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    Re: Brass vs Bronze for sextants [Was: Hooke's Quadrant
    From: Will O?Neil
    Date: 2004 Jan 28, 19:55 -0500
    At 18:30 1/28/2004, Kieran Kelly wrote:
    Also on a recent post I asked if anyone was aware of sextants being cast in
    bronze other than the Plath Classic and Royal. Why would you cast in bronze?
    What is the advantage over brass?

    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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