What is "acid-free"

Usage examples of "acid-free".

But the rag paper, acid-free, marked by special noncorrosive inks, would last far longer than any wood-pulp paper.

Then she carefully covered the desktop with white acid-free paper, taped it to the corners, laid another sheet on top, and placed a series of sample bags, stoppered test tubes, tweezers, and picks along one edge.