If a strip of leather is slowly heated in water a sudden shrinkage occurs at a temperature which is characteristic of the tannage. Generally, the higher the shrinkage temperature, the better the heat resistance of the leather, so measurement is useful for judging the suitability of leather for different footwear manufacturing processes and particularly for moulded-on footwear.
Frequently, the shrinkage temperature is above 100°C and has to be determined in water under pressure. This is the special feature of the SATRA apparatus which measures shrinkage temperatures up to 115°C.
The apparatus consists of a vertical sight glass, connected by tubes to a smaller boiler which can be heated by a bunsen burner (not supplied), so providing a convection circuit of heated water.
A strip of leather and a thermometer are suspended in the sight glass, the upper end of the leather is fixed and the position of the lower end is indicated by an adjustable marker outside the tube to help judge when shrinkage occurs.
A safety valve is fitted to the apparatus which limits the pressure to that corresponding to a temperature of 120°C.
A perspex guard is provided around the sight tube as a safety precaution.