THE CLAY-MINING INDUSTRY. 337 



The Middlesex district supplies a greater variety of clays than 

 any of the other three. Among these certain well-marked types 

 are recognized, but the names of these are of little significance in 

 indicating the use of the material. The most important varieties 

 obtained in the Middlesex district are the following : 



No. 1 fire clay. — This term refers to the most refractory clay 

 to be found in any fire-clay pit, arid in some cases indicates a 

 material of high refractoriness, fusing at cone 34 or 35. The 

 term does not refer to any standard type, but is used by each clay 

 miner to indicate his best grade of clay, and therefore it does not 

 follow that the No. 1 fire clays obtained in the different pits of 

 Middlesex county are all of the same degree of refractoriness. On 

 the contrary, they exhibit as much variation, as there is among 

 the different No. 1 fire bricks made at different works. In some 

 cases the term A r o. 1 fine or No. 1 bhie is used. Around Wood- 

 bridge some of the better grades of fire clay are often quite sandy 

 in their character, and consequently the terms No. 1 sandy, top or 

 bottom sandy, or extra sandy are employed. 



No. 2 fire clay. — This refers to the second grade of fire clay 

 found in any given bank. 



The chemical composition and fusibility of many of these Mid- 

 dlesex county fire clays will be found in Chapter XIX, but since 

 the term No. 1 and No-. 2 fire clay is so loosely used, they are 

 grouped there as highly refractory, refractory and semirefractory 

 clays. 



Retort clays. — This type of clay is recognized only in the 

 Woodbridge district. As there recognized it is very plastic clay, 

 burning dense, usually at cone 3 or even lower, but it is not as 

 refractory as the No. 1 fire clay. It was probably extensively used 

 in former years in the manufacture of gas retorts, and in this man- 

 ner has obtained its name. At the present day, however, its chief 

 use is in the manufacture of stoneware, especially of the sanitary 

 variety and of other objects where a dense-burning clay of good 

 bonding power is required. 



Stonezvare clay. — This term is applied in some pits to a dense- 

 burning plastic clay of less refractoriness than the ball clay, which 

 is used to some extent in stoneware manufacture as well as for 



22 CI, G 



