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The Evolution of Glass-Making Techniques

Cheryl Owens

Published on: 7/18/2023

Throughout the development of early glass, the crucible was made from natural clay. Ancient Egyptian crucibles beginning about 1370 BC were shallow and had large amounts of alkali and magnesia and 6 to 8 percent iron oxide. This type of crucible could hardly withstand modern melting temperatures of 2,000 degrees F and higher, and they contaminated the glass with iron. The discovery of the blowing iron brought in the development of pot furnaces, which have remained almost unchanged to the present time. The pot furnaces were made of a mixture of raw clay mixed well to remove bubbles. The pot floor was made first, before the sidewalls and the cover, and a side opening was added last.Compartmentalized furnaces were developed by the 9th and 10th centuries. Wood fires burned within a lower compartment, directly underneath a compartment which held a glass melting pot. After the product was formed, it was left to cool slowly in a third compartment located above or to the side. Many of the early designs of furnaces failed to understand the need for air drafts. During the late 17th century, cone-shaped, or “English,” glass furnaces using coal as fuel appeared. The cones rose as high as 114 feet and were 32 feet in diameter. In these furnaces, covered pots for glass melting were placed on a middle level below the ground, and an underground tunnel drew in high-velocity upward air drafts. Much higher temperatures could be attained with this design.