These leather sand or shot bags are made of heavy weight but supple suede. They are perfect for placing a pitch bowl, or pitch tray on to add mass for a more solid blow, reducing vibration, and creating a positionable surface to work on. They are also great for bracing metal to be hammered on, or for supporting vessels that are filled with pitch for chasing.
 
They come empty and have a small velcro opening, so that you can fill them with sand, or lead shot. The reason they come empty, is that you can fill them up to your desired volume, and with your desired filler. This also makes them ideal for transport. If you are taking a workshop, you can simply empty it, stop by a home depot, (or beach) at your desination, and fill it up as desired.
 
Sand is cheap and easily available, but lead shot will add much more mass, for a much denser blow.
 
Available in 10", 14" and 18" sizes
For more info, there is a tutorial here