Dr. A. Hubert Cox — !:^outh Staffordshire Fire-clays. 59 



any case it is evident that this fine quartz-flour is of true detrital 

 origin, and not an authigenous constituent arising as the result of 

 secondary changes in the clay itself. 



Among the minor constituents of the clays, carbonaceous material 

 can invariably be recognized in the hand-specimen ; this probably 

 pla5^s an important part in determining the porosity of the burnt 

 clay. Pyrites occurs dispersed as a fine flour, and a certain amount 

 of oxides of iron may also be recognized. 



The first stage of ignition consisted in heating the clay (made 

 plastic with water and then dried) to a temperature of 1000-1080° C. 

 for about fifteen minutes. The main changes during this preliminary 

 heating were : (1) the burning-off of carbonaceous material, (2) the 

 oxidation of ferrous compounds, involving a colour-change from the 

 original dark-grey to a paler and more yellowish tint, (3) an almost 

 complete loss of the combined water. The originally soft clay is 

 thus converted into a " biscuit-clay ", as it is termed by the pottery 

 manufacturers. The biscuit is highly porous, and therefore can 

 absorb water, but it cannot be reconverted into a plastic material 

 comparable with the original clay. Some important change has 

 therefore taken place during the burning, and the biscuited clay has 

 lost all power to combine with water. Curiously enough this change, 

 presumably chemical, is not accompanied by any corresponding 

 change in the physical characters as revealed by the microscope, 

 although the macroscopic change is obvious enough. The biscuited 

 clay presents in fact, so far as I could observe, very much the same 

 appearance under the microscope as the original clay. 



When, however, the ignition is carried out at a higher temperature, 

 about 1500° C, various interesting chemical and mineralogical 

 changes begin. The clay now gradually "vitrifies", owing to the 

 partial fusion of the less refractory constituents, resulting in the 

 formation of material that is practically a glass. The amorphous 

 nature of the new material is not, however, readily established in the 

 early stages of the process. The diffi(;ulty arises both from the 

 minuteness of the particles, and the extent to which the newly- 

 formed vitreous material is crowded with as yet unaltered crystalline 

 material. Also the glass, as first formed, is in a state of strain, since 

 the vitrification takes place without the material ever becoming 

 actually liquid in the popular sense. Accordingly the glass is 

 birefringent in the early stages of the process. As the heating is 

 continued, more and more of the original crystalline material loses 

 its identity, and the resulting vitreous substance thus has more 

 chance to adjust itself. This latter factor is, however, partly 

 counterbalanced by the fact that the newly-formed glass occupies 

 more room than the crystalline material, as is shown by the respective 

 densities. Owing to slight flow-movements, a minute, irregular 

 streakiness soon appears. The strain is thereupon somewhat lessened, 

 so that the material becomes, over small areas, quite isotropic. 



The vitreous material can then be seen gradually to corrode and 

 absorb even the larger sand-grains, so that an almost uniform product 

 is finally obtained. As stated above, all these changes take place 

 "without the material ever becoming really liquid, always provided that 



