STRAINS IN GLASS PRODUCED BY APPLIED COLOR LABELS *
Authors: Ablard J. E. 
Published: June 2006

Abstract

The fact that “applied color lettering” can weaken a bottle has been recognized from the beginning of the practice, and the fact that it normally does not but may even increase the strength slightly is also well known. Weakness arising from the “color” causes fractures with origins on the outer surface of the bottle wall and within the colored area. They are thus readily distinguished from those fractures which arise from weaknesses of the inside surface of the wall.

The author investigated this subject with commercial and experimental colors on pressure bottles of various manufacturers. A special thermal‐shock test was used, which tests the strength of the barrel of the bottle without producing large strains on the base. By correlation of such shock tests with the strain patterns observed in ring sections examined under the polarizing microscope and with thermal‐expansion data, it has been possible to set up a suitable procedure for insuring the proper “fit” of a ceramic color to a glass. With minor exceptions, the problem is considered to be similar to those of applying enamels to metals, glazes to pottery, and in sealing metals to glass, which have already been discussed in the literature.

By an analysis of the respective thermal‐expansion curves for the glass and the ceramic colors, the author suggests the reason for the experimentally found fact that the ceramic color must be of slightly lower coefficient of expansion than the glass in order to minimize the strain between the color and the glass when the color has been fired on to the glass and the two cooled down to room temperature. The suggested reason is that the fluxes used in the manufacture of ceramic colors are of necessity softer than the glass and, therefore, their annealing range is below that of the glass. From the shapes of the respective total expansion curves, it is shown that, when the color passes through its lower critical annealing temperature, its curve falls below that of the glass. If the coefficient of expansion for the color is less than that of the glass, the lower rate of contraction of the color balances out this initial high contraction so that at room temperature no residual strain is left.

A brief discussion of the interface between the glass and the ceramic color is also included, with particular reference to the case of a flux containing lithia. The small‐diameter, highly mobile lithium ion migrates into the glass, creating an interface tension cord, visible under the polarizing microscope. Present information shows that this is permissible, provided the other factors mentioned have been properly adjusted.

A considerable amount of work has been done to determine the cause of the weakening effect of some applied color labels on the strength of glass bottles. The strength of the bottles was found to be unchanged after the label had been applied except when (1) the difference in thermal expansion between the bottle and the applied color label was such as to produce a tension strain in the label or (2) the applied color label was so unstable that it devitrified during the firing process, crystallizing out a silica‐rich phase and leaving a matrix consisting of a high alkali phase having a high thermal expansion. The difference in thermal expansion between the matrix and the glass gave rise to a tension strain in the applied color label.

It has long been recognized that tension strains on the surface of glassware are undesirable because they reduce the strength of the glass. The present work has affirmed this contention in the case where the outside surface is an applied color label. When the label is not in tension, the ware is as strong after the label has been applied as it was before.

A rapid method for determining the suitability of the applied color for use with a particular glass is suggested. It is also shown that an applied color label which contains lithia, if properly compounded, will not reduce the strength of the glass to which it is applied. A tentative explanation is presented of the factors involved during the decoration process which result in strains in the glass under the applied color label.