THE PROPERTIES OF MODELLING- GLUES 271 



melted, add the glycerin gradually, as the glue will take it up ; 

 and, when united, add the oil, also in small quantities, and boil 

 all together, keeping thoroughly stirred. Some time will elapse 

 before the whole is satisfactorily united, sometimes, as for 

 Formula 95, a whole day's boiling being insufficient ; especially 

 is this the case if oil floats on the top, or if it can be seen as 

 bubbles or shining patches in the mass, and it then requires 

 more boiling and stirring, even to the extent sometimes of two 

 days over an ordinary fire, but over a gas-jet the mass will be 

 done in a day. For Formulae g6 and 97, however, three hours 

 should be sufficient when half the quantities are taken. 



The properties of each are dissimilar, inasmuch as, although 

 the modelling-glue Formula 95 is a fairly good medium, and 

 flows and delivers well, it has rather too much oil in its 

 composition ; Formula 96 is much the better of the two ; it 

 flows and delivers much more easily, is very elastic, and of a 

 better and lighter colour, approaching that of candied sugar, 

 and takes tube oil-colour finely when a small quantity is stirred 

 into the hot mixture. Formula 97 is, however, the very best, 

 and is of a fine, light, whitish-yellow, flows and delivers beautifully, 

 is very elastic, and comes out of a properly oiled mould (colza- 

 oil) finely. It takes tube oil-colour admirably when it is hot, 

 and, being many tones lighter than the compositions made with 

 oil, is so much more valuable for colouring upon its surface 

 afterwards. In some cases it is advisable to colour the com- 

 position to the groundwork of the object — say of such as a 

 star-fish — and to touch up with other pigments those parts 

 not partaking of the general colour. Further, this composition 

 does not seem so liable to " blows " as any of the other glue- 

 compositions, and probably this is due, to the chemical affinity 

 between the balsam and the glycerin, both uniting perfectly 

 with each other and with the glue, which is more than can be 

 said for oil and glycerin, which only appear to unite by the 



