Oddy test

Agnes Gall-Ortlik <agaor [at] caramail__com> writes

>I am carrying out Oddy tests on gap-filling materials for enamels on
>metal. I have found some references in the literature describing the
>directions for setting up the test, but of those, just one gives
>information on how to evaluate and interpret the results--color
>shift, surface condition, localization and morphology and changes:
>...
>I would be thankful for any answer that would enable me to improve
>interpretation clues.

I am working at the British Museum, I run the accelerated corrosion
testing and I am actually researching the method at the moment. To
help you to interpret the result it is important to always run a
blank (with no material) for comparison. The change depends on the
coupon :

    *   Silver is considered as unsuitable when it is blackened.

    *   Copper is considered unsuitable also when it is blackened.
        You get sometimes on copper coupons different colours from
        pale orange to shiny red, which are not considered as a
        change due to the material tested as we get the same change
        on the blanks.  This is due to oxidation of the copper
        surface because of the elevated temperature and RH.

    *   Lead is the more reactive. It is considered unsuitable when
        you get spots of white corrosion. The colour change you get
        sometimes on the coupon,  from grey to blue or black, is not
        considered as a change due to the material tested as again
        we get the same change on the blanks.

Concerning your specific questions, I have been studying the effect
of the condensation on the coupon. Indeed, when you open the oven
you get condensation on the edges of the tube. If your coupon is in
contact with the edge, you will get water on the coupon which tends
to corrode the lead coupons. In that case you get a mixture of red
yellow and white corrosion located mostly at the contact point of
the coupon. The way to avoid a contact of the coupon with the edge
is to make sure that the coupon is suspended right in the middle of
the tube and by putting the tube vertical.

    *   at high RH the corrosion produced on the coupon is usually
        plumbonacronite.

    *   The aspect of corrosion does not permit identification of
        the corrosion product type.

    *   The odor can not be used as a criteria, sometimes you have a
        strong smell and you do not get any corrosion.

    *   Other criteria have been studied like the weight gain of the
        coupon or weighting the stripped corrosion and
        electrochemical analysis which could be used to quantify the
        corrosion.

If you see corrosion around a material it is evolving corrosive
gases.  The lack of corrosion under the material is usually because
it is blocking access of oxygen (or sometime moisture).  In that
sense it is protective, *but* this relies on a complete surface
layer with no breaks.  If breaks occur this can lead to dramatically
accelerated corrosion.  Also the material has the potential to
damage other objects inside the enclosure from the gases evolved.

Ms Laurianne Robinet
The British Museum
Conservation research Department
WC1B 3DG London - UK



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                  Conservation DistList Instance 14:48
                 Distributed: Thursday, March 22, 2001
                       Message Id: cdl-14-48-001
                                  ***