From the manufacturer:
Pigmented India ink that is both acid-free and archival (PH neutral) Smudgeproof and waterproof when dry. Our ink contains the highest quality pigments for unsurpassed lightfastness. Lightfastness rating and color name noted on barrel of each pen. Artist brush pens are ideal for graphic artists, illustrators, designers and architects. Perfect for sketching, journals, cartooning, fine-ink drawing, scrapbooking and rubber-stamping.