309 



NOTES ON THE TENACITY, ELASTICITY, AND DUCTILITY OF RAW 



SILK. 



By Philip Walker. 



The tenacity of a silk filament is that propeity by virtue of which it 

 resists rupture when stretched. Its ductility is the property which per- 

 mits it to stretch without rupture -, and its elasticity the virtue which en- 

 ables it, after stretching, to recover to a certain extent its initial length. 

 In ordinary i)arlance the tenacity of a silk thread is the weight, in grams, 

 necessary to i^roduce rupture. In determining the tenacity the force is 

 generally exerted on a thread 50 centimeters long. 



The properties of elasticity and ductility come into play simultane- 

 ously in the operation of stretching. 



This fact was first determined by M. Eobinet,* a Frenchman, who in 

 1838, and for ten years following, devoted himself to this and similar 

 subjects with great advantage to sericultural science. 



Thirty years later the subject was again attacked by M. Persoz,t of 

 the Paris conditioning house, and at about the same time by M. Paul 

 Francezon, of Alais (Gard). I owe to them a large part of the informa- 

 tion contained in these notes. 



If a thread a meter long is suspended from a fixed point, and to the 

 lower end a weight is attached so as to stretch it 50 millimeters, this 

 stretch will, upon the removal of the weight, diminish to about 35 mil- 

 limeters. This 15 millimeters of recovery represents the elasticity of 

 the filament and the 35 the ductility under the tension of the weight 

 employed. If, on the other hand, we stretch it still farther, the propor- 

 tion of ductility becomes greater and of elasticity less, until at between 

 15 and 20 per cent, (usually) the limit is passed and the filament breaks. 



When the stretch approaches the limit of elasticity the recovery is 

 very slow in a dry atmosphere, but more rapid in the presence of moist- 

 ure. Thus, a thread 50 centimeters long, stretched 5 centimeters, or 

 10 per cent., when relieved of its load, recovered at once 25 millimeters j 

 in the first half hour this increased to 3 centimeters. It was then moist- 

 ened, and immediately recovered another half centimeter; during the 

 following minute it shortened until but 1 centimeter of stretch remained 

 and then the recovery stopped. 



But although -there is evidently a very distinct action of elasticity 

 and ductility in the stretching of a silk thread, it is found sufiicient in 

 the commercial testing of raw silk to examine their joint action and to 

 determine the total stretch of which a given thread is capable without 

 breaking. 



*Memoire sur la Filature de la Sole, Paris, 1839, and a series of memoirs published 

 from 1843 to 1848, which are unfortunately not in the library of tbe Department. 

 + Essai sur le Titrage et le Decrensage de la Soie, Persoz, Paris, 1878. 



