September 1, 1915.] 



THE INDIA RUBBER WORLD 



643 



Kneading and Pressing Gutta Cakes 



Coagulating Gutta Latex. 



Fiisceptibilitj of passing from the cellular to the fibrous condi- 

 tio ii varies in the different kinds of guttas. The more markedly 

 this quality is developed the ,_ 



greater the separation from 

 the 1"\\ grade guttas known 

 as mitta caoutchouc. I 

 of the fibrous structure renders 

 gutta percha too brittle and 

 unsuited to cable making. In 

 it- normal condition the spe- 

 cific gravity of gutta percha 

 is 0.9?' t.. 0.999. ('..m! i. ed 

 specimens show 1.010 to 1.020. 

 Gutta percha is preserved by 

 immersion in cold water. By 

 the action of light and air it 

 rapidly oxidizes to a hard, 

 brittle- resin, completely losing 

 it- toughness and emitting an 

 at id odor. Tins change is 

 more rapid in air at 77 to 86 

 degrees Fahrenheit, if the ma- 

 terial is exposed in thin sheets 



ami frequently moistened and dried in the sun. When thus 

 converted into a resin it increases in weight and in solubility 



in alcohol and alkalies and becomes a - nductor of elec 



tricity. All the gutta perchas of commerce contain these oxida- 

 tion products up to 15 per cent., which arc insoluble in 

 and benzene. 



In this connection it should In- state. 1 that oxidation under 

 the influences of air ami light only proceeds slowK ,\hcn the 

 gum has been well compacted by thorough mastication. This 

 oxidation docs not proceed in a constant manner. Some sam- 

 ples will resisl all deterioration, while under the same conditions 

 - will crumble on handling. The explanation is offered. 

 in such case, that the commercial gums used are of diverse com- 

 position, largely because not standardized by the collector- or 

 ti aders. 



Freshly cut surfaces of gutta percha are not tacky, and will 

 i o1 unite with each other at ordinary temperatures, bul if gently 

 heated and pressed in contact strongly they will adhere per 

 manently. 



A low temperature, several degrees below zero Fahrenheit, 

 has no effect on gutta percha. The increase of its pliability by 

 heating is noticeable between 85 and 1(K) degree- Fahrenheit, and 

 at 122 degrees Fahrenheit it yields readilj ' slight pressure 

 and is capable of receiving and retaining tlu most delicate im- 

 pressions. At 194 degrees Fahrenheit it becomes adhesive and 

 pasty, permitting the mass to lie freely molded into any desired 



shape, winch it retains at normal temperatures. This charac- 

 tc-ristic property is due to the air interposed in thi f the 



material. At 212 degrees Fahrenheit pasty fusion terminates 

 and the substat lifies in the air, absorbing 2^ per cent. 



volumi Melting occurs at 266 degrees Fahr- 



i and the material remains permanently 

 viscous, application of higher hi and distilla- 



tion with a carbonaceous residue. The distillate- consist ■ I 



!: It'll'-- ■ 111. 



Mi. application of a flame to gutta percha cause- it to ij 

 quickly and burn with a shov tks and the droppii 



if. in. lie. 1 material, after the manner of burning sealing wax. 

 The impermeabilitj of gutta percha i- an extremely important 

 characteristic, Waur. either I ■ ■ nly a 



very limited depth into it- pores. Such mechanically included 

 water in no wa\ chai t ties of the material 



if it dors not exceed two to three per cent, by weight of the 

 gum. 



It i- a 1 r conductor of heat and electricity, being the 



valuable dielectric plastic known. Gentl villi -ilk. 



t will emit electric sparks practically an inch in length. 

 Faraday, in 1843, discovered the insulating property of gutta 



.in.! I. in saw it- ap- 

 plicatii n ruder water or in 

 tin ground it retains, prac- 

 tically permanently, it- value 

 as an insulator. According to 

 Wunschencjorff, under stand- 

 ard condition- of temperature 

 ( 7?. 2 ahrenheit i and 



dimensions, the insul 



gutta percha, referred 

 as unity, is ex- 

 ilic lllllll: ■ 



m percha res 

 solvents and is completely in- 

 soluble in water at all tem- 

 peratures. In boiling water it 

 swells and absorbs about 5 to 

 6 per cent., which it parts with 

 -lowly. Ibis mechanically in- 

 cluded water may be expelle'd 

 at 302 degrees • ■ 



without constitutional change in the gutta percha. Practically 



insoluble 



cold dilute alcohol, its solubility rapidly in. i 



I.ATEX. 



