Investigation of cat scratches on container glass

Abstract

Cat scratches have become a chronic quality problem for the container glass industry but there is still controversy about their origin and how to eliminate them. Many tests related to appearance, shape, location, and chemical composition of this type of cord have therefore been made to get more information about them. As a result it can now be said that cat scratches are surface cords containing higher amounts of alumina and especially zirconia than the host glass. The cords form 10 to 20 µm thick bands located 10 to 60 µm below the surface. The origin of the problems is the corrosion of the refractory material. It is impossible to locate exactly the primary source because different mechanisms can create the cords. However, the main source is probably in the hot zone of the melter. The refractory-rich glass accumulates on the bottom of the tank where the overall flow makes it creep along the bottom of the whole furnace to the feeder outlet. The best ways to reduce the frequency of cat scratches are either appropriate drainage or mechanical stirring in the feeder channel.