Ultrasound

I must thank Stefan Michalski for responding to my query concerning
my articles on ultrasound and the use of ultrasound in general as an
appropriate modality in conservation.  It is good to get responses
from colleagues even if 5 or 6 years go by, as it provides an
opportunity to explore the role of scientific research in the
conservation community.

Michalski argues that my articles were not relevant to the
application of ultrasound by misting and therefore were not
pertinent to the modality he was describing.  Again, I wrote two
articles, one a critical review of ultrasound in conservation with a
historical outline and the other a report on the effects of
ultrasonic misting of paper with type.  The central argument was to
show how ultrasonic cleaning can destroy or alter organic residues
on archaeological objects which could, if left intact be analyzed by
immunochemical methods. Methods of retrieval of these residues have
increased in sensitivity in recent years (Fullagar, 1996; Petraglia,
1996; Neff, 1993).

Michalski discounts the review based on his understanding of the
effects of ultrasound as discussed in a recent review of the
research in ultrasound by one of the major scientists in the field,
Dr. K.S. Suslick.  Michalski quotes from Suslick's review (1998) on
the effects and nature of the major delivery of energy by ultrasonic
radiation, which is cavitation.  He includes a quote which he
believes severely limits the amount of energy which can be produced
without a liquid medium. This is interpreted by Michalski as meaning
that no affects can occur without a continuous concentration of a
liquid between the source of the radiation and the object.
He also discounts the effects of the ultrasonic scaler used in the
dentist's office by a similar belief.

Two effects of the use of ultrasonic radiation are referred to here:
One is does this radiation produce effects outside of complete
liquid phases?  Since we live in a sea of gases which contain
varying concentrations, we hear noise due to the effect of sound
traveling by transferring energy through molecules over distances.
Cavitation occurs in liquids, not just water, but oil, organic
reagents, etc.  Cavitation depends on the concentration of
radiation.

My experiments indicated that some effects of the use of a
ultrasonic mister produced energy effects at a distance.  If this
was by means of cavitation, or other means, through a mist (ie
highly concentrated mist) then this should be investigated by
others.  Abdulla (1988) noted in his review of research that
temperature and vapor pressure are important factors determining a
limit on the frequency  of ultrasound that can result in chemical
effects.  These factors vary in all applications.  In dismissing my
results Michalski states that

   "Thus without a liquid carrying the ultrasonic wave, there is no
    means of bubble formation, no impressive cavitation phenomena.
    All sound waves, and particularly high frequencies like
    ultrasonic, do not cross an air/liquid or air/solid interface
    well at all, due to severe impedance mismatch. They are
    reflected.  What little energy does cross such interfaces
    diminishes rapidly in intensity."

This is not supported by research; this energy can cause effects,
and the phase of liquid at the surfaces can support cavitation, as
is shown by the action of ultrasonic cavitation produced by a
propeller blade of boats.  The wear was once thought to be formed by
water alone but the action of the propeller and the currents formed
by the turbulence cause a resonance producing ultrasonic radiation
and cavitation in the foamy interface (not continuously liquid) but
still subjected to ultrasonic energy. Andrea Prosperetti has been
investigating such environments for more than 20 years applying
Daniel Bernoulli's principle relating to fluids and surface motions.
Suslick also reports bubble collapse near an extended surface which
can also produce localized, high-speed jets of liquid that impinge
on the surface (Suslick, Science, 2 March 1990:1067.

Michalski also misinterprets the production of hot-spots in
sonoluminescence and in all defines the "wet" or "dry" nature of the
sonic event in an unrealistic fashion. A more realistic perception
and correct interpretation of these events is clearly defined by
Jeffries, Copeland, Suslick and Flint in Science, 10 April 1992:248

The other effect is that on the reagent used. We know that
ultrasonic radiation denatures proteins and thus the application of
consolidants by ultrasound could have such an effect on the film
produced.  The obvious step to take then would be to test films
produced by ultrasonic misters vs other misters, etc.

One expects scientists to operate on the level of fact and not
belief, that when data which appears in the research of other
scientists contradicts their own, the time honored tradition in
science is to attempt to replicate it.  Unfortunately Michalski
decided instead to simply interpret the evidence of my experiments
as not relevant.  In my 1987 review of conservation techniques in
anthropology and archaeology, I showed how materials and methods were
invented and quickly spread into the field for application prior to
experimentation to determine their effectiveness, their durability
and their appropriateness.  Cathrine Sease has contributed several
articles on this subject, especially regarding soluble nylon.  The
main questions which have been of most concern are is the
treatment effective, does it provide as good a visual restoration as
older techniques, does it change visually over time and what is its
reversibility? The other main concern has been only one part of
appropriateness:  is the new treatments effects appropriate from a
curatorial viewpoint and an ethical one.  The other part of an
appropriate treatment is can it be applied by  the average bench
conservator?   The third concern, and one I have been pursuing in
recent years (one example of which I published last fall in Studies
in Conservation) is the durability of the treatment.  How long will
it last and does this justify the treatment in terms of initial
expense and long-term effects on the object?

The introduction of new methods or materials into the field can be
done by bench conservators or by conservation scientists.  Bench
conservators, however, seldom have the resources to test the
long-term effects of their treatments and this should be the task of
the institution scientists.  They serve the community best by acting
as a check . One would hope then, that when conservation scientists
introduce new methods or materials, they could subject their results
to rigorous testing to eliminate any potential problems in
application by conservators in the field. We might expect that their
answers would be complete, allay fears of damage now or in the
future and be readily available.   We might also, expect, that when
contradictory results appear in the literature or are communicated
at meetings or other venues, that these would be investigated in a
spirit of mutual respect and trust.

Niccolo Caldararo
Director and Chief Conservator
Conservation Art Service

                                  ***
                  Conservation DistList Instance 11:96
                   Distributed: Friday, May 29, 1998
                       Message Id: cdl-11-96-009
                                  ***