and slopes sharply the actuator 21 is operated to incline the resting surface of the sole so that the surface of the sole which is to be treated still remains with its heel portion in correspondence with the rear centering block and consequently with its heel area and toe area substantially at the same height as can also be seen in fig4 whenever the waist of the sole arches above the resting surface the front mask can have a body 61 shaped in such a way as to still follow the shape of the sole and support it the body 61 can also be perforated to constitute an extension of the suction duct as shown by the broken line in fig4 as shown in fig5 in the case of soles which are sufficiently flexible to be easily flattened out to substantially adhere to the resting surface 26 it is possible to use the alternative solution shown in fig5 in which the rear movable plate 32 is lowered to receive the heel the operator can press the specimen sole against the front plate so as to lower the rear plate by the correct degree and then operate the means blocking the rear plate to keep it firmly secured in the required position once the adjustment has been carried out it can be maintained throughout the treatment of all the soles of the same type after.