Picture molten metal rushing in fast right near the gate and scraping sideways across the sand floor of the mould, much like a fast stream washing soil off a riverbank. That scouring action erodes loose sand grains away from the drag surface close to the gate, and the metal then fills that eroded space, leaving a shallow raised patch on the finished casting. Because the look and the cause both resemble water washing material away, foundrymen simply call this defect wash. It is distinguishable from cold shut, swell, and blow holes because wash is tied specifically to high velocity erosion happening near the gate in bottom gating.