TINNING COPPFR, &C. 303 



The common method of tinning consists in making the surface 

 of the copper vessel quite bright, by scraping it, and by washing 

 it with a solution of sal ammoniac ; it is then heated, and the tin, 

 or metallic mixture designed for (inning, is melted, and poured 

 into it, and being made to flow quickly over every part of the sur- 

 face of the vessel, it incorporates with the copper, and, when cold, 

 remains united with it. Rosin or pitch is sometimes used, to pre. 

 vent the tin from being calcined, and the copper from being 

 scaled, either of which circumstances would hinder the sticking 

 of the tin. 



I had the curiosity to estimate the quantity of pure tin, which is 

 used in tinning a definite surface of copper. The vessel was ac. 

 curately weighed before and after it was tinned, its surface was 

 equal to 254 square inches ; its weight, before it was tiniu-d, was 

 46 ounces, and its weight, after the operation, was barely 464 

 ounces; so that half an ounce of tin was spread over 254 square 

 inches, or somewhat less than a grain of tin upon each square inch. 

 How innocent soever pure tin may be, yet the tenuity of the coat 

 of it, by which copper vessels are covered, in the ordinary way of 

 tinning, cannot fail to excite the serious apprehensions of those 

 who consider it; for in the experiment which I have mentioned, 

 the tin was laid on with a thicker coat than in the common way ; 

 instead of a grain, I suspect that not a quarter of a grain of tin is 

 spread over a square inch in the common way of tinning. A dis. 

 covery has been lately made at Paris of a method of giving to cop. 

 per or iron a coat of any required thickness, by tinning them ; 

 the composition used for the tinning is not mentioned ; but it is 

 said that a piece of copper, which in the common way of tinning 

 only absorbed 21 grains of tin, absorbed of the new composition 

 432 grains, or above twenty times as much*. Till this discovery 

 is generally known, our workmen should study to cover the cop. 

 per with as thick a coat as they are able of pure tin. The dan- 

 ger from the corrosion or solution of the tin by vinegar, juice of 

 lemons, or other vegetable acids, if any at all, cannot it is appre- 

 hended, be sensibly felt, except in very irritable habits, or 

 where sour broths, sauces, or syrups, are suffered to stand 

 long in tinned vessels before they are used. And, indeed, a pro. 

 per attention to keeping the vessels clean, mi^ht render the use of 

 copper^itself, for the boiling of food, especially of animal food, 



* J. 'Esprit An Jonrnaox, Mai, 1785. 



