The Science of Brickmaking - George Frederick Harris

Chemical Composition of China-clays.[8]

	Kaolin.	Kaolin average.	Sandy Kaolin.
Silica	46.32	44.60	66.68
Alumina	39.74	44.30	26.08
Iron oxide	.27	.20	1.26
Lime	.36}	1.60	.84
Magnesia	.44}	1.60	trace
Water	12.67	8.74	5.14

The kaolin alluded to in the first column is a remarkably pure material, perfectly white, and contains an enormous quantity of water. It refers to one of the finest washed china-clays in the market, and is extensively used in porcelain manufacture. It is quoted here principally to give an idea of what a really pure clay is like chemically. We notice that, in spite of its relative purity, it contains .27 per cent. of iron oxide. This could have been well done without, from the manufacturer’s standpoint, but is of course a very minute proportion. Small as it is, it must exert a slight amount of colouring influence. The lime and magnesia are present in slightly larger proportions, and a little more of either would be advantageous rather than otherwise, as assisting to flux the material. This is an earth with which practically anything may be done by judicious blending and careful preparation.

With reference to the second column, the figures do not refer to any particular clay, but they have been compiled to show the average composition of kaolins as used in the market. It will be observed that the silica and alumina are present in approximately equal proportions, which is a characteristic of fairly good china-clays. The iron oxide remains as before, but there is a larger proportion of lime and magnesia—as much as can be permitted except in a second-rate clay.

The evidence of the third column shows that the sand in the china-clay is to a large extent quartzose, and this is at the expense of the alumina. Such a material would be suitable for making a species of white fire-brick, and it might do for the commoner kinds of china-ware. The earth is really of the nature of a loam—a sandy clay. There is too much iron in it for the production of perfectly white goods. The proportion of lime might be increased to advantage.

Chemical Composition of Fire-clays from Newcastle-on-Tyne.[9]

	1	2	3	4	5	6	7
Silica	51.10	47.55	48.55	51.11	71.28	83.29	69.25
Alumina	31.35	29.50	30.25	30.40	17.75	8.10	17.90
Iron oxide	4.63	9.13	4.06	4.91	}2.43	1.88	2.97
Lime	1.46	1.34	1.66	1.76	}2.43	1.88	}1.30
Magnesia	1.54	.71	1.91	trace	2.30	2.99	}1.30
Water, etc.	10.47	12.01	10.67	12.29	6.94	3.64	7.58

The reader will see at a glance that the range of variation permissible in fire-clays is very wide. These earths are all found close together, and are utilised for similar purposes, though often blended to produce desired results. It will be noticed that one of them (No. 6) contains as much as 83.29 of silica, whilst another has no more than 47.55 per cent. The range with reference to alumina is very wide also, from 8.10 percent. (No. 6) to 31.35. The refractory character of any sample of fire-clay is determined by the proportions in which the silica and alumina are contained, and by the absence of lime, iron, and other easily fluxible substances. The proportion of iron discovered in sample No. 2 is certainly much in excess of the requirements of the material, as a fire-clay, and this no doubt is tempered by admixture, unless utilised for inferior goods. The iron oxide in the other samples is about sufficient for general purposes. The amount of lime present in all the samples constitutes a good feature; much lime cannot on any account be allowed in earths for fire-clay goods. With so much iron present, and the fair proportions of magnesia (except in sample No. 4) these clays may be regarded as typical, with the exception of No. 6. They have been utilised for many years in the manufacture of fire-bricks and the like.

Chemical Composition of Fire-clays, from Welsh localities.

	1	2	3
Silica	50.35	56.90	54.80
Alumina	23.50	24.90	27.60
Iron oxide	10.40	2.83	2.56
Soda	1.55	3.00	2.00
Magnesia	1.45	1.07	1.00
Water, etc.	11.85	11.60	11.80