Loading ...
Sorry, an error occurred while loading the content.

Re: thermoluminiscence laboratory

Expand Messages
  • tribalarts
    Many thanks to all those who responded. In a few days I will post images of the object in question for comments, along with other pottery figures from Nigeria
    Message 1 of 6 , Nov 28, 2007
    • 0 Attachment
      Many thanks to all those who responded. In a few days I will post
      images of the object in question for comments, along with other
      pottery figures from Nigeria I have acquired over the years. As a
      new member to the group, I am appreciative of the valuable
      information that has been shared, and in particular to Mr. Rasmussen.

      Dave Liebl

      --- In African_Arts@yahoogroups.com, "Mark Rasmussen" <mark@...>
      > Hello Dave,
      > I have posted a new "group shot" of 47 CT images of terracotta
      objects on my
      > web site (www.rare-collections.com <http://www.rare-
      collections.com/> ). Of
      > the group, 75% were pastiche (fake or otherwise so altered.) - all
      had TL
      > certificates from various labs.
      > The following is a copyrighted excerpt from my contribution to an
      > book (the proceeding from http://www.rare-
      > on authentication techniques that speaks to some of the issues:
      > Thermoluminescence Dating
      > TL dating is an important (and often misunderstood) scientific
      > that is applicable to such materials as pottery (baked clay
      > porcelain, burnt stone, burnt flint, and volcanic products. The
      > principle is that certain minerals (mainly quartz, feldspar, and
      > essentially "record the passage of time" through the cumulative
      effect of
      > prolonged exposure to the weak flux of nuclear radiation emitted by
      > radioactivity in the surrounding burial soil and within the object
      > (there can also be a small contribution from cosmic radiation).
      This "clock"
      > is reset to zero during the firing process (if sufficiently heated)
      and will
      > subsequently be indicative of the time since the last
      firing/heating. TL
      > testing can also be used to date casting cores (e.g., inside Benin
      > as well as fired objects that may not have not been buried, and the
      > and limitations of each context must be fully understood. TL
      testing is
      > performed by having a qualified conservator (trained in TL sample
      > collect (typically by drilling) an appropriate number of samples
      from the
      > object and submitting them to a TL laboratory for testing. Making
      sure that
      > the samples are representative of the object as a whole and that
      they are
      > not contaminated is critical to the process. Forgers are aware of
      TL testing
      > procedures and often place ancient/authentic fragments where they
      > sample-takers will be most likely to take samples. CT scanning when
      > with TL testing is considered the most thorough and accurate method
      > evaluating valid TL sampling locations.
      > Specific TL Testing Issues
      > Many supposedly ancient ceramic objects have been determined to be
      > pastiche objects constructed from age-appropriate/authentic
      fragments (often
      > unrelated) combined with various fill materials. This fill material
      > frequently comprised of age-appropriate/authentic material that has
      > ground up and mixed with binders/adhesives in an attempt to
      simulate the
      > original ceramic. The rationale for using ground up
      > age-appropriate/authentic material is that it will create a similar
      look and
      > may still pass a TL test if the laboratory does not screen the
      samples for
      > binders/adhesives. This issue is further complicated by the fact
      > samples that contain binders/adhesives are not necessarily
      problematic -- it
      > is common to find objects that have been "stabilized" with
      > during the normal course of conservation. Interpreting the intent
      > context is the real issue.
      > Some TL reports mention the fact that adhesives/binders are present
      > still provide a date for the samples and leave it up to the reader
      to guess
      > at the intent, context, and meaning. In some instances, the report
      > mention that this is normal/routine and may further minimize the
      > Objects with this type of notation on the TL report always require
      > evaluation and testing. The next major challenge is that
      > age-appropriate/authentic materials are often strategically placed
      > objects) where the TL sample-taker is most likely to take samples.
      There are
      > many potential issues associated with TL testing. Specifically:
      > . TL samples are rarely taken from important locations (such as the
      face) or
      > from highly decorative areas in order to avoid potential damage.
      > locations are often the areas that contribute significantly to the
      > (monetarily/historically) of the object. Restoration to these areas
      > common -- many featureless or highly damaged objects are enhanced
      > although they may be "real" in a general sense, the original
      aesthetic has
      > been changed. The repair of a TL hole is typically a very minor
      procedure by
      > a qualified Conservator and unwarranted concerns should not
      > appropriate testing.
      > . Often the surfaces of objects are obscured by clay/slip/mud
      (which may be
      > mixed with binders/adhesives) or even completely painted. This
      issue can
      > make site selection for TL sampling very difficult and often leads
      > results that are not representative of the object as a whole.
      > . Some TL labs charge according to the number of samples to be
      tested and
      > have an additional charge to screen samples for binders/adhesives.
      > often results in an inadequate number of samples being taken.
      Samples should
      > be obtained and thoroughly screened (for binders/adhesives) from
      > significant portion of the object -- especially from areas that
      > significantly to its value (monetarily/historically). This sampling
      > be undertaken only by a fully qualified Conservator and must be
      > in light of other techniques such as CT. This creates a bit of a
      > while CT can illustrate the exact condition and composition of the
      > and can serve as a guide in taking accurate TL samples, it is not
      > performed first. While it is true that CT can contribute to the
      > dose of the object, testing has demonstrated that it does not
      normally alter
      > the TL dates produced owing to the low levels of radiation used and
      > relatively low precision of TL. The minimal risk associated with
      > radiation exposure can be further mitigated by "reserving" samples
      prior to
      > CT scanning and including an appropriate dosimeter with the object
      > the CT scan, then providing a record of the radiation dosage to the
      TL lab
      > with the samples. This procedure must be done with the prior
      > cooperation, and guidance of the testing laboratory and its
      limitations must
      > be fully understood.
      > . Most TL labs do not routinely screen samples using Fourier
      > infrared (FTIR) spectroscopy or other sophisticated screening
      methods to
      > check for binders/adhesives. They typically must subcontract (at the
      > client's request) for this analysis, and additional charges and
      > requirements are necessary. Some contaminants may reveal themselves
      by their
      > effects on the TL glow-curve, but not all are so easily detected.
      This can
      > create a further problem in that some TL labs rely solely on the
      presence of
      > residue after evaporating the acetone used during sample
      preparation to
      > detect binders/adhesives. This method may only detect
      binders/adhesives that
      > are immiscible with acetone or have discernable amounts of solids.
      > this technique may be useful when residue is actually found, it is
      > considered inadequate. In the event that binders/adhesives are
      detected, the
      > challenge of interpreting their presence/context remains.
      > . One further drawback of TL testing is that it cannot reveal
      whether the
      > features of an object (or the entire object) have been recently
      carved from
      > age-appropriate/authentic material. Featureless and highly damaged
      > are sometimes enhanced in this way -- a common example of this
      (seen in
      > altered/fake Nok figures) is the carving of new eyes into a highly
      > face. The addition of incised and enhancing elements is a further
      > Another example is when decorative elements made from
      > age-appropriate/authentic material are added to the object. This is
      > generally accompanied by a unifying wash of clay/slip/mud over the
      > surface to obscure the issue. Again, these elements/objects may TL
      > correctly but are inauthentic.
      > . One final area of fraud that can cause TL to report misleading
      results is
      > artificial irradiation. In principle, the apparent TL age can be
      > by exposing the object to a calibrated dose of ionizing radiation.
      > problem of artificial irradiation is sometimes detected by suspect
      > glow-curves or anomalous dates although independent testing has
      proven that
      > it can go undetected. With that being said, the artificial
      irradiation issue
      > may be more prevalent than we currently appreciate. Fake (modern)
      > bronzes/brasses that have had their casting cores artificially
      > have been seen to produce seemingly ancient and valid TL dates.
      There has
      > been research into methods that have the potential for detecting
      > irradiation (the zircon technique, etc.) but routine TL testing
      does not
      > typically include them.
      > Additional TL Testing Issues
      > Further problems with TL include the frequency of forged or altered
      TL lab
      > reports. Although some alterations/issues concerning these
      documents are
      > easy to detect, some require forensic document examination
      techniques to
      > discover. It is always advisable to secure a certified copy of the
      > TL report (with its original photograph) from the issuing lab.
      Verifying the
      > photograph (with the report) is an important element, as the
      > mismatching of photos and reports is a known fraud technique.
      > complication of TL is that it sometimes produces a date that
      indicates age
      > but does not fit into generally accepted or statistically relevant
      norms for
      > the material. In these cases, other techniques should be employed
      to further
      > evaluate the object -- no one should ever simply rely on the fact
      that it
      > appears old.
      > All TL labs are not created equal. It is very important to work
      with a lab
      > that that is recognized for its reputation and experience. Some TL
      labs may
      > not have the skills necessary to fully evaluate samples and deal
      with the
      > complex technical issues all too common with potentially
      > objects. Further, labs that allow unqualified sample-takers to
      > samples or labs that do not have controls in place to deal with
      fraud, chain
      > of custody, and quality control issues must be avoided.
      > Thank you,
      > Mark Rasmussen - Rare Collections
      > 5865 Neal Avenue North, Suite 345
      > Stillwater, MN 55082 USA
      > 612-961-4747 (Cell)
      > mark@...
      > www.rare-collections.com
      > Notice: The information in this email is confidential and may be
      > privileged. It is intended solely for the addressee. Access to this
      email by
      > anyone else is unauthorized. If you are not the intended recipient,
      > disclosure, copying, distribution or any action taken or omitted to
      be taken
      > in reliance on it, is prohibited and may be unlawful. Information
      > in this email may be subject to the terms and conditions of a
      > Agreement. If you received this message in error, or do not agree
      with its
      > terms, delete it and all of its contents from your system(s).
      > _____
      > From: African_Arts@yahoogroups.com
      [mailto:African_Arts@yahoogroups.com] On
      > Behalf Of Alexander Bortolot
      > Sent: Monday, November 26, 2007 9:44 PM
      > To: African_Arts@yahoogroups.com
      > Subject: Re: [African_Arts] thermoluminiscence laboratory
      > Lee has done a wonderfully thorough job assembling information on
      > topic, but since Bortolot Daybreak has been mentioned without a web
      > here it is:
      > http://www.daybreak
      > <http://www.daybreaknuclear.us/bortolot_daybreak_frameset.html>
      > nuclear.us/bortolot_daybreak_frameset.html
      > [Full disclosure: I'm related to Daybreak's lab director, Victor
      > Any responsible TL lab will tell you that TL test data indicating
      > authenticity is merely one kind of evidence that must be combined
      > information derived from other approaches. The Daybreak site
      provides a lot
      > of useful information on TL and other authentication methods, as
      well as
      > links that you may find helpful now or in the future.
      > And yes, Daybreak is the only commercial TL lab in the US. Which in
      > practical terms means: don't expect immediate results!
      > Cheers,
      > Alex
      > Moderator's Note: Thanks, Alex, for including the link; I omitted
      > web-site link because I found it not to be operative -- earlier --
      at least
      > on my browser. Now it seems to be working fine. Thanks, Lee
      > ----- Original Message ----
      > From: Lee Rubinstein <LeeRubinstein@...>
      > To: African_Arts@yahoogroups.com
      > Sent: Monday, November 26, 2007 6:08:57 PM
      > Subject: Re: [African_Arts] thermoluminiscence laboratory
      > Dave:
      > For the purposes of visual comparison and to view on-line some
      > tested examples of Nok/Sokoto/Katsina terra cotta -- as well as
      > from other African traditions, see http://memoiredafri
      > <http://memoiredafrique.com/en/nok/galerie-amis.php> que.com/en/
      > nok/galerie- amis.php.
      > Other on-line resources include these articles and web-sites,
      including that
      > of Oxford Authentication* -- which Mark previously mentioned -- as
      well as
      > Daybreak Nuclear**, which may be the only (???) US-based laboratory
      > providing thermo-luminescence testing for African cultural
      > Articles:
      > *A power-point presentation on "TL Testing":
      > http://www.nd. edu/~nsl/ Lectures/ phys178/pdf/ chap3_11. pdf
      > <http://www.nd.edu/~nsl/Lectures/phys178/pdf/chap3_11.pdf>
      > A 2003 article by NMAfA conservators Stephen Mellor and Dana
      > http://www.thefreel ibrary.com/ The+curator- conservator+
      > <http://www.thefreelibrary.com/The+curator-
      > ering+Roy+Sieber-a0111847724> collaboration: +remembering+
      > a0111847724
      > "Poking Holes in Beautiful Art, and Getting Paid for It, Too" (NYT,
      > 11, 2004) regarding the work of Bortolot Daybreak**
      > 3-3299> Laboratories in Guilford, CT:
      > http://query. nytimes.com/ gst/fullpage. html?res=
      > <http://query.nytimes.com/gst/fullpage.html?
      > 9C8B63&n=Top/Reference/Times%2520Topics/Subjects/A/Art%
      > 9D02E2DE1538F932 A25757C0A9629C8B 63&n=Top/Reference/ Times%
      > Subjects/ A/Art%20Restitut ion
      > A relevant 2001 article by Michael Brent from the archaeology. org
      > http://www.archaeol ogy.org/0101/ abstracts/ africa.html
      > <http://www.archaeology.org/0101/abstracts/africa.html>
      > Laboratories:
      > The Museo d'Arte e Scienza in Milano:
      > http://www.authenti cafricanbronzesa ndceramics. com/
      > <http://www.authenticafricanbronzesandceramics.com/>
      > Marc Ghyssels:
      > http://www.marc- <http://www.marc-ghysels.com/> ghysels.com/
      > Labor Ralf Kotalla:
      > http://www.kotalla. de/englisch/ indexhaupt_ e.htm
      > <http://www.kotalla.de/englisch/indexhaupt_e.htm>
      > Oxford Authentication:
      > http://www.oxfordau <http://www.oxfordauthentication.com/>
      > com/*
      > ASA Laboratory of Archaeometry:
      > http://www.asa- <http://www.asa-art.de/> art.de/
      > Additionally, for a broader consideration of the commerce in
      > antiquities, you may also enjoy reading the article, "Stealing
      History: The
      > Illicit Trade in Cultural Material" by Neil Brodie, Jenny Doole and
      > Watson -- (commissioned by ICOM and available on-line at the site
      of the
      > McDonald Institute for Archaeological Research) at
      > .cam.ac.uk/ projects/ iarc/research/
      > illicit_trade. pdf.
      > Lee
      > _____
      > Be a better pen pal. Text or chat with friends inside Yahoo! Mail.
      > <http://us.rd.yahoo.com/evt=51732/*http:/overview.mail.yahoo.com/>
    Your message has been successfully submitted and would be delivered to recipients shortly.