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The skill in casting thick-walled vessels such as these ladles is in producing sound cross-sections with no voids or gas porosity. With heat resistant castings, low carbon steel can be poured at very high temperatures and this permits the molten mass to purge itself of air entrapment, allowing metal to fill shrinkage voids before the entire mass solidifies. The final ladle castings were inspected for defects using ultra-sonic inspection techniques. Ultra-sonic inspection is a non-destructive testing technique used to detect imperfections (voids, cracks) below the surface of solid materials. This process uses high-frequency sound waves that pass through the solid material and monitor the echo (bounce-back) distance of the sound wave within the material. The theory supporting this method is that if the material is sound, the echo distance equals the material thickness.