How can we clean corundum crucible?

How can we clean corundum crucible?
As we all know, crucibles are mainly used in labs to hold and heat chemicals. However, after using it for a certain period, there will be residuals of various chemicals in it. These foreign matters will seriously affect the thermal performance of crucible. As a result, it is necessary to clean it thoroughly. Nextgen Advanced Materials suggest you try the following method to clean up crucibles.
Add some acid to clean residuals that stick to the bottom of crucible. However, you must pay attention to the corrosivity of acids and wash up the crucible with running water. 
But for some stubborn impurities in a crucible, you have to take a more effective approach. You can heat the crucible with nitric acid. During which the temperature should be controlled carefully to avoid high temperature increase, because crucible may break due to sudden change of temperature.