Most ancient silk fabrics have deteriorated and have been reduced in strength. The author showed that the older the silk fabrics, the greater the loss of strength. He conducted experiments on raw silk, exposing them to different levels of humidity and to ultraviolet radiation. The results obtained show that raw silk is greatest in strength at the relative humidity of 20% and shows reduction in strength with the increase in relative humidity, and that irradiation with ultraviolet light deteriorates it.