CLAY 813 



decomposition derived from imbedded organic remains. Details of the 

 analysis of red and other clays will bo found in a paper by the writer ' On 

 the Disposition of Iron in Variegated Strata,' Quarterly Journal of the Geo- 

 logical Society, vol. xxix. p. 351. 



The Colouring of Burnt Clays. The colour of burnt ferruginous clays is entirely 

 due to the amount of iron present, irrespective of its previous state of combination, 

 but subject to certain conditions in the general composition of the clay. The action 

 of the kiln, with some exceptions referred to below, is uniform on nearly every state 

 of combination in which the iron occurs ; viz., to reduce it to anhydrous sesquioxide 

 associated as silicates in a more or less intimate state of combination with the other 

 silicates developed in. the process of burning. 



Yellow clays coloured with hydrous sesquioxide (e.g. yellow ochre), and red 

 clays coloured with anhydrous sesquioxide, and the lower hydrates merely lose their 

 water of combination and become bright brick reds (e.g. red ochre and Venetian red). 



Grey clays containing finely-divided pyrites or bisulphide of iron are also converted 

 by the kiln into bright reds, the sulphur being driven off, leaving the terra cotta 

 charged with the red anhydrous sesquioxide. 



In clays charged with grey carbonates of iron the following reaction takes place : 

 The carbonic acid (CO 2 ) is driven off as carbonic oxide (CO), part of its oxygen per- 

 oxidising the iron. 



Grey clays containing less than 1 or l per cent, of iron change in the kiln to 

 various shades of cream colour and buff, whilst those containing from 2 to 10 or 12 

 per cent, range in colour from yellowish fawn to dark reds ; from 3 to 4 per cent, of 

 iron produces in the kiln the bright red bodies used in the manufacture of red terra 

 cotta, encaustic tiles, red building bricks, &c. There seems to be no essential diffe- 

 rence (with the exception noticed below) in the colouring matter of the clays that 

 burn buff and those that burn red in the kiln, the depth of colour depending merely 

 on the amount of iron present, the buff shades regularly graduating into the deeper 

 shades of red. 



The brightest shades of red and buff are, however, produced with but a partial vitrifi- 

 cation of the body. At a heat sufficient to insure its complete vitrification a further 

 change of colour takes place. The bright buff shades are changed to neutral greys, 

 and the reds to a slaty-greyish-black, which probably results from a partial reduction 

 of the metallic colouring matter and its more intimate combination with the other 

 vitreous silicates produced at the higher temperature. In clays containing a large 

 proportion of carbonaceous matter the complete peroxidation and consequent colouring 

 power of the iron seems to be arrested. In a black carbonaceous clay from Bovey 

 Tracey, containing 13 percent, of organic matter, the combustion of the carbon in con- 

 tact with the ferruginous oxides seems wholly or partially to have reduced them to a 

 metallic state, or lower oxide having less colouring power than the sesquioxide, and 

 a remarkable bleaching of the burnt clay has been the result. The presence of the 

 alkaline earths in ferruginous clays, especially of lime and magnesia, has also a singular 

 bleaching power in the kiln, arresting the development of the bright red colour. A 

 Permian marl, containing 6 per cent, of sesquioxide of iron and 35 per cent, of carbon- 

 ate of lime, burned of a greyish buff instead of the rich red such a proportion of iron 

 would otherwise have produced. From some experiments made by the writer, it has 

 been ascertained that as small a proportion as 5 per cent, of caustic magnesia mixed 

 with a red clay entirely destroys its red colour in the kiln, probably from the produc- 

 tion of a pale-coloured double silicate of iron and the alkaline earth. A familiar 

 example of this reaction occurs in the process of manufacturing yellow bricks in the 

 neighbourhood of London, the colour of which is dependent on the admixture of 

 ground chalk with brick earth, the brick earth by itself burning of a red colour.' 



The composition of ordinary clay will be seen from the following analyses, by Mr. 

 T. H. Henry : 



1. Fire-clay (Stourbridge, Brierly Hill) : 



Silica 61-80 



Alumina . . 30-40 



Protoxide of iron 4-14 



Magnesia . -50 



Water and organic matter 13-11 



99-95 

 With trace of soda. 



2. Three samples of Fire-clay from Wales : No. I. inferior ; the other two good ; 

 No. III. the best. 



