176 GEOLOGY OF OHIO. 



2d. The burning, which may be so managed as to destroy this 

 quality wholly. 



3d. The cooling or annealing process, which, more than any- 

 thing else, makes a brick tough or brittle. 



Uniformity is a quality affected by two forces; 1st. The process of 

 manufacture, which ought to secure by its machinery material of very 

 great homogeneity; 2d. The burning, which can be managed so as to 

 make but little variation in the different parts of one kiln, and also 

 between the product of different kilns. 



So that in the burning more than in anything else, the value of the 

 material for paving purposes depends. 



The chemical processes of burning are those which have been pre. 

 viously set forth. The only point of variation consists in the large size 

 and area of the clay body which is to be vitrified and the greatly increased 

 danger of bloating in driving off the combined water. 



The custom of glazing paving material has been much discussed and 

 agitated. A popular impression, carefully fostered by the manufacturers 

 who do not glaze, is to the effect that the glaze is a surface veneer which 

 merely adorns the surface of an otherwise unprepossessing material, and 

 that, as a rule, it is used to allow bricks which are too soft in the center 

 to pass inspection. There is no foundation for this belief. As a matter 

 of fact, the glaze of a paving brick is harder than the substance of the 

 brick ; and in addition to this it is a fact that very few clays can be suc- 

 cessfully and handsomely glazed which have not been thoroughly burnt. 

 Instead of casting suspicion on the quality of the ware, it is more often 

 a proof of its quality. Salt glazing cannot be made to take effect on 

 clay ware at any low temperature; and the presence of any salt glaze is 

 presumptive evidence, though not a positive proof, not only that the brick 

 has been at the vitrifying point, but has been held at that temperature 

 until thoroughly soaked with heat; for practical experience of those who 

 have used salt glaze invariably goes to show that their glazing is only 

 satisfactory when a kiln is thoroughly burnt. Thus the salt glaze in 

 paving material is a source of benefit to the public and makers on the 

 following accounts: First it delays the wear of the surface for a time, 

 and assists in preventing absorption. Second, it is an indication but not 

 a sure proof of satisfactory vitrification, and thereby assists in culling out 

 soft material. 



The attention of the maker or public is always directed to that por- 

 tion of the product which is "off color" and different from the rest; in 

 salt glazed goods this portion included most of the soft material. All 

 the unglazed portions of a kiln are not necessarily soft, because many 

 clays will not readily glaze, owing to the presence of lime salts. So that 

 it is not fair to count glazed material as good, or all unglazed as bad. 

 Nevertheless on a j^ard where salt glazed goods are made, the good 

 material is largely found among the glazed, and the bad is largely found 

 in that devoid of 2:laze. 



