THE LONG RECORD OF FUSIBILITY

The fusibility of minerals has long been recognized as a useful property. Ores were smelted, metals worked, glass and ceramics made, and cloth dyed. The resultant store of practical and folk wisdom was part of investigations in mineralogy in the 18th century and before. Test fusions of rocks and minerals were done in conjunction with those technologies and the blowpipe was in very early use. Things became more scientific in the early part of the 18th century with the work of Pott, Wallerius, and others. In 1784 Wedgwood published a table of fusibilities in order to calibrate his pyrometer. In 1794 Kirwan published a ranking for fusibility of 1 to 5, tied to degrees of Wedgwood, but described in terms of appearance. De Saussure published an extensive table of fusibilities in which he used a novel method to relate visual results from blowpipe analysis to Wedgwood temperatures. Fusibility was defined in Cadet's Dictionaire de Chemie of 1803, and was listed in mineralogy texts of the time. Cordier published his own list in conjunction with his ground-breaking work on mineral identification in fine-grained basalts. Von Kobell published a fusibility scale consisting of successively less easily fusible minerals, that is still, like Moh's hardness scale, included in current textbooks. Seger's cones provided another method of determining fusibility.

There were many innovations in high temperature pyrometry. Many were designed to make conditions for technologies better known and efficient, but fusibility tables were still inaccurate and inconsistent. Barus reviewed the situation in 1889. Noting the unreliability of the earlier fusion scales for minerals, in 1891 Joly originated what he called a meldometer, which depended on the thermal expansion of a platinum ribbon. Optical pyrometers, thermocouples, and gas pyrometry also have a long history beginning in the 19th century. Geologists and mineralogists were still concerned with conditions of rock origin and metamorphism, and originated equipment of increasing complexity. However, von Kobell's scale is still used and continues to be cited in mineralogy textbooks and web sites.