These two terms are not interchangeable. “Percent” means “per hundred” and can either be written out or expressed by the symbol %. Ideally, “percent” is always associated with a specific number:

The maximum error that can be introduced by over-mixing is 10%.

“Percentage” is used to refer to a general relationship rather than a specific measure:

A large percentage of the people voted, but only 20 percent of the votes counted.

In published literature, many writers use “percent” as an adjective (“percent quartz”) for economy, especially in figures and tables. If this is done, the same phrasing should be used consistently to refer to the same thing.