Copper outside, dust inside

The sacrificial altar in the Mishkan was very different to the Mizbeach in the Beis Hamikdash.
 
In the desert, the Mizbeach was constructed as a hollow shell built out of acacia wood. The wood was covered with copper-plate.
 
The Torah says that the Mizbeach had to be made as a מזבח אדמה, an altar made of earth[1]. How did the Mizbeach in the Mishkan meet this criterion?
 
At each place that Bnei Yisroel camped the hollow of the Mizbeach would be filled with earth[2]. So, the Mizbeach had dust on the inside and copper on the outside.
 
In one of the prophesies of Yishayahu[3], Hashem says about the Jewish peopleמדעתי כי קשה אתה וגיד ברזל ערפך ומצחך נחושה – “I know how stubborn you are, that your neck is like an iron sinew and your forehead is like copper”.
 
A copper forehead is a metaphor for brazenness – ones holds their head up strong and does not show shame or humility. Rashi explains that the Mizbeach was covered in copper to atone for the sin of brazenness.
 
But like all Middos, the Middah of brazenness can also be used in a holy sense. It represents being strong in one’s service of Hashem and to not be embarrassed from those who mock at us[4].
 
Like the copper coating this brazenness should only be an outward and external show. On the inside, we need to be filled with earth, reflective of humility, as we say in our daily Shemoneh Esrei “let my soul be like dust before all”.
 
~ Based on Reshimos booklet 108