54 MADRAS FISHERIES BULLETIN VOL. XIII, 



etc., are usually struck in this way, as also the cheap penny tins of 

 fish paste. 



39. This system has the following advantages: — 



(1) comparative absence of joints and seams; 



(2) the freedom from contamination by the lead in the solder 

 though, in properly soldered cans, this is negligible; 



(3) comparative absence of leaks, provided the double seamer 

 is properly adjusted, and the rubber washers are in good condition ; 



(4) great speed ; one machine will close as many cans as a 

 dozen solderers ; 



(5) the dished cover of the tin is forcibly pressed down in 

 the double seamer on to the contents, so that air is practically 

 excluded and exhausting is seldom necessary, at least for sardines, 

 etc., which can be closely packed ; 



(6) as solder is not used, the cans and covers can be prepared 

 at leisure and stored since the condition of the edges of body and 

 cover does not affect the hermetical sealing ; 



(7) the nature of the joint between the body and lid is a 

 mechanical lock, so that there is no risk of joints giving way in 

 processing even at high pressure ; 



(8) the operation of double seaming being mechanical, it may 

 be carried out by any one after slight teaching, so that there is no 

 lengthy apprenticeship as for solderers. 



40. The disadvantages are — 



(1) that the machines are costly and require somewhat delicate 

 adjustment, especially in the drawing press and double seamer, 

 and do not work to perfection unless perfectly adjusted ; hence a 

 good mechanic or mechanical engineer is a necessity ; 



(2) that the composition rings are apt to deteriorate especially 

 in India, unless protected from the air; hence leaks are possible by 

 breakage of the rings or by their being squeezed out of place : it is 

 found necessary at Beypore to heat the lids and rings immediately 

 before double seaming ; 



(3) oil rapidly deteriorates rubber or composition containing 

 rubber, and since sardines are preserved in oil and all fish are 

 more or less oily and since the act of double seaming forces out 

 some of the oil and over the rubber ring, the rubber must deteriorate 

 within a comparatively short time, and thus cause damage to goods 

 if kept long. 



Disadvantages (2) and (3) can be obviated by using a solution 

 instead of a rubber ring ; this solution often contains no rubber, 



