A tensile testing machine has a jaw which includes a unitary ‘H’ section member having a cross piece and two uprights. The cross piece is mounted to a cantilever beam which is supported on the test machine and has force measuring elements. A pneumatic actuator applies tensile forces to the first ends of the uprights via strands to urge the first ends apart. When first ends of the uprights are urged apart, their oppositely disposed second ends are urged together to grip a sample deposit to be pulled off a substrate in a tensile test. In use, the jaws can be moved at speed, in the order of 500 mm/s to pull the deposit off of the substrate. The force required to pull the deposit off the substrate in this tensile test is measured by the force measuring elements on the beam.