468 FARMERS' INSTITUTES. 



requires a heat of over 1000° F. When this water is once driven off the 

 material of which the brick is mide can never be soaked up so as to become 

 plastic. Yet if the operation be stopped at this point we shall not have a 

 hard burned brick or hard burned tile. It is necessary to go further still, and 

 to partially fuse, at lewst in an exceedingly small degree, the particles which 

 compose the brick. If the heat is too great the particles are all fused and 

 the brick becomes vitrious and misshappen. Just how much heat is needed 

 for these operations is not known, and no doubt largely depends on the 

 impurities in the clay, — iron oxide, sand and lime all making the brick more 

 easy to burn. In the orJinary open top kiln, the heat is often sufficient in 

 the arches to melt cast iron, while on top it rarely rises to more than 

 sutlicient to drive off the hygroscopic water, say 1000° F. In this case we 

 will say the range of temperature is from 2500° to 1000° F., although there 

 is nothing certain in that assumption. 



This sketch of the operations of burning calls to mind an important precau- 

 tion, and that is with respect to the admission of cold air. While the ware 

 is very hot, the effect of cold is to suddenly contract it; this result is almost 

 -certain to break and crumble the ware. This fact renders it very necessary 

 to take precautions to prevent the admission of cold air after the kiln is 

 much heated. 



The addition of sub-arches outside a kiln is a device of much value in 

 preventing the admission of cold air into the kiln. It supplies the kiln with 

 hot air instead of cold air. 



The kilns in common use are of three kinds: the open-top kiln, or in 

 reference to the draft, the up draft kiln; the closed-top or down-draft kiln 

 and a kiln in which the direction of draft may be changed at pleasure. 



THE UP-DRAFT KILN. 



This is the old form of kiln. For brick burning the walls are usually 

 temporary and put up and taken down with the kiln; for tile burning the 

 walls are usually erected to stay and are made thick. The only top to this 

 kiln is the plattnig laid on top of the brick or tile after they are in place. 

 It is, however, unnecessary to say anything about the construction of this 

 form of kiln; it is well understood. 



The rising temperature tends to increase the draft through this kiln from 

 the bottom to the top, and also as the heat is not uniform but is largely ap- 

 sorbed in passing through the kiln, the draft will vary in different places, 

 thus increasing the difficulty of managing it and making it absolutely impos- 

 sible to secure an even temperature throughout. Thus, for instance, if one 

 part becomes much hotter than the surrounding parts, the draft becomes in 

 that place correspondingly greater. The result is a tendency to draw the 

 whole heat of the kiln to that point. This may go on to such an extent as to 

 actually produce a down draft, in other portions of the kiln, in order to 

 replace the air carried out in the hottest place. Such things are well 

 understood by practical burners and they know well what to do. They will 

 stop either the rush of hot air upward or that of the cold air downard by 

 covering the top of the kiln with earth at those points. 



Again it is extremely difficult to get the heat to distribute itself evenly with 

 such a kiln. The only way it can be approximately accomplished is by pre- 

 venting the ingress of all possible air from below or from the sides, making 

 the combustion as slow as possible. 



