But a locality which furnished silica, alkali, and lime would still be badly off as regards the needs of glass-making if it were out of reach of adequate supplies of substances refractory enough when fashioned into crucibles to permit the fusion of the mixture. For this purpose fire clay is the material par excellence, since it withstands both the chemical action of the molten glass and the disintegrating effect of the intense heat of the furnace. It is an essential to glass-making. Bulk for bulk, however, much less fire clay is needed than crude material for the batch, so that it is less needful that the fire clay shall be a local product. It can be brought to the batch more economically than the batch can be taken to it. It does not happen, therefore, in the history of the glass industry, that the mere presence of suitable clay ever determines the location of works. At the present time much of the clay used in both England and America comes from Germany. It is significant, in looking over the columns of our trade journals, that the advertisements are for the most part of the imported rather than the native article. There are, however, large deposits of excellent clay in northeastern New Jersey, in western Pennsylvania, in Missouri, in Ohio, and in other parts of the country, which must eventually be utilized. The American clay is, if anything, purer than the foreign, but it is less dense, and will probably require somewhat different treatment from the German. The attempt to substitute it for the imported in the earlier days, before the requirements of the pot clay were so well known and our own deposits had been so well exploited, led to financial disaster, and even to the suspension of a large works at Boston, where the experiment proved absolutely fatal. Our knowledge of refractory materials is less scientific than of any of the other materials used in glass-making. In consequence we are the more dependent upon rule-of-thumb methods in working them, and pay the more dearly for the experience when we venture any innovation.
The third element involved, fuel, is of all the most important, both as regards quality and cost. In America it has been the dominant element, and largely determined the location of our glass-houses and the measure of their success. The choice lies between four varieties—wood, coal, petroleum, and natural gas. In the earlier days a fifth fuel is found on the list, North Carolina rosin, but it can hardly be said to figure in the present production. In England coal does not seem to have been used as a fuel to any extent until the beginning of the seventeenth century. About 1623 Sir Robert Mansell obtained a patent for a "method of making glass with sea coal, pit coal, or any other fuel not being timber or wood." The patent was probably for some particular method, as the simple use of coal was well known, even in the preceding century, though by no means common. The early Virginia glass