A novel test has been developed to measure the friability of the primary coat of ceramic shell moulds. It has been used to demonstrate how the friability is affected by the refractory loading and polymer content of standard shell systems used for casting aluminium alloys and steels and to establish the effect of the mould firing temperature. It has also been used to assess a yttria primary coat used for moulds for casting titanium alloys and has shown that this is significantly more friable than the primary coats used for casting aluminium alloys and steels. However, increasing the firing temperature to 1400°C dramatically reduces the friability of the yttria primary coat to a level comparable with more conventional mould materials. The friability test to assess the surface strength of a mould complements the Modulus of Rupture test which measures its bulk strength.