Dear Peter,
I forgot an important point of the description. The samples have been
examined on either sections or surfaces. One of the samples was polished
from the surface at first, showing dendrite. With more and more working on
the surface, the structure in the core showed recrystalised grains. I think
it is difficult to be reason the phenomenon as the annealing treatment.
Looking forward to your reply.
William Qian
-----Original Message-----
发件人: Peter Northover <[log in to unmask]>
收件人: William Qian <[log in to unmask]>
抄送: arch-metals <[log in to unmask]>
日期: 1999年9月16日 20:15
主题: Re: unreal texture of arsenic bronze
>Dear William,
>
>The combination of residual coring in the form of a mor or less deformed
>dendritic structure and recrystallised grains is quite common in
>archaeology. The explanation is very simple: in many of the copper alloys
>used in antiquity the recrytsallisation temperature is substantially below
>that at which the as-cast segregation is homogenised. The
>recrystallisation temperature for a 4% arsenical copper is (off the top of
>my head) something like 400-450 degrees Celsius, while the homogenisation
>temperature may be something like 700 degrees. Thus, if a piece of cold
>worked arsenical copper is annealed between these temperatures the
>structure will recrystallise but the deformed cored structure will remain.
>This gap is very useful for determining the conditions in which a copper
>alloy was worked and annealed.
>
>Etching such structures can be difficult depending on the impurities
>present. FeCl3 and ammoniacal hydrogen peroxide used alternately can be
>useful. The ammoniacal peroxide is a relief etch and brings up the coring,
>while the FeCl3 can bring out the grain boundaries and the slip traces.
>Ammoniacal peroxide used after an FeCl3 lightens the colour typically
>produced by the etch and can make the grain boundaries easier to see.
>
>David Dungworth at English Heritage is also interested in this phenomenon
>and I am sure will have something to add.
>
>Yours,
>Peter Northover
>
>
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