Write on! School Smart Black Extra Fine Line Markers write like a pen, but with the boldness of a black permanent mark. Quick drying black ink is resistant towards heat, water, and smearing. Tip measures 1/2 millimeter for legible handwriting or sketches. Marker is ideal for labeling on most surfaces like acetate films, plastic, ceramic, and glass. Non-toxic. Conforms to ASTM D-4236 standards. Sold as 12 per pack.