Bead size is inversely proportional to welding speed at the same current. Higher speeds reduce bead width, increase the likelihood of porosity, and if taken to the extreme, produce undercutting and irregular beads. At high welding speeds, the arc voltage should be kept low to minimise the risk of arc blow. If welding speed is too low burnthrough can occur. A combination of high arc voltage and low welding speed can produce a mushroom-shaped weld bead with solidification cracks at the bead sides. For a given arrangement of wires and wire diameters, welding speed is limited by the welding current which can be tolerated by the flux. Some fluxes are specially formulated to allow high speed operation. Higher speeds are possible with multiple wire operation or by holding a more acute electrode angle.