another leaner material, sometimes even sand itself is mixed in just 

 such an amount as to reduce the shrinkage without too far lowering the 

 other useful qualities. 



FIRING QUALITIKS. 



Many difficulties must be overcome in order to obtain a day mixture 

 which will hum well in the kiln and produce the desired result with the 

 utmost economy. Of course the selection of the right kind of kiln and 

 the proper control of the fire is of paramount importance. Such ques- 

 tions can not be treated in an article of this nature and larger works on 

 the subject must be consulted. 



Fire xhr-inhige. After a clay mass is thoroughly air-dried it under- 

 goes a further shrinkage on heating as the water of combination already 

 referred to is driven out and the particles softened by incipient fusion 

 followed by vitrifaction, draw together and adhere. By incipient fusion 

 is meant the point at which the particles become soft in the fire and by 

 vitrifaction the point at which they fuse together. Common bricks and 

 earthenware are baked only to this first point and retain an open and 

 porous structure, the bond of incipient fusion being sufficient to form 

 the clay into tough, stone-like materials. Paving bricks and many kinds 

 of stoneware are vitrified. 



Fusibility. It is well known that if pure crystalline substances are 

 heated. they become gradually hotter without apparent softening until a 

 certain critical thermometric point is reached at which the e^stals sud- 

 denly melt and the mass assumes a liquid condition. If, on the other 

 hand, a noncrystalline, amorphous substance like glass is heated, it 

 begins very soon to soften and continues to grow continually more vis- 

 cous until it finally flows. In other words, crystalline substances have 

 sharp and definite melting points while amorphous substances have 

 none at all, but soften gradually to the point of flow. As has already 

 been mentioned, there is evidence to show that clays are mixtures of 

 crystalline and amorphous particles in varying proportions, and it may 

 now be further pointed out that, in the fire, clays behave just as we 

 should expect such mixtures to do. If clays had a sharp melting point, 

 owing to the difficulty of exact control of temperature, the potter would 

 be in continual danger of melting his ware to formless lumps, while on 

 the other hand he could obtain no vitrified bond unless the particles 

 first softened to. the point of incipient fusion. As a matter of fact in 

 most clays the point of incipient fusion is lower by from 100 to 500 

 F., than the point of vitrifaction, and this point in its turn may be many 

 hundred degrees below the point of deformation and flow. Common 

 brick clays will usually fuse at 2,000 F., while many of the better 

 grades of clays will stand 3,000 without viscosity or deformation, while 

 fire clays go as high as 3,500 and even more. It will be apparent from 

 what has been said that a clay which will stand a considerable increase 

 in temperature above a given point without softening to the point of 



