50 NEW YORK STATE MUSEUM 



nary blue clays require about 29 per cent of water to make them 

 stifficiently plastic for brick manufacture by the soft-mud method. 

 They shrink by air drying about 6 per cent, and when burned at 

 cone I (about 1150° C) show no fire shrinkage. The burned product 

 is a relatively soft brick of pink color. In working the brown clays 

 about 26 per cent water is necessary and the air shrinkage is 7.5 per 

 cent. At 1150° C they show a fire shrinkage of 1.5 per cent and 

 give a dense vitreous product of red color. 



As a variation of the blue brick clays that are found along the 

 middle section of the Hudson valley, the Albany slip clay presents 

 some features of interest. Its occurrence is not distinguished by 

 any local characteristics different from those surrounding the brick 

 clays, in fact the two occur together. The slip clay is an excep- 

 tionally soft " greasy " type of the blue clay in which the sandy 

 partings are reduced to a minimum. The two can hardly be 

 differentiated, however, by physical appearance alone, and actual 

 tests are required to determine whether a particular bed is slip clay 

 or not. From a number of chemical analyses Jones^ found that 

 the principal difference in composition lies in the alimiina content 

 which is lower for the slip clays. There is, however, a considerable 

 range in this respect among different samples obtained in the same 

 locality, the percentages of aluminum oxide varying from 11.80 to 

 15.65 per cent in five samples taken from an Albany bank. The 

 clay is worked mainly in the vicinity of Albany, and seems to be 

 restricted in occurrence to a stretch of a few miles above and below 

 that city which lies near the middle of the old Mohawk delta, where 

 the clays attain their maximum areal development. Its use as a 

 slip in glazing pottery wares is best known. Much of the domestic 

 stoneware is thus glazed. Electric insulators for high-tension 

 transmission lines are often glazed with slip clay. A mahogany 

 color is usually sought for in glazes and is natural to some of the 

 Albany clay. A larger use for the clay has developed in the 

 manufacture of carborundimi and emery wheels; the granular 

 abrasives are mixed with a small proportion of slip clay and then 

 molded into forms, after which they are fired. The slip clay is 

 preferred to other clays for the purpose on account of its low fusion 

 point. 



Brick manufacture is carried on in the Hudson valley by the 

 soft-mud process, and in 191 5 the canvass conducted by this office 

 showed a total of 127 such yards located along the river from 

 Saratoga county to Westchester and Rockland counties. The 

 combined capacity is well over a billion brick a year. In 1906 t.ie 



^ Op. cit. p. 28. 



