The term black glass mainly refers to very dark English bottles from about 1690 to the 1850. Black glass bottles are usually not actually black, but the colors (ranging from dark brown to olive-green) become visible when the bottle is held against the light. One reason for the production of black glass was that it protected the contents of the bottle against the harmful effects of light. From the late 1600s to the mid 1800s, the majority of English bottles were made of black glass. The most typical bottles from that period were onions (ca. 1690-1730), mallets (ca. 1730-1770), squat cylinders and cylinders (from about 1770). The English melting technology, based on the use of coal, was later introduced in the United States and in continental Europe, and also resulted in almost black bottles. Sizes: 3 1/4 x 3 1/4 x 10 1/2 in. 3 x 3 x 9 in.