20 Apr Material Analysis
Spencer Hall ACR
Analysis can be an incredibly useful tool when trying to identify existing materials with a mind to undertake like for like repairs or as a way to understand why substrates are behaving in a certain manner. There are a number of differing types of analysis open to the conservator, with certain situations and needs lending themselves to very specific approaches… Analysis can be useful for reviewing a number of substrates including, but not restricted to; mortars (in both lime based and cementicious forms), painted surfaces, stone, metals and efflorescence. The list below tries to shed some light on some of the complex alternatives, allowing the individual to decide which is most appropriate to their project needs.
Completed using photography and/or visual inspection on site. This requires no intrusive sampling and allows consideration in context, especially with reference to performance and decay. Restrictions relate to the fact that this is wholly interpretive and so human error can occurs.
Chemical Analysis (wet chemistry)
The constituents of a mortar can be determined using various chemical tests after dissolution in acid. The downside to this is that the binder is in effect dissolved and so it is down to the skill and experience of the analyst to ‘interpret’ the results. It IS possible however to note key characteristics of the properties created as a result of this ‘loss’ and so this is often the standard method of conducting initial ‘scientific’ analysis.
Petrographic Analysis by Optical Microscopy
The detailed analysis of minerals by optical mineralogy in thin section. This method is generally referred to as “polarized light microscopy” (PLM). Polarized light microscopy of stone and ceramics, is a crucial tool for the study of ancient and historic objects and building materials. The technique is used to identify materials and their possible sources, understand production technology and object function. It can study deterioration mechanisms and inform preservation strategies and conservation treatments. Specialised expertise is however required to use this technique effectively.
Spectroscopic Techniques – X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF)
X-Ray diffraction (XRD) is a technique that is really only appropriately applied to pure amorphous or crystalline substances and allows the user to learn a great deal about the structure of the compound.
1. An XRay beam is fired at a sample over a series of angles keeping the source and target (sample) at a consistent distance (called a goniometer).
2. The reflected or diffracted X-rays provide a 2 theta angle and as such Bragg’s Law (nλ= 2d SinФ) can be solved. We then use the displacing values to define the h, k, and l indices of the unit cell.
3. Using the three indices, it is then possible to calculate the dimensions of the units cell.
X-ray Fluorescence (XRF) is a technique which has broad application to mineralogy and petrology as multiple elements/oxides can be analysed at the same time
1. High voltage accelerates electrons toward a metal target to produce a specific fixed wavelength X-ray beam that hits the sample
2. Inner shell electrons in the sample are ejected and photons are emitted as outer shell electrons drop to inner orbitals.
3. Photons have characteristic energies and elemental concentrations can be calculated with this technique by comparing sample intensities to a known standard. XRF can analyse most elements heavier than oxygen.
Spectroscopic Techniques – Electron Probe Micro Analysis (EPMA) and Scanning Electron Microscopy (SEM)
Electron Probe Micro Analysis (EPMA) consists of a beam of electrons generated and sent from the electron gun through a series of lenses and apertures toward the target sample.
1. The diameter and strength of the beam can be modified based on the current and accelerating voltage used to generate the beam.
2. Electrons in the specimen are excited and then, as electrons relax and fall back from an excited to a ground state, will fluoresce and generate X‐rays
3. X‐rays generated from the beam‐specimen interaction are characterized by energy and wavelength to identify the element. Intensity is used to analyse concentration
4. Energy Dispersive Spectrometers (EDS): counts the number and energy of emitted X‐ray photons B) Wavelength Dispersive Spectrometers (WDS): use specifically tuned crystals to pick‐up particular wavelengths
Scanning Electron Microscopy (SEM)
1. Very similar to EPMA, in that SEM uses an electron gun to generate and fire electrons in a beam at a sample.
2. SEM is generally considered only semiquantitative, but can produce incredible images using both secondary electron scatter or back scattered electrons (BSE).
a) Secondary electron images can reveal a great deal about the topography of a sample.
b) BSE reveals contrastsbased on average atomic number, such that images will reveal higher rates of backscatter based on the relative density of atoms composing the material. 3. EDS spectrometers are most commonly used on SEM equipment while microprobes are often used with both EDS and WDS.
Fourier transform infrared spectroscopy (FTIR)
FTIR is a technique which is used to obtain an infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer which measures intensity over a narrow range of wavelengths at a time.
In conclusion; for most material analysis (mortars specifically), a combination of visual interpretation and (wet) chemical analysis should normally be sufficient. Where greater information is required, defer to analysis laboratory and discuss specific requirements with them as they will normally recommend what is most suitable and what will give the answers to the questions posed.