PAINTING. 251 



cal one, sharp-grained sand being employed under about 15 

 pounds compressed-air pressure at the nozzle, to cut away the 

 rust and mill-scale, by being directed to the desired point 

 from the end of a rubber tube or hose. While a certain 

 method of cleaning when intelligent care is exercised, and the 

 penalty for negligence not being so severe as where acid is 

 used, the objection recorded to the use of sand is that a 

 special building must be provided, from the fact that, unless 

 the sand is confined, it is likely to prove damaging to ma- 

 chinery and become generally a nuisance. 



The last and most popular method of cleaning plates and 

 shapes is by the use of scrapers and brushes, either by hand 

 or mechanically, electric revolving brushes being considerably 

 used of late. The loosened material is wiped away with oiled 

 waste or rags. Nearly all of the larger bridge- works clean their 

 shapes in this way. The objection to this is that although 

 the surfaces may seem bright and free from rust and scale, 

 under a glass it will be seen that only the microscopic met- 

 allic points have been burnished, the depressions showing 

 minute rust-specks which have not been touched by the scraper 

 or brush, and may therefore become points or foci for corro- 

 sion. For these reasons, it would seem that specifications for 

 the cleaning of metals should be drawn to include the use of 

 the sand-blast, the cost of which is about the cost of a coat of 

 good paint, and is said to be about $1.50 per ton of metal, 

 exclusive of handling. During its evolution, the time at 

 which the metallic member should be cleaned and primed is 

 of great importance. In an investigation of this question, 

 a testing-bureau, having a wide experience and facilities for 

 observation, writes as follows: " In rolling a plate, a slab is 

 drawn from the heating-furnace or soaking-pit, and it passes 

 through the rolls. As it is being reduced, salt is thrown upon 

 the slab; it causes a loud explosion, and loosens the scale 

 formed and a steam-jet is turned on the slab, which blows 



