Calks were used to improve a horse's traction in mud, on icy surfaces, etc. There are different shapes of calks for different conditions. Obviously, the shoe has to be one that will accept calks. Diamond Calk Horseshoe Company of Duluth, Minnesota may still manufacture horseshoes and calks. Neverslip and Giant Grip are two other calk manufacturers.
The J. Caspers calk wrench is 13" long. Mine still has green paint on it.