The Brainliest Answer!
The coffin will brake due to moister in the dead body(when it stared to dry) inside a air tight chamber. 
usually to  dry the dead body until r unless u have to use proper mummification. otherwise it will decompose ,due to that fume and water vapor will buildup the pressure inside the coffin ,if glass can with-stand  this high pressure than nothing will happen.if not it will break .  
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actually it brakes due to the soul coming out right
its my doubt
soul.....u mean spirit///
Acc to Hindu and various religious aspect A body with out soul is called dead body... When a person dies his sole will exit