closed retort and a current of superheated steam passed through for twenty minutes followed by a current of producer gas (carbon monoxide), to reduce any higher oxides that may have been formed.  In the Gesner process a current of gasoline vapor is used as the reducing agent.  The blueing of watch hands, buckles and the like may be done by dipping them into an oxidizing bath such as melted saltpeter.  But in order to afford complete protection the layer of black oxide must be thickened by repeating the process which adds to the time and expense.  This causes a slight enlargement and the high temperature often warps the ware so it is not suitable for nicely adjusted parts of machinery and of course tools would lose their temper by the heat.

A new method of rust proofing which is free from these disadvantages is the phosphate process invented by Thomas Watts Coslett, an English chemist, in 1907, and developed in America by the Parker Company of Detroit.  This consists simply in dipping the sheet iron or articles into a tank filled with a dilute solution of iron phosphate heated nearly to the boiling point by steam pipes.  Bubbles of hydrogen stream off rapidly at first, then slower, and at the end of half an hour or longer the action ceases, and the process is complete.  What has happened is that the iron has been converted into a basic iron phosphate to a depth depending upon the density of articles processed.  Any one who has studied elementary qualitative analysis will remember that when he added ammonia to his “unknown” solution, iron and phosphoric acid, if present, were precipitated together, or in other words, iron phosphate is insoluble except in acids.  Therefore a superficial film of such phosphate will protect the iron underneath except from acids.  This film is not a coating added on the outside like paint and enamel or tin and nickel plate.  It is therefore not apt to scale off and it does not increase the size of the article.  No high heat is required as in the Sherardizing and Bower-Barff processes, so steel tools can be treated without losing their temper or edge.

The deposit consisting of ferrous and ferric phosphates mixed with black iron oxide may be varied in composition, texture and color.  It is ordinarily a dull gray and oiling gives a soft mat black more in accordance with modern taste than the shiny nickel plating that delighted our fathers.  Even the military nowadays show more quiet taste than formerly and have abandoned their glittering accoutrements.

The phosphate bath is not expensive and can be used continuously for months by adding more of the concentrated solution to keep up the strength and removing the sludge that is precipitated.  Besides the iron the solution contains the phosphates of other metals such as calcium or strontium, manganese, molybdenum, or tungsten, according to the particular purpose.  Since the phosphating solution does not act on nickel it may be used on articles that have been partly nickel-plated so there may be produced, for instance, a bright raised design against a dull black background.  Then, too, the surface left by the Parker process is finely etched so it affords a good attachment for paint or enamel if further protection is needed.  Even if the enamel does crack, the iron beneath is not so apt to rust and scale off the coating.