In the present state of civilization the use of iron has reached a very
wide extension, and in a great number of cases iron is used where wood
or stone was formerly used. It is certainly an important question how
this metal can be protected under all circumstances against rust or
oxidation, so that the many costly iron structures may retain their
usefulness and strength, and be handed down uninjured to posterity.
Wherever bright iron comes into contact with air and moisture it
immediately begins to rust, and this rust is not content to continually
rob it of its substance in its persistent progress by scaling off the
surface, but at the same time it injures the remainder of the iron by
making it brittle. Attempts have hitherto been made to protect the iron
by covering it with other and less easily oxidizable metals. For this
purpose tin was first selected, then lead and zinc, and recently nickel.
Furthermore, earthy glazings and enamels, such as are used on stone
ware, have been applied to iron vessels, and they have already found
extensive use in the household. In most cases these coatings, either
metallic or vitreous, are inapplicable, either because they cannot be
applied or are too expensive, so that on a large scale recourse must be
had to paints made by mixing oils with metallic oxides, earths, etc.,
for protecting the surface of the iron from air and moisture.
It has been observed that iron does not rust in _dry_ air, not even in
dry oxygen. In like manner it frequently happens that unpainted iron,
such as weather vanes, fences, etc., is exposed to the air for a century
with very little injury, being covered with a thin coating of the
magnetic oxide (proto-sesquioxide), which acts as a protection and
prevents farther action. Hence it has been proposed to produce a layer
of this magnetic oxide on the surface artificially, and it was found
that superheated steam furnished the means for doing this. But it is not
to be supposed that such a process would find use on a large scale, and
besides this protection could only serve for iron tolerably exposed to
the open air and not for that in direct contact with carbonic acid and
water.
An interesting observation has been made on railways that the iron
rails, ties, bolts, etc., rust until the road begins to be used. Here we
must assume that anything made of iron is more inclined to rust when at
rest than if occasionally caused to vibrate, when an electrical action
probably comes into play and decreases the affinity of iron for oxygen.
In tearing down old masonry iron bonds and clamps are often found which
are as free from rust, so far as they are covered with mortar, as they
were the day they left the blacksmith's hands. A French engineer met
with such a phenomenon when he uncovered the anchor plates of several
chain bridges which had been built about thirty years. Where the anchors
were covered with the fatty lime mortar of the masonry they showed no
traces of rust, but the prolongations of the anchors in empty spaces
were rusted to such an extent that they were only one-third of their
original thickness.
It has been repeatedly observed that iron does not rust in water in
which are dissolved small quantities of caustic alkalies or alkaline
earths, which neutralize every trace of acid. It seems that these
experiences are the basis of A. Riegelmann's (Hanau) new protection
against rust. The paint that he uses contains caustic alkaline earths
(baryta, strontia, etc.), so that the iron is in a condition similar to
the iron anchors of the chain bridges that were embedded in lime mortar.
Although a paint is not thick enough to inclose so much alkali as the
masonry did that the iron was embedded in, nevertheless the alkaline
action will make itself felt as long as the coating has a certain
consistence. Under all circumstances, however, these new paints will
be free from active acids, which is more than can be said of our iron
paints hitherto in use. Besides this, the rust protector has such
a composition that it could serve its intended purpose without the
addition of any alkali. If experience confirms this claim, it will be an
interesting step forward in the preservation of iron, and contribute to
an extension in the use of iron.--_Polytechn. Notizblatt_.
* * * * *
[Illustration: SUGGESTIONS IN DECOTATIVE ART.--A CUPBOARD IN ITALIAN
WALNUT WITH DARKER PANELING.--_From The Workshop_.]
* * * * *