 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 . 
