When determining the empirical formula of Magnesium Oxide, why are the crucible and lid heated before weighed?

When you carry out any gravimetric ysis, you must make sure that the equipment used will have constant m. It could be possible that the crucible and lid have been contaminated with oil, dust, moisture or some other substance. Therefore before using it you subject it to the same conditions that would be used in the actual experiment. So you heat the crucible and lid, allow it to cool and then determine the empty m. In strict practice you should repeat this process until what is called constant m is obtained.