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Jared Sherman wrote:

Kieran,

"If you want to have real fun [sic] try obtaining bronze through-hulls for a
boat. The odd thing is, when you start asking manufacturers what alloy they
are using some of them pretend this is a great trade secret. (Which any
university chemistry lab could unveil over a cup of coffee or a pizza
delivery.) Others will tell you what they use, and if you check the
alloys out some of their brasses are bronzes, and their bronzes are brasses.
All sorts of trade names (naval bronze, red brass, etc.) exist for alloys,
and some include both tin and zinc leaving anyone to guess which yellow
metal they should be called."

An excellent confirmation of Jared's posting was sent to the Sextant list as
part of the same query so I thought it would be worthwhile re-producing it
here. We may never know, it seems, what Plath meant when they described the
Classic as a "bronze" sextant. too bad there is no-one left to ask.

Kieran Kelly

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Will O'Neil wrote:

First a bit of clarification regarding terminology. In modern usage, metal
is said to be copper if it has no more than 2% of elements other than Cu in
its composition. If it contains more than 2% zinc, and there is no other
major constituent other than Cu and Zn, then the material is termed brass.
And if it contains major constituents other than Cu and Zn then it is a
bronze. All of which is to say that there are a great many more choices in
making bronze than in making brass.

Traditionally, of course, tin (Sn) was the primary alloying element in
bronzes before the 20th century, often accompanied by significant amounts of
lead (Pb) and sometimes a good quantity of Zn as well.

Brass has the advantage that it is cheap and easily worked, as well as
taking a good finish. Until the mid 19th century, brass was often made by
co-reduction of Cu and Zn rather than by direct alloying, resulting in quite
variable composition and properties. In general, brass is not a terrific
casting metal, as it is difficult to get consistent mold fill and low
porosity unless mold sections are pretty generous. Early sextants were
almost all made from plate brass, which in practice tended to be
insufficiently stiff to maintain the necessary tolerances. Edward Troughton
won fame by developing his double-frame sextant construction which overcame
the limitations of brass plates. It was a sound design but too expensive in
terms of labor for general use.

Another problem with brass is that it is not highly resistant to salt-water
corrosion.

Early in the 19th century, makers shifted toward cast frames. If cast in
brass the sextant needed fairly thick webs in order to give good foundry
results. Later, many makers turned to tin bronzes. Because tin is costly,
this raised materials prices, but there were compensations. Tin bronzes
characteristically have superb foundry properties, permitting very fine and
intricate castings to be made. This allowed makers to go to "spiderweb"
frames having a multitude of thin, deep web elements, which could minimize
weight (and expensive material) while providing good stiffness.
Additionally, tin bronzes are inherently stiffer than brasses. Also, there
are tin bronzes with very good corrosion resistance.

In the 20th century, many sextants frames were cast in silicon or aluminum
bronze, which is cheaper than tin bronze and can offer good foundry
properties along with excellent corrosion resistance and high strength.

In practice terminology has often been loose and there has been a tendency
to apply the "brass" label to many materials that are technically bronzes.
It is often not really possible to tell without cutting off a piece for
chemical analysis or microscopic examination -- something which is very
rarely done, naturally. One clue is that normally a bronze frame will give
off a rather brighter ring when tapped than will one of brass, owing to the
greater stiffness and generally lower porosity of bronze. Also, if cast in
brass the frame will seem fairly heavy, and will usually show at least some
visual evidence of porosity or pitting.

Will O'Neil

   

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