On a question like this, it helps to read a few lines before and after the quoted text to be sure that you have the context clear.
The language at issue is "Noguchi . . . pondered the fact that sculptors through the ages had relied exclusively upon negative light - that is, shadows - for their conceptual communication, precisely because no metals, other than the expensive, nonoxidizing gold, could be relied upon to give off positive-light reflections."
In context, the author is comparing gold - which is highly reflective and non-oxidizing, meaning that oxygen does not cause its reflective quality to fade over time - with other metals "that can be made highly reflective but lose their reflective properties over time."