5.3 Powder X-ray diffraction of the Mary Rose timbers
For the samples of the Mary Rose timbers with the mysterious yellow precipitate, the results of X-ray diffraction (XRD) analyses were compared with peak patterns from suitable standards. International standard databases are available that contain XRD patterns for thousands of crystalline samples, without the need to analyse standards at the same time as samples. Figure 10 shows the results from XRD analyses of the Mary Rose timbers compared with these standards (Wetherall, 2008). Note that the results are illustrated according to 2θ, since this is characteristic of each crystalline structure. (2θ is explained further in Box 2.)
Box 2 2θ and the diffraction of X-rays.
A model for the diffraction of X-rays by crystalline lattice planes was originally put forward by father-and-son team William Henry (1862-1942) and William Lawrence Bragg (1890-1971). They suggested that the interaction of an incident X-ray beam with a lattice plane could be viewed in the same way as light reflecting from the surface of a mirror. This is shown in Figure 11 in which the incident X-ray beam has an angle of incidence of θ (theta), with respect to the lattice plane, and is reflected at the same angle. This is called specular reflection. (Note, the lattice planes are two-dimensional and represented sideways-on in Figure 11 by the dotted lines.) During reflection, there is no change in the wavelength of the X-ray, so there is no energy change: this is called elastic scattering or Thompson scattering. Specular reflection will also apply to X-rays that penetrate to the second and subsequent lattice planes, that are separated by a distance, d. The reflected X-rays from the various layers will either constructively or destructively interfere with one another.
In the case of constructive interference (Figure 12a), the X-ray waves are in-phase with one another, so their displacements add. By contrast, destructive interference (Figure 12b) occurs when the X-ray waves are out of phase and their displacements cancel.
A simple calculation, based on Figure 11, can determine whether constructive or destructive interference occurs at a particular angle (θ). The path difference between the beams that have been labelled 1 and 2 is the length AB + BC and can be written as:
Note: sin is an abbreviation for sine, a mathematical function you will find on your calculator.
For constructive interference (Figure 12a), the path difference must be equal to an integral number of wavelengths, λ, since the waves will then be in-phase:
This is the famous Bragg Law where n represents the order of diffraction. (The strongest diffraction peaks arise from first-order diffraction when n = 1.) Only when this law is satisfied will the detector in the diffractometer give a response. This information is then converted electronically into a series of peaks representing X-ray intensity, angle of incidence and reflection (2θ).
The output from an XRD is a series of peaks representing X-ray intensity as a function of the angle 2θ. The pattern of peaks can be used as a 'fingerprint' for a given crystalline chemical compound so the diffraction pattern generated by an unknown sample can be compared with known patterns stored on a computerised database to identify the sample's composition.
At 2θ = 35° in Figure 10, there is a peak of high intensity from the Mary Rose precipitate. This corresponds with a peak characteristic of the iron oxide magnetite (Fe3O4) standard. A low intensity peak at 2θ = 29.5° also corresponds with a magnetite peak, so it is fairly certain that magnetite is present. Similarly, at 2θ = 28.5°, there is a low intensity peak, which corresponds with a peak from the calcite (CaCO3) standard. Unfortunately, other calcite peaks cannot be seen on the trace, so calcite may be present. At 2θ = 27.5°, a low intensity peak corresponds with one from the jarosite (KFe3[(OH)3SO4]2) standard but no other jarosite peaks can be seen in the Mary Rose precipitate, so it may be present.
The presence of magnetite was not surprising because of the oxidation of iron bolts in the timbers. However, the possibility of minerals rich in sulfur, such as jarosite, was intriguing since, in order for jarosite to form, it required a source of sulfur. It also required a source of oxygen since jarosite is produced from the oxidation of sulfur, so it must have grown once the ship was salvaged.
Certain types of anaerobic bacteria thrive by consuming decaying organic matter (which includes S-containing molecules) on the seabed, to produce hydrogen sulfide (H2S) as a by-product. This H2S can diffuse into wood and be converted by other bacteria into sulfur. In aerobic conditions, such as those that prevailed after the Mary Rose was salvaged, oxidation of sulfur to precipitates was accompanied by conversion of sulfur into sulfuric acid (H2SO4). This can easily degrade the wood's cellulose in a process called acid hydrolysis.
Estimates suggest there was over two tonnes of sulfur present in the Mary Rose timbers, which was not good news for the ship's conservation.
In 1994, the cooled water sprayed on the ship was replaced with a mixture of water and a wax called polyethylene glycol (PEG) which would pervade the wood and solidify, thus sealing it from further decay by bacteria or acid and preserving its structural integrity. PEG is a polymer composed of ether monomers (CH2CH2O), with an alcohol (-OH) terminal group at one end and a hydrogen at the other (Figure 13); the general formula for PEG is H(CH2CH2O)nOH, where n is the number of monomer units and can be up to 500. Thus, PEG binds, by hydrogen bonding, to the wood's organic molecules in the same way as water. Over time, the number of monomer units in the PEG sprayed onto the Mary Rose's timbers has been increased, meaning more PEG and less water becomes bound. This PEG-preservation technique was halted to allow the ship to be sufficiently dried out to be displayed in an accessible public gallery from 2012.
The Mary Rose and its artefacts continue to be analysed, enabling its future preservation and further investigations into Tudor life and the processes that operate on the seabed. Similar investigations of other ships, such as the Vasa, a 17th century Swedish warship, have benefited from the development of analytical techniques established and applied to the Mary Rose. However, further analysis of her intrinsic structure will be limited because of the damage this may do to her, particularly when she is on display to the public. For this, non-invasive techniques will need to be applied, similar to those used to analyse works of art.