Spot testing

Karin Abelskamp <k.abelskamp [at] archeologie__nl>

>I am looking for a spot test to demonstrate the presence of tin in
>an archaeological object made of a copper alloy object or a lead
>alloy. Does anyone know if such a test is commercially available? If
>so, where can I purchase it, preferably in the Netherlands?
>Furthermore, is such a test perhaps also available for zinc?

There are Merkoquant spot test kits for both tin and zinc available
through

    Fisher Emergo BV
    Zaandam, The Netherlands
    +31 20 4877000
    <URL:http://www.emergolab.com>

Unfortunately these commercial tests are not really suitable for the
tasks you want them to perform. The zinc test gives a false positive
result when copper is present! Probably the same problem will occur
with the tin test. These tests are (unadapted) therefore not
suitable for detecting either zinc or tin in bronze, brass or other
copper alloys.

A method for detecting tin in copper alloys that works pretty well
is the "cacotheline" test. This test uses cacotheline
(Nitrobruciquinone hydrate) available through Instru-Labo in
Udenhout (+31 13 5111982). For this test you'll have to get the
metal ions from the object (either from the corrosion layer or the
metal-core) into solution this can be achieved by dissolving the
sample in a drop of diluted hydrochloric acid (20% in water). You
can also put a small acidified piece of filter-paper (first soaked
in the acid-solution) on the object for approximately. 2 minutes.

Another method is using electrolyses (please see Odegaard 2000 for
this method). Add a large drop of cacotheline solution (=1 gr of
cacotheline in 15 ml water) to the dissolved sample or the
filter-paper taken form the object. If tin is present a purple
colour will form. Please note that whenever copper is present there
will also be (a weaker) purple colour. *But*, the positive for
copper will disappear within 15-20 minutes (as long as the sample
stays wet, so do use a large drop of cacotheline solution)

I somewhat time consuming method for detecting zinc in the presence
of copper using Merkoquant spot test papers is described by G.
Eggert (see Eggert 1988), it should work but be aware of false
positives and negatives.

Whenever performing spot tests on metals be sure to have reference
metals available so you can see what the reaction should look like
(and test whether or not all the prepared solutions still work) When
testing archaeological metals be aware that sometimes not all
compounds of the alloys may be represented in the corrosion due to
preferential corrosion processes as dezincification and
destannification. The metal core on the other hand can look like
copper due to the same corrosion processes but may be a brass or
bronze with less or no zinc or tin present on the surface....

I would advice you to get a copy of "Material Characterization Tests
for Objects of Art and Archaeology" (Odegaard 2000) and possibly
read the literature mentioned below (available trough the ICN-libary
in Amsterdam <URL:http://www.icn.nl>. You can also contact me
off-list for more information and a Dutch manual on spot test for
archaeological metals.

References:

    Eggert 1988
    Eggert, G., "Qualitative Analyse von Kupferlegierungen durch den
    Restaurator" in Arbeitsblatter, Heft 2, Gruppe 2, Bronze, 1988

    Townsend 1988
    Townsend, Joyce H, "The Identification of Metals: Chemical Spot
    Tests" in Modern Metals in Museums, Institute of Archaeology
    Publications, London, 1988

    Odegaard 2000
    Odegaard, N., C. Scott and W.S. Zimmt, Material Characterization
    Tests for Objects of Art and Archaeology, London, Archetype
    Publications, 2000

Michiel Langeveld
M Langeveld Metaalrestauratie
Zamenhofstraat 150, UNIT 236-238
1022 AG Amsterdam
+31 6 50218260


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                  Conservation DistList Instance 19:23
                 Distributed: Friday, October 28, 2005
                       Message Id: cdl-19-23-004
                                  ***