The Pitt Artist Pens feature pigmented India Inks that are lightfast, acid-free and pH Neutral. The ink is odorless, waterproof and permanent once dry on porous surfaces. The ink will also not bleed through paper making them ideal for sketch books. The pens are excellent for building color and achieving a vast array of techniques and details. The flexible brush tip retains its shape and works beautifully on all types of papers and canvas. Use the 0.7mm Medium pen for outlines and adding detail.