Removing lichen from lithic artifacts

Carrie F. Jackson <carjacks [at] prairie__nodak__edu> writes

>I am seeking information for our laboratory on the removal of lichen
>from lithic artifacts.  We have a number of artifacts that were
>surface collected from an archeological site in North Dakota in the
>summer of 1999.  They have been stored in paper field sacks since
>their collection.  We are looking for direction on how to safely
>remove the lichen from the artifacts, if possible.


    Margot Brunn, Museum Conservator at the Provincial Museum of
    Alberta has asked me to reply to a question about the removal of
    lichens on lithic artifacts. As the museum's curator of Botany I
    have worked with the Archaeological Survey of Alberta in using
    lichens to date stone features and as a cryptogamic plant
    specialist have developed some familiarity with lichens.

    There are a number of issues to consider here including lichen
    biology, the nature of the lithic feature, microclimates and the
    reasons one would want to remove the lichens.

    Relevant Lichen biology

    Lichens are composed of algae and fungi. The fungi give the
    lichens their shape and the lichens attach themselves to the
    stone surface by the fungal hyphae. Now each hyphae is
    microscopically small, and mushy, like a common garden mushroom,
    but when thousands of these guys grab onto each square
    centimeter of rock surface they are almost impossible to break
    free. A small force multiplied thousands of times becomes
    significant.

    Lichenologists travel with geological rock hammers and a set of
    chisels when they collect lithic lichens! Manual removal of the
    lichen from the rock surface is virtually always destructive to
    the lithic surface.

    The second point to consider is that lichens are acidic. In fact
    we use the complex acids produced by the lichens as a means of
    species identification. These acids are in part responsible for
    the weathering of the rock surface covered by the lichen and the
    immediate surrounding area. In rainy or even very humid
    conditions some of the acids are washed out of the lichen, onto
    the rock surface and this causes a breakdown of the rock.

    On the other hand, lichens can act as an effective shield,
    blocking a large amount of the sun's radiation from reaching the
    underlying rock surface. In relatively dry regions lichens can
    actually protect a rocks surface from damage by insolation.

    This leads us into a consideration of lithic geology for the
    weathering effects are relative to the nature of the bedrock.

    Lithic Geology

    If your artifact is composed of acidic rock, such as granite,
    rhyolite, most igneous rock, etc, then the weathering affect of
    the lichens acid can be expected to be negligible. It is an acid
    acting on an acid. By contrast if your artifact is composed of
    basic rocks, such as limestones, sandstones from the Great
    Plains, in fact most sedimentary rocks from the Great Plains,
    and basalt, then the weathering effects of the lichens may be
    significant. The bottom line: lichens on acidic rocks are likely
    to cause little weathering. If removed they will not likely
    leave a stain on the rock surface. Lichens on basic rocks may
    lead to significant weathering; they often stain the underlying
    surface. You will never get this stain off without further
    damaging the lithic surface.

    The proceeding discussion is based on field conditions. Now we
    consider microclimate.

    Microclimate

    Under dry conditions, e.g. in your lab or storage area there is
    very low relative humidity and further your relative humidity
    and temperature probably stays fairly constant. This is
    critical. For lichens to grow, and for the acids to be washed
    out of them, they need to be wet, either from rain, dew or just
    very high relative humidity. If a lichen is found growing on a
    rock these conditions are being met. However, in your museum
    these conditions will not (should not) be met. The lichens will
    stabilize, become dormant, no chemical action will occur. Your
    lichens will not continue to grow, cover or damage your surface.
    I've had lichen covered lithic artifacts in my lab for 5 years
    and in my herbarium for a decade without them growing a
    millimeter.

    Why remove them?

    Why do you want to remove them?  It is very important that you
    remember that the lichens are part of the artifact. They
    naturally grow on stone surfaces that are available to them,
    whether these surfaces are naturally occurring or are artifacts
    of human activity. If you remove them you are potentially
    tossing away valuable information about the feature. You can
    never recover the data once it's gone. In my view, it's a bit
    like knocking the notches off of a Besant arrowhead or buffing
    the patina off of a bronze sculpture!

    In the work of Plains Archeologist Rod Vickers and myself on
    medicine wheels in southern Alberta we found that although we
    couldn't put an exact date on features, we could do relative
    dating within a site. This told us a nice story about how the
    wheels were constructed, and added on to, over a considerable
    period of time. We've used lichens to solve mysteries of cairn
    features in Jasper National Park where we couldn't dig to get to
    datable material. Lichenometry goes in and out of favour but
    there has been a considerable revival in recent years, there are
    more dating curves available, and our statistical analysis are
    much more powerful. Lichens can virtually always tell you the
    relative time of surface exposure of any lithic surface in a
    given site, lichens are highly habitat sensitive so they can
    also tell you about the nature of the lithic substrate. The use
    of lichens as artifacts is vast, but under-rated; it just takes
    some imagination. But if you remove the lichens you can't do
    anything.

    If you still feel your concerns over ride any scientific data
    you may generate then here are some things to consider:

    1.  You'll never get a crustose lichen off a rock and keep the
        rock's surface intact. As I said above, we use rock hammers
        to get the critters off the surface. You could try wetting
        them and then scraping them off. Foliose lichens, the second
        most common lithic lichens can usually be removed by careful
        prying with a sharp bladed knife. But then we still have the
        staining problem. Lichens cause differential weathering on
        the rock which is visible as stains. On basic rocks the
        lichens will stain the rocks by their acids. The lichens
        also shield the rock from radiation which can lead to
        differences in color even on acidic rocks.

    2.  Once a lithic artifact is in the museum no more weathering
        is going to occur. The artifact will not deteriorate any
        further. Basic lithic artifacts in field conditions will
        continue to deteriorate with lichen growth and a case can be
        made for removing the lichens from these intact features,
        but removing the lichens is likely to add the equivalent of
        hundreds of years of natural weathering.

    3.  A colleague from the Geological Survey of Canada can measure
        the surface exposure of metamorphic rocks by the amount of
        radiation absorbed in their crystals. It only takes a single
        crystal to get a date. Because lichens block radiation it is
        theoretically possible to obtain the date of exposure of a
        lichen covered surface. We have not followed up on this
        project but the theory is sound and potentially a major
        benefit for archeological research. It is likely to become a
        dating method in the future, but again, no lichens, no
        dates.

    4.  If you do decide to remove the lichens at the very minimum
        identify as many species as possible on each artifact and
        record the size of the largest specimen of each species on
        each lithic artifact. There is a correlation between age of
        surface exposure both with the number of lichen species and
        with the size of the largest specimen of each species.

    I am open for discussion about the relative merits of
    recognizing lichens as artifacts or scraping them off for
    aesthetic or conservation reasons. If you convince me of the
    relative merits of removing them I could probably even help you
    develop a good method for doing so.

    Roxanne Hastings
    Curator of Botany
    Provincial Museum of Alberta
    Edmonton, Alberta, Canada
    Rhastings [at] mcd__gov__ab__ca

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                  Conservation DistList Instance 13:31
                Distributed: Tuesday, November 23, 1999
                       Message Id: cdl-13-31-004
                                  ***