PEPIN Postcard Watercolor Books contain 20 cards, made of the finest acid-free watercolor paper. The 250 gsm paper is reasonably thick to make the cards suitable for mailing. Apart from watercolor, practically all other techniques can be used to apply color to the delicate designs in these books. PEPIN Postcard Watercolor Books match the quality of the finest professional watercolor pads. In order to get the best results, we recommend that you use high-quality coloring materials.