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?

I have a reference and some notes for the spot-testing of metals,
including tin and zinc.  The notes are third-generation copies from
lectures by Henry Hodges.  The book you may want to consult is:

    Marilyn Laver.
    "Spot Tests in Conservation: Metals and Alloys,"
    ICOM Committee for Conservation 5th Triennial Meeting, Zagreb
    1978

Hodges's notes are complex, and you'll need a well-stocked
laboratory (with fume hood!) and centrifuge, but if you have access
to such, here are the instructions:

For tin:

   "1.1. Make a solution: Put a small scraping of test material in a
    test tube with a little distilled water and shake continuously
    for several minutes. If no solution has taken place, bring to a
    boil. If sample remains insoluble, add a little dilute nitric
    acid and shake.  If rate of solution still slow, warm solution
    (An effervescence on adding acid probably indicates the presence
    of a carbonate or sulphide).

   "1.4. Group II cations: To this acidified test solution add a
    little thioacetimide solution and boil.  The thioacetimide will
    liberate hydrogen sulphide. A precipitate will indicate the
    presence of copper, mercury, bismuth, antimony, cadmium, tin, or
    arsenic. Continue to boil until there is no longer any smell of
    hydrogen sulphide. Centrifuge; retain the precipitate for the
    tin test and retain the liquid as the test solution for the zinc
    test.

   "1.5. Separation of Group II into IIa and IIb by solution of some
    sulphides in sodium hydroxide (NaOH) solution: To Group II
    sulphide precipitate add NaOH solution. Sulphides of tin,
    antimony, and arsenic will dissolve.  Centrifuge.  Keep the
    liquid for the tin test. 1-5-5) Confirm for tin: To the above
    NaOH solution add dilute hydrochloric acid (HCl) until the
    precipitate re-forms.  Centrifuge and wash. Add a little
    concentrated HCl, and place a drop on filter paper. a) add some
    zinc dust and then 1 drop cacotheline solution.  A violet color
    indicates tin. Or, b) Add 1 drop toluene 3:4 diothiol solution.
    A brick-red color indicates tin."

For zinc:

   "1.6. Group III and IV cations: To the solution produced in step
    1.4 above, add ammonia until alkaline, then a little more
    thioacetimide. Any precipitate formed may be a mixture of
    hydroxides (Group III) and sulfides (Group IV). The precipitate
    is washed and dissolved in as little aqua regia as possible (To
    make aqua regia mix 40 ml nitric acid and 60 ml hydrochloric
    acid. Always use fresh mixture).

   "1.8. Group IV cations: Divide the above test solution.  To one
    half, add thioacetimide. A precipitate indicates zinc,
    manganese, cobalt, or nickel. Add dilute HCl. If the precipitate
    dissolves, zinc or manganese is indicated.  Use the other half
    of the test solution to confirm for zinc:

   "1.8.1. Confirm for zinc: Mix 2 drops ammonium thiocyanate and 1
    drop mercuric chloride solutions on a black tile. Add 1 drop
    test solution. Slowly-forming white needle-like crystals
    indicate zinc."

Best of luck,

Elizabeth Hendrix
Laboratory Supervisor
Center for Materials Research in Archaeology and Ethnology
Massachusetts Institute of Technology
Cambridge, MA 02139


                                  ***
                  Conservation DistList Instance 19:23
                 Distributed: Friday, October 28, 2005
                       Message Id: cdl-19-23-006
                                  ***