60 Dr. A. Hubert Cox — South StaJ^ordslnre Fire-clays. 



one is dealing with a truly refractory clay, and accordingly the ignited 

 article retains its original shape. The resulting product, in fact, 

 does not present to the naked eye any marked difFei'ence to the 

 biscuit-clay, save that it appears, perhaps, to be a little more compact. 

 A careful examination, however, reveals slight traces of fusion 

 having taken place, and the material no longer tends to crumble 

 easily as does the biscuit-clay. Actually, however, the specific 

 gravity is lowered, as is only to be expected when crystalline material 

 passes over into the amorphous condition. 



A high-grade clay will then remain in much the same condition, 

 even when subjected to considerably higher temperatures, tempera- 

 tures up to 1600° C, or even in certain special cases up to 1700° C. 

 Vitrification is naturally progressive, but still the clay does not soften 

 appreciably until these temperatures are reached. An inferior clay, 

 on the other hand, under the same conditions melts down more or 

 less completely, naturally changing its shape completely during the 

 process. All stages from readily fusible clays to highly refractory 

 ones may be obtained. It was found that the softening-point of 

 lower-grade fire-clays was considerably raised if the coarse sand- 

 (quartz-) grains were first removed by washing. Conversely these 

 coarser portions from the clay, when treated alone, w'ere found to 

 have a very low softening-point, becoming quite fluid at the higher 

 temperatures. This point will be further referred to below. 



lieturning to the more refractory clays, with increase of tempera- 

 ture an entirely new change begins, the results of which become 

 more and more marked the longer the clay is subjected to the higher 

 temperature. The new change consists in a devitrification of the 

 glass owing to the separation of tiny needles of some crystalline 

 substance. This devitrification may set in before the vitrification is 

 quite complete, in that the larger quartz-grains may not have been 

 finally absorbed by the fluxing groundmass. In specimens exposed 

 to a very high temperature for many weeks the devitrification has 

 proceeded to a very considerable extent, although it still remains 

 incomplete. The material so obtained consists of a colourless and 

 completely isoti'opic glass enclosing great numbers of small crystals. 

 The crystals, in fact, are so numerous and so small as to render the 

 glass opaque except in very thin section, when the glass appears as 

 if it contains a vast number of little globules. Under favourable 

 conditions, and with high powers, the apparent globules are seen to 

 be really angular, and exceptionally they are seen to be elongated in 

 prismatic form. 



The crystals were isolated by means of cold dilute hydrofluoric 

 acid, which remained without action on them. They all proved to 

 be acicular in form, with a high refractive index — about 1'66 — 

 straight extinction, and -\- character. The}' consist, therefore, of 

 a sillimanite. There appear to be no substances present other than 

 sillimanite and the clear glass. In one sjiecimen, which had been 

 exposed to a high temperature for many weeks, and in which, 

 therefore, the crystallization may reasonably be assumed to he 

 complete, the amount of sillimanite was found to vary between 29*0 

 and 33'3 per cent. 



