152 GEOLOGY OF OHIO. 



Hand-molded bricks are irregular in shape, generally more or less 

 porous or open grained, owing to the lack of all pressure in their form- 

 at on and to the large amount of water used in tempering. They are 

 devoid of any cracks, cleavages or laminations resulting from the arrange- 

 ment of the particles of clay by the power that formed the brick. The 

 material used is generally poorly prepared and not homogenous but if 

 clays of suitable composition were used and were ground fine and temp- 

 ered with the proper machinery, a brick could be produced by hand- 

 molding whose structure would more closely approximate our ideal than 

 can be made by any other method. 



Soft mud bricks have most of the good points of the hand-molded 

 bricks so far as structure are concerned; they very rarely show faults 

 like laminations or cracks; they are usually open grained and porous on 

 account of the large quanties of water used in their tempering. This 

 loose knit structure can only be remedied by the use of compressive 

 force after the brick have been partly dried and become stiff enough to 

 benefit by such treatment. 



A brick which has had this treatment becomes one of t e best in 

 structure that it is possible to make. But such a process of manufacture 

 becomes an expensive one, on account of the necessary division of the 

 drying into two stages. 



The stiff mud bricks have usually a dense compact arrange- 

 ment of their particles and possess good strength, but are apt to 

 show evidences of the influence of the machinery in their structure. 

 This process is the cheapest way to manufacture bricks from tempered 

 clay, but no machine has yet been devised which is capable of contin- 

 uously producing material whose structure is even approximately 

 what is desired and needed by the paving brick industry. The 

 advantages of the process are such however as to indicate that success 

 will be attained by overcoming the obstacles which interfere with this 

 plan, rather than by developing the useful points of any of the othei 

 y stems of brick making. 



Dry press brick possess the most dense, even grained structure 

 and the highest specific gravity of any kind made. But as a result of the 

 process of manufacture, the brick is an agglomeration of grains which 

 owe their conjunction to pressure. If the clay is easily enough vitrified to 

 unite its various particles at a safe heat and form one coherent mass in- 

 stead of mam^ thousand separate masses, then the dry pressed brick 

 would be the best and cheapest known. But the facts are, that no one 

 has ever succeeded in producing to an}^ profit a dry press brick in which 

 this has been continuous^ accomplished. The grains of clay may 

 vitrify, but do not unite. The green brick is an agglomeration of sepa- 

 rate particles which can be separated by friction. The burnt brick is the 

 same, and while each grain may be vitrified to non-absorption and the 

 brick as a whole be nearly non-absorbent, still the particles will easily 

 " i Id. to the disintesratins: effect of friction. 



