302 TINNING COPPER, &C. 



ferent customs. Vessels tinned with pure tin, or with (he best 

 kind of pewter, which contains no lead, do not stain the fingers 

 when rubbed with them : whilst those which are tinned with com. 

 position, into which lead enters as a constituent part, colour the 

 fingers with a blackish tinge. 



Zinc was long ago recommended for the (inning of copper ves. 

 sels, in preference both to the mixture of tin and lead, and to pure 

 tin* : and zinc certainly has the advantage of being harder than 

 tin, and of bearing a greater degree of heat before it will be 

 melted from the surface of the copper; so that on both these ac- 

 counts it would, when applied on the surface of copper, last longer 

 than tin ; just as tin, for the same reasons, lasts longer than a mix. 

 ture of tin and lead. But whether zinc makes any part of the com- 

 pound metal for tinning copper, so as to prevent the necessity of 

 repeated tinning, for which a patent was granted some years ago, 

 is what I cannot affirm. Whatever may the excellence of that 

 composition, or of any other composition, which may be invented 

 with respect to its durabilily, and its not contracting rust ; still 

 it ought not to be admitted into general use, till it has been proved, 

 that it is not soluble in vegetable acids, or that its solutions are 

 not noxious+. A method has of late years been introduced at 

 Rouen, of applying a coat of zinc upon hammered iron sauce, 

 pans. The vessels are first made very bright, so that not a black 

 speck can be seen ; they are then rubbed with a solution of sal 

 ammoniac, and afterwards dipped into an iron pot full of melted 

 zinc, and being taken out, the zinc is found to cover the surface of 

 the iron ; and if a thicker coat of zinc is wanted, it may be ob- 

 tained by dipping the vessel a second time. This kind of covering 

 is so hard, that the vessels may be scoured with stand without its 

 being rubbed off J. Kitchen utensils, which are made of cast iron, 

 are usually tinned to prevent the iron's rusting ; and, as great 

 improvements have been lately made in rendering cast iron mallea- 

 ble, it is not unlikely, but that tinned iron vessels may become of 

 general u s e. 



Mem. de V Arad. des Scit-n. a Par. 1142. 



f- This doubt with respect lo zinc is said to have been removed. M. dp la 

 Planche, a physician at Paris, tried the experiment on himself: he took the 

 tall* of zinc, formed by the vegetable ar ids, in a much stronger dose than the 

 aliments prepared in copper vessels, lined with zinc, could have contained, 

 and he felt no dangerous effVcts from them. Fourcroy's Chem. vol. I. p. 442. 



f Jouro. de Phy. Decoem. 1778. 



