The three metals used in the temple were
• Copper
• Silver
• Gold.
They form a hierarchy - for example, Gold is the best metal while copper is the lowest metal. The copper altar corresponds to the ordinary people and atones for their sins. The Golden altar corresponds to the priestly people and addresses their needs.

The Bible does not give us much information on the symbolism of copper, silver, and gold. It is possible to give various symbolic associations: For example, copper vessels are used with fire while silver,gold vessels are more precious and not used with fire. We may therefore say that copper atones for fiery emotions. Rashi states that Copper atones for brazenness. Perhaps Rashi focuses on the Hebrew root of copper, Nun-Cheth-Shin which is also the root of the word snake, which possibly symbolizes brazenness or similar emotions.

I think it important to emphasize that the symbolic interpretation should be objective. Therefore we are not focusing on lingual coincidences. We are instead focusing on something common to all interpretations: Copper, silver, Gold form a hierarchy of metals. Using this basic idea we symbolically interpret: There are three stages of people: Ordinary, medium and spiritually advanced corresponding to copper, silver, and gold. Thus the copper altar atones for the ordinary people. Such an approach which doesn't emphasize particular traits of ordinary people - such as brazenness - seems the most acceptable; something that can be agreed to by all people. That is all people can agree that at least the Bible is talking about atonment for ordinary people. Then each person can add more detail such as acts of brazenness. The idea of ordinary could be accepted by everybody while the focus and specficitiy on particular emotions, like brazenness, would only be accepted by those people who believe them.