines also have a specific melting point which is above the glass transition . Above this melting point, the viscous liquid will become a free - flowing liquid (see rheological weldability for thermoplastics). Examples of thermoplastics include polyethylene, polypropylene, polystyrene, polyvinylchloride (PVC), and fluoroplastics like Teflon and Spectralon . </P>

The technique of cross friction could be described as all of the following except