In inustry, etching (sometimes called chemical milling or wet etching) is the process of using acids, bases or other chemicals to dissolve away unwanted materials such as metals, semiconductor materials or glass. This process has been used on a wide variety of metals with depths of metal removal as large as 12mm (.5 in). Selective attack by the chemical reagent on different areas of the workpiece surfaces is controlled by removable layers of material called masking or by partial immersion in the reagent. It has applications in the printed circuit board and semiconductor fabrication industries. It is also used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes.