MANUFACTURE OF PLASTERS. 39 



"kettles". Heat is applied both at the bottom of the kettle and by 

 flues passing entirely through the cylinder. 



A heavy stone or brick masonry support is built for the kettle, in- 

 closing a fire-space in the form of an inverted cone about 4 feet high. 

 At the top of this cone a cast-iron flanged ring is set in the masonry. 



FIG. 10. Four-flue kettle, with accessories, dismounted. (Butterworth & Lowe.) 



On this flange is placed the "kettle-bottom", which is an iron casting, 

 concavo-convex in shape, a little less than 8 or 10 feet in diameter, 

 with the convexity placed upward, the rise being 1 foot. This bottom 

 has a thickness of f inch at the edges and 4 inches at the crown. Kettle- 

 bottoms must be made of the best scrap-iron, as ordinary scrap-iron does 

 not last as long as pig. Sheet steel has been tried, but does not serve 

 as well as the best scrap. "The life of a kettle-bottom is terminated 

 by cracking. The cracks can be calked with asbestos cement, but 

 the expense of stoppage and repairing soon overcomes the saving." 



