Wrought Iron is the purest form of iron; although it may contain traces of carbon. It is usually made by ‘‘puddling process’’. It is very costly and its use has been almost totally replaced by cheaper steel. However, for some components like chain-links and Crane-hooks wrought iron is still the preferred raw material.

Slip gauges are graded according to their accuracy as Grade 0, Grade I and Grade II.
Grade 0 is used in laboratories and standard room which serves as standard for periodically checking the accuracy of Grade I and Grade II gauges.
Grade I is of higher accuracy and used in inspection departments.
Grade II is intended for use in workshops during the actual production of components, tools and gauges.