CLAY WORKING INDUSTRIES. 135 



None of these shales require a high heat to vitrify them ; many of 

 them will cinder hopelessly if they get a high heat for a short time only. 

 They all of them owe their fusibility partly to the large amounts of earths 

 and alkalies present. It is probable that the iron retards rather than 

 assists in bringing about the fusing action, as it is observable that vitrifi- 

 cation begins in all these clays while the iron is still in the sesquioxide 

 form as evidenced by the handsome color, and that long exposure at a 

 comparatively low heat will to a large extent perfect the vitrification ; but 

 as the heat is raised the color darkens until the iron finally breaks up the 

 combination which has existed, by its conversion to the lower oxide. 



Thus while we have in this table the records of a set of very easily 

 fusible clays, we also have, as is determined by the experience of those 

 using them, that peculiar quality or balance between refractoriness and 

 fusibility which allows the vitrification to be made practically complete 

 without going over the danger line of temperature. 



The shales constitute a new and valuable addition to the mineral 

 wealth of the state, especially valuable in the production of vitrified 

 wares, but the importance ot the fire clays of the state which have for 

 years constituted almost the entire source of material for vitrified wares 

 must not be overlooked. 



The table of analyses here appended shows the composition. of a few 

 of the fire clays in use in the state : 



