An Examination of the Cause of the Humidity Sensitivity of the Lightfastness of Dyed Materials

Light fading of some typical sulfonated acid wool and direct cotton dyes has been assessed, visually against the blue standards, on cotton, wool, silk, and anodized aluminum, and spectrophotometrically in films of regenerated cellulose and gelatin, at various respective relative humidities of the surrounding atmosphere. Some tests were also made in which the films were exposed to light when sealed against entrance of air. The known, lower susceptibility of fading rate to relative humidity of dyes on wool compared with cotton is confirmed and shown to apply also to silk and gelatin. Fading of dyes on anodized aluminum, after sealing, is unaffected by any change in the humidity of the surrounding atmosphere. It is suggested that on cotton, increase in RH causes more rapid fading because of increased fiber swelling, which accelerates transport of oxygen to the excited dye molecule. The anodic film on aluminum does not swell and so is unaffected by change in RH. Though the swelling effect operates with proteins, it is less noticeable because fading thereon is only partly caused by oxidation; mainly it is due to reduction, hydrogen being abstracted from the substrate itself, and therefore fading is less dependent on oxygen transport into the fiber. This suggestion is consistent with the observation made in this work—that fading on cellulose is considerably retarded by completely sealing the dyed substrate against air, whereas fading on a typical protein (gelatin) is hardly affected at all.