No. 3 (1921) LACQUERING OF CANS 263 



boiling water with a little soda ash or washing soda (sodium 

 carbonate) or of caustic soda, dissolved in it ; if caustic is used the 

 solution should be very weak. When dry, the varnish should be 

 painted on the tin with a soft brush (badger or camel hair) of 

 sufficient size to enable quick working and to avoid undue 

 streakiness. 



The object of adding turps which thins the fluid, is to permit of 

 easy working ; thick boiled oil or thick varnish is difficult to apply 

 inside a can, and if very thick its use may not only be unecono- 

 mical, but may result in unduly thick and uneven layers, especially 

 at the bottom of the cans. By adding turps working is much 

 easier, and as the turps rapidly evaporates the oil-varnish is left as 

 a moderately thin layer to harden by the heat. 



Before putting the cans into the stove the oil-varnish should be 

 somewhat set ; otherwise it will run unduly in the stove. 



The heat should be applied moderately at first to allow the turps 

 to evaporate gradually ; it may then be continuously increased to 

 the required degree. 



As stated above, the heat for soldered cans must not rise above 

 300° F. 



When one layer has been applied, and stoved, it is usual to 

 repeat the whole process so as to double the layer of stoved 

 lacquer; it is better to give two somewhat thin layers rather than 

 one thick one. In commercial japanning of the better sort five or 

 six thin layers are sometimes given, but this is unnecessary in the 

 lacquering now under consideration. 



II. It is of course possible to buy tin-plate in the flat ready 

 lacquered, the lacquer being, on good samples, so adherent and 

 elastic that it does not in the least crack when the tin-plate is bent 

 into cans. But this lacquer is usually thin and, for jam purposes, 

 may be insufficient, though it is used by various jam-making 

 firms even in their export goods. But in such cases it is probable 

 that the tin-plate is specially lacquered, possibly in the jam factory 

 itself. 



