I don't have a clear idea of what "temperature doesn't manifest itself on to the beaker" means. If the beaker isn't exposed to 1000°, it doesn't seem like you actually have a question. If it does, your question is apparently (I'm guessing): Why in air at 120° doesn't a beaker at 1000° crack? The answer MIGHT be that the temperature rise of the beaker was slow and uniform enough to not cause excessive stress. The answer MIGHT be that beakers (kimax, pyrex, borosilicate, possibly even quartz) are designed to tolerate wide temperature ranges. Or perhaps the answer is that the beaker never comes into contact with 1000° material (or that the energy transfer of any such contact is very small). I will say that a reaction started at 120° and which runs up (exothermally) to 1000° is either out of control or must be controlled to be quite gradual - so gradual as to cast doubt on the attainment of 1000° (since there are heat losses involved).