Beginning in the 1880s, American manufacturers added manganese to the glass as a decolorizer. After exposure to the sun's ultra-violet rays the manganese would change the color of the glass to purple. Manganese worked fine until the outbreak of World War I in 1915 caused another defining temporal characteristic to occur in bottles. Prior to 1915, Germany was a main supplier of manganese. When the war broke out, the supply of manganese was no longer available to the bottle makers and they resorted to using selenium as a decolorizing agent.
Selenium causes the glass to turn an amber color when exposed to light.
Techniques for labeling glass bottles also changed through time. Etching was common in the late nineteenth century, followed by Applied Color Labeling. Developed in the 1920s, Applied Color Labeling was a technique which used panels or lettering, embellished with enamel. The technique was used more commonly in the 1930's, primarily for bottles that were reused, such as soda and milk bottles, but still is still in limited use today. After 1930, when manufacturers were finally able to produce inexpensive containers that would not change colors after long exposure to the sun, clear glass available to the general consumer made a comeback.