Shell cores are made using a resin coated silica sand that is invested (blown) into an iron core box. The core box is preheated to about 550 degrees Fahrenheit and the core(s) cure and become rigid due to the reaction between the resin and heat. The cores are yellow in color and are hollow, having a wall thickness of 1/8 to 3/4 inch depending on casting size and configuration. The hollow shell that forms the core gives it its name. Shell cores are made on vertically parted semi-automated machines yielding 20 to 60 cycles per hour. Shell cores provide the best cosmetics and dimensional tolerance of all the core processes.