A Method for the Identification and Assessment of the Presence of Bronzing in Printing Ink Films

This paper examines the phenomenon known as bronzing. The phenomenon has historically been ascribed to unusual morphological defects in the surface of an ink film. The most common explanation proposes that at some mass concentration, the individual pigment particles are pushed up through the surface of the varnish and light strikes the dry pigment resulting in a diffuse reflectance that differs from that observed when the pigment is surrounded by varnish. The measurement data reported in this paper clearly shows that this model cannot be correct. Not only do the extensive measurements reported here point to a more fundamental cause for bronzing, but they provide the framework from which to establish a measurement protocol for quantitatively assessing the level of bronzing in an ink film. This new framework can also provide the necessary additional information to a colorant formulation algorithm to allow the computer to predict at what concentration a colorant will begin to exhibit significant bronzing, either in a monochromatic ink or in a mixture of other colorants.