These artist quality tools are designed for use with Pan Pastels , water-based colors, polymer clay, or ceramics. Just pull the Micropore sponge cover completely over the knife tip, load with color, and gently apply to the surface. Use light pressure- do not dig. They are easy to control for lines or details, and are reusable. Clean with soap and water. Each packaged knife comes with five replaceable covers. There are also packs of replacement covers sold separately. The mixed pack includes ten covers in each of the four shapes. Sold as PACK OF 2.