These grey sheets / blocks of lino are especially made for "linocut" relief printmaking. They are 3.2mm thick, have a fabric backing, and are available in a range of sizes and packs.
The surface of the lino is cut away using special lino cutters or a knife to form the basis of the lino print. The block is inked up with a roller (known as a brayer) and this creates a positive image when printed onto paper or fabric. The printing process can be done by hand or with a press. You can use water-based lino ink, oil-based lino ink, or a block printing medium mixed with paint.
