This brings up something that has troubled me for years:
Pete collects a bronze nail, in about 2005, which is 85% copper, 10%
tin and 2.5% zinc. What was the composition of the nail in 1825?
More generally: If we retrieve metallurgical samples that have been
under seawater for many decades (or longer) can we assume that their
composition after retrieval is the same as it was when they were
dropped into the sea? If so: Under what circumstances and with what
precision?
The various alloys used for sheathing hulls (in the post-1780 period)
were all predominantly copper. I'll take a wild guess that the zinc
content in all of them was between zero and 10%. If a modern sample
has a 2.5% zinc content but all we can deduce is that the original
material had _at_least_ 2.5%, then the analysis may not tell us
anything that wasn't already obvious.
Can anyone offer an authoritative comment on how much, or how little,
those percentages would change with immersion in seawater?
Trevor Kenchington
Pete Johnson wrote:
> I can supply a sample from what we think is the steamboat
> Washington located in Long Island Sound, Northeast US. The
> Washington was built in 1825 and lost in 1831. A sample of the
> sheathing and a sheathing nail were collected and were tested using
> Energy Dispersive X-Ray Analysis (EDX). This is a version of X-ray
> diffraction that can perform a non-destructive analysis of a
> metallic sample. Testing results of the copper sheet showed that
> the alloy was almost pure copper with only some trace copper
> compounds as contaminates. Tests on the nail found it to be an
> alloy of copper and tin with trace amounts of zinc and silicon. The
> actual analysis of the nail was:
>
> Copper 85.23%
> Tin 10.31%
> Zinc 2.60%
> Silicon 1.86%
>
> I do not have extra samples of the nail but can send a square
> centimeter of the sheathing.
>
> Pete Johnson
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