PAINTING. - 253 



and the punching and handling of untreated plates and shapes, 

 as well possibly as the jar of railway transportation, and the 

 several handlings, loosen more mill-scale than enough to com- 

 pensate for any rusting in transit, and that therefore the 

 proper time to clean and prime is at the shop, after the me- 

 chanical work has been completed, and immediately before 

 shipment to the point of erection, any grease which may re- 

 sult from the machining being also subject to removal at the 

 same time. The facilities for cleaning and painting being 

 usually superior at the shop to those likely to obtain at the 

 point of erection, is another consideration in favor of shop- 

 cleaning and priming. Structural metal, when carefully 

 cleaned of all rust, mill-scale, grease, and dirt, should be im- 

 mediately protected by some covering as nearly impervious to 

 moisture as possible in order to prevent further corrosion 

 from chemical and galvanic action. 



Zinc Coating. It. has been found that the application of 

 molten zinc, called "spelter," as a bath, forms a coating 

 which is electrically positive to iron or steel, and which in 

 the presence of galvanic action results in the corrosion of 

 the zinc and the protection of the ferric body. Such a coat- 

 ing is very effective, but with the larger plates, where the 

 dipping is done by hand, the process is very expensive, f in. 

 plate being the thickest material so far galvanized for practi- 

 cal purposes, the cost being from $14.00 to $16.00 per ton. 

 Besides the expense, unfortunately the process reduces the 

 strength of plates and shapes to an extent that galvanized 

 metal is generally considered as being " rotten : ' and unfit for 

 use where certain and considerable strength is required. 



Again, it has been asserted that water in galvanized re- 

 ceptacles or reservoirs becomes unfit for use, which, if true, 

 would debar this method of protection either for the towers 

 and members where strength was required, or for the tank, 

 where the storage of water was the purpose of the structure. 



