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.
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