The bottle’s shape, color and labeling style are the most direct ways of authenticating a vintage milk bottle. Factors that affect the bottles’ casing and paint are also indications of authentication.
The first telltale sign of a bottle’s age is its shape since milk bottles before the 1930s were rounded before taking on the square squat shape of the '40s. Because these bottles were often reused, manufacturers added labels to discourage reuse by other manufacturers and to ensure the bottles returned for refilling. These labels were etched, embossed, all-over raised or painted on. Rare bottles were made of an opaque white milk glass or a green glass, whereas more common milk bottles were either clear or amber.
Throughout time the rattling of the bottles themselves scratches the casing in which they are held. Such marks are common, but value declines with greater wear. Vintage bottles are often reproduced by applying new paint over an old bottle, but these counterfeits are easily caught since the paint scrapes off with ease. Original bottles were painted before being heated, making the paint difficult to erode.