GUTTA PERCHA MOULDS. 55 



economical ; but there are cases in which, the moulds being very 

 large, the use of plaster is unavoidable. 



Mould* in Fusible Alloy. The next means of taking moulds is by 

 fusible metal : this name is given to alloys of two or more metals 

 which melt at very low temperatures ; it suits the purpose of taking 

 moulds of small objects very well. The following are examples of 

 such compositions: 



Tin. Lead. Bismuth. Zinc. 



11 20 



12 30 

 10 11 



These all melt at a temperature below that of boiling water ; the 

 ingredients are melted together in an iron ladle, poured out upon a 

 flat stone, broken up, and re-melted in the same way two or three 

 times, in order that they may be thoroughly mixed. The medal from 

 which the mould is to be taken is prepared in the same manner as 

 described for wax. 



The fusible alloy is melted and poured into a saucer, or, what 

 does better, a small wooden tray : the operator now watches till it 

 cools down into a semifluid state, or to the point of setting, when he 

 brings the medal suddenly upon it, face downwards, and holds it 

 there until the alloy has fairly set ; he then allows it to cool, and 

 undoes the slip around the medal, from which the mould will easily 

 separate. The height of the slip of paper above the surface of the 

 medal determines, of course, the thickness of the mould. The 

 beginner very seldom succeeds with his first attempts at making 

 moulds in fusible alloy; but as a little experience teaches more than 

 the reading of an essay upon the subject, he will soon find both his 

 patience and labour rewarded with gratifying success. Some of the 

 finest moulds are taken by this process, but, from the constant loss 

 of the materials by oxidation, &c., it is expensive ; so that its use 

 amongst electro-metallurgists is very limited. 



Moulds in Gntta Percha. Gutta Percha, as a material for mould- 

 ing, serves the purpose most admirably. We have seen moulds of 

 this substance equal, if not superior, to any that we ever saw taken 

 in wax; and of a depth of cutting which it would have been very 

 difficult to have taken in wax. The method adopted for taking 

 moulds is to heat the gutta percha in boiling water, or in a chamber 

 heated to the temperature of boiling water, which makes it soft and 

 pliable. The medal is fitted with a metallic rim, or placed in the 

 bottom of a metal saucer with a cylindrical rim a little larger than 

 the medal ; the medal being placed back down, a quantity of gutta 

 percha is pressed into the saucer, and having as much as will cause 

 it to stand above the edge of the rim : it is now placed in a common 

 copying press, -and kept under pressure until it is quite cold and 



