i 



604 THE POPULAR SCIENCE MONTHLY. 



cally a definite chemical compound, the proportions in which the 

 ingredients of the " batcli" are mixed vary in every establishment. 

 Sand is the basis of the operation. It is the commercial repre- 

 sentative of silicic acid. With this are mixed lime and alkali 

 (either carbonate or sulphate of soda, or both) in sufficient quan- 

 tity to furnish an easily fusible mass that, on solidifying, shall be 

 both clear and transparent. There is very wide range in the 

 choice of materials. The purest grades have the disadvantage of 

 costliness, while the inferior glass has the equal disadvantage of 

 commanding but an indifferent price. Between these two consid- 

 erations swings the balance of expediency. 



When the batch has been made up, it is melted in large clay 

 crucibles, or glass pots, as they are commonly called. 

 The manufacture of the pots is the most tedious and 

 exacting process connected with glass-making. It is 

 one of the few industrial opera- 

 tions in which machinery has 

 not been able to supersede man. 

 A mixture of raw and burned 

 fire-clay is employed. It is 

 necessary that this should be 

 A ^^^^ ^y prepared with the greatest care. 



I ) ^^"""^"-^/^ Once a day for at least four 



'-'' weeks the mass must be turned 



The Operation OP " Marveb:ng." ^ it- t , , •, 



and worked, m order to get it 

 free from air and give it the proper toughness. For this knead- 

 ing process no tool has been found equal to the bare foot. There 

 are a warmth and an elasticity about it that better than any- 

 thing else develop the required plasticity in the clay. Bare- 

 footed men, pacing up and down in lead -lined troughs, pre- 

 sent a very primitive industrial picture. The impression is not 

 removed when one goes up-stairs and watches the transformation 

 of this much -worked material into crucibles. The hand here 

 occupies the place that the foot does below- stairs'. By equally 

 slow stages the crucible is built up. First the bottom is formed, 

 a circular slab about four inches thick and some forty inches in 

 diameter ; then the sides are gradually raised, a little addition 

 being made each day, until at the end of about six weeks the 

 work is completed, and a heavy, tub-shaped crucible is the result. 

 Meanwhile the incompleted walls are kept constantly covered 

 with damp cloths to prevent premature hardening. The temper- 

 ature and humidity of the work-room are also objects of unremit- 

 ting attention. But, though the crucible has now taken form, and 

 its material been under treatment for more than ten weeks, it is 

 not yet ready for the trial by fire. Several months must pass be- 

 fore it is considered sufficiently dried to withstand even a prelimi- 



