CASTING METALS. 23 



if too much moisture is present there is a liability to the sudden 

 formation of steam when the melted lead is poured into the hole, 

 which will blow the metal violently out of the hole and perhaps 

 cause a severe burn : terrible accidents sometimes happen in this 

 way in foundries and casting-houses when due care is not taken 

 to avoid the cause. 



The lead may be melted in a clay crucible on the fire or over a 

 gas or spirit-lamp ; but a more convenient arrangement is a ladle 

 made for the purpose, consisting of an iron basin with a spout for 

 pouring, and a long iron bar attached as a handle ; this handle 

 generally keeps sufficiently cool at the far end to prevent burning 

 the hands, even though the bowl be full of nearly red-hot lead. 

 The scum should be removed from the melted lead by skimming with 

 an old knife or a piece of cardboard ; the freshly skimmed surface 

 will be perfectly bright and silvery, but whilst you watch it, it 

 tarnishes and loses its lustre and becomes covered with a film often 

 showing the colours of the rainbow, whence the term iridescence. 

 The film thus formed results from the combination of the lead with 

 the gas oxygen contained in the air forming oxide of lead, and is 

 due to a chemical change termed oxidation, which will be examined 

 more closely hereafter (Chapter XIY. ) ; most ordinary metals 

 behave in much the same way on heating, whether they become 

 fused or not in the process ; thus a polished plate of copper held 

 horizontally over a Bunsen or spirit-lamp flame so as to heat the 

 centre of it will develop rings of iridescent colours owing to the 

 analogous formation of oxide of copper ; similarly a polished steel 

 knife blade treated after the same fashion will give iridescent films 

 of oxide of iron ; in these two instances the metal remains solid, 

 whilst with lead as above, and bismuth (Expt. 18), the metal is 

 melted by the heat. 



The skimmed lead should be allowed to cool until it is nearly 

 on the point of setting or solidifying, and should then be carefully 

 poured into the holes in the sand which form the moulds. After 

 a few minutes the lead will have solidified and can be withdrawn 

 in sticks or rods from the sand, preferably by means of a pair of 

 tongs, as it may be still too hot to handle comfortably. Zinc, tin, 

 and several other metals may be cast into sticks in just the same 

 way. 



Smoother castings may be obtained by using polished iron 

 moulds sold for the purpose of casting metals, &c., into sticks, 

 made in two halves like the wax-lemon moulds in Expt. 5. 

 Bullets are cast by means of two hemispherical moulds held 

 together like the blades of a pair of scissors. 



Expt. 16. To Granulate Zinc. Melt some lumps of zinc in 



