Thursday, 23 February 2012

Hunting for Crystals

A significant part of the project to investigate the Cook-Voyage collections at the Pitt Rivers is to attempt to identify some of the materials used to produce the objects.  Over the years, there have been many informed guesses about what materials are present, but no physical identification has been carried out on the majority of the collections.  Once materials have been identified, it is possible that they may have some characteristic that could help identify similar material in future.  One of these is the presence of crystals in plant cells.  These crystals, or phytoliths, are found in many plant species, and their presence could be one of the features that allows for a positive identification of a sample.  Phytoliths are commonly made from calcium oxalate, but other crystals have been found that contain silica.

This week I took samples of plant material from two objects to Dr. Debra Carr at Cranfield University.  Deb has worked extensively on methods for characterising plant fibres, particularly those used by the Maori (see for example her article 'Approaches for Conservators to the Identification of Plant Material used in Maori Artefacts'  in Studies in Conservation Vol.53 (2008)  The samples I took to her were from Tahitian objects.  One was a piece of aerial root from a type of palm (Freycinetia sp.) that was used to stabilise the structure of the cloak from the Mourner's costume (1886.1.1637 .4).  The other was a piece of hibiscus (Thespesia populnea) fibre from a woven mat (1945.11.130).  Both samples had already been identified, and Deb was interested to see if they contained phytoliths.

The samples were first examined by X-ray diffraction (XRD).  This technique allows for the identification of materials with a regular crystalline structure by focusing a beam of X-rays onto them.  The X-rays are diffracted by the crystals and the diffraction patterns detected and analysed - these are characteristic for different crystals.  The small size and dispersed nature of many of the phytoliths in plant material means that it's hard to detect them using this technique, although it was possible to say that there were crystals in the samples.

The sample of Freycinetia in the XRD machine

Next, X-ray fluorescence (XRF) was used to examine the samples.  Again, the sample is bombarded with X-rays.  The elements within the sample release energy when they are excited by the X-rays, and the amount of energy released is characteristic of each element.  The XRF machine at Cranfield can build up an image of the sample, mapping out where the different elements of interest, such as calcium in this case, are located within the physical structure.  Again, there was some evidence, particularly in the sample of Freycinetia, that calcium was present just under the epidermis of the aerial root, where the presence of crystals might be expected.

Dr. David Lane carrying out XRF analysis
A map of the elemental composition of the hibiscus fibres obtained from the XRF

Finally, we looked at the samples under the electron microscope to see if we could visualise phytoliths within the plant structure.  The microscope uses a beam of electrons to illuminate the sample and to provide the image, and has a much greater resolution that a light microscope.  Although I was surprised at how much of the physical plant structure remained after nearly 250 years, we did not conclusively find crystals in either sample. 

Looking at the hibiscus sample under the SEM

It's possible that with more time we could find phytoliths within the plant material, and this would help us carry out identification in the future.  It's also possible, particularly in the case of the hibiscus mat fibre, that the way the fibre was processed before being woven, by being soaked in seawater for several days, could have removed any evidence of phytoliths.