CLAY WORKING INDUSTRIES. 189 



the qualities which paving material must possess must be vitrification, 

 toughness and uniformity. Of the latter point, inspection is the only- 

 test of the ability of the manufacturer to deliver bricks by the thousand and 

 hundred thousand of one uniform hardness, toughness and solidity. 



Of the vitrification, the ability to absorb water has been constituted 

 the official test. As usually carried out, two whole bricks and one broken 

 in halves are dried carefully at a low heat, as over a register or in a 

 radiator or on top of a boiler setting. This drying should be continued 

 not less than forty-eight hours. This drying is necessary to expel water 

 accidentally introduced into the samples, or introduced for purposes of 

 fraud. When dry, the bricks should be weighed separately, and in the 

 aggregate,in scales fit to distinguish the influence of one fourth of an ounce. 

 The bricks shonld then be soaked forty-eight or seventy-two hours in 

 clean water and then removed, wiped dry and weighed again. The 

 increase should be reduced to a percentage of the dry weight, and reported 

 as such. The average of the three is to be taken as the correct figure, pro- 

 vided none oi the samples fall conspicuously below the required standard. 



Under such treatment as this, vitrified material ought not to show a 

 gain of over two per cent. In Cincinnati and Louisville, the specifica- 

 tions of the city engineers make two per cent the maximum limit which 

 will be received. This practice is gaining ground in other cities where 

 brick pavements are being adopted. Certainly there is no safety in allow- 

 iug the limit to be raised much over this point. There is some material 

 which might stand very creditably in actual use which would show an 

 absorption higher than this amount, but it is only where extreme tough- 

 ness is combined with high refractory qualities that these conditions are 

 met, and such material while possibly good in some cases, is apt to prove 

 dangerous in the long run. 



On the other hand, it is not. politic to insist on too high a degree of 

 non-absorption, as manufacturers are likely to burn their material past its 

 best physical strength and toughness, in their endeavors to produce a 

 thoroughly vitrified ware. 



It should be the aim of the paving brick maker to use a clay which 

 vitrifies early in its burning; that is one which vitrifies easily; such clays 

 often show their best physical strength, coupled with their best vitrifica- 

 tion. Refractory clays like sandy fireclays, while they are capable of 

 being made into paving material of very great excellence, require to be 

 burned relatively much harder to come inside the two per cent limit, and 

 are constantly liable to show themselves above it if the samples are taken 

 at ramdom. 



The value of the absorption test is unquestionable. It is, more than 

 anything else, a measure of the fitness of the material for the street. 



To test the physical quality of toughness, or ability to withstand 

 abrasion and wear, various tests have been devised. 



The commonest and perhaps the fairest test of this nature is the "tum- 

 bling" test or "rattling" test as variously called. The value of this test 



