Aleister Crowley said:
The Altar represents the solid basis of the work, the fixed Will of the Magician; and the law under which he works. Within this altar everything is kept, since everything is subject to law. Except the lamp.
With that picture in mind consider III x 30 = 90, which is Tzaddi - The Emperor. He is fixed will and law. But notice how the light/lamp shines on him. The law of the Emperor governs the four elements but not the fifth.
The most obvious and literal way to interpret this altar is as a physical object. But it may also be possible to interpret it as the aspirant him/herself. This altar is made of "open brass work".
Brass is an alloy of Copper and Zinc. Copper is sacred to Venus who is Love. Zinc, if I remeber my school boy chemistry right, is a transition metal whose properties include an ability to conduct electricity, when heated it becomes malleable, becomes magnetic if placed within a megnetic field, and readily forms combinations with other elements. It doesn't take much effort to see how these properties can be interpreted in a more symbolic way. The basis of the aspirants work is Love under Will. He/she is idealy able to conduct magical force (Fire), allow change to occur through open Union (Water), be open and perceptive to subtle (magnetic-like) influences (Air), and to be flexible or sturdy as required (Earth).
Burn thereon in Silver or Gold? Burn brings to mind the fiery, aspirational qualities of Aries, which is of course the Emperor. Silver and Gold relate to Yesod and Tiphareth. This makes me think of the silver-microcosmic pentagrams and gold-macrocosmic hexagrams that "burn" upon the magician, who is the Altar.