I recently got an Ember Mug and I wanted to see whether or not I was able to heat up cold coffee in it using nothing but the mug.
It turns out you can heat up cold coffee in the Ember Mug but it takes an extremely long time, is extremely laborious task and uses almost all the battery. You're much better off using a microwave or stove to heat up your coffee or tea faster and then putting it in your Ember Mug afterwards.