X-ray fluorescence (XRF) spectroscopy
XRF can help identify heavier elements indicative of pigments; impurities in and sizing on paper; specific alloys of metals, metal mounts on furniture, and metallic leaf; and tannages and dyes in leather, just to name a few applications. This is a non-destructive technique and the instrument that we use at West Dean College is portable.
Abby's recent article in the Icon Journal of Conservation details the use of XRF to identify iron as the cause of selective discoloration in two 17th century codices.
Fourier-Transform infrared (FTIR) spectroscopy
FTIR is useful for identifying unknown organic materials (we often use it for adhesives) as well as for comparing between two similar organics to find differences. Here the client wanted to know the difference between discoloured and non-discoloured paper as well as some that had been washed and some that had not yet been washed, to determine whether washing would improve the condition or not. The technique may be destructive or nondestructive.
A spectrophotometer can be used both to define an individual color and also to quantify color change, such as before and after treatment or exhibition. It takes a (nondestructive) reading of the surface and compares it to the ambient light in the room and to a standard white tile in order to give a value in the L*a*b* system.
For microscopy, we take a tiny sample of a decorative surface (eg. paint or varnish) and examine it in cross-section. Under the microscope, we can see the individual layers of paint or varnish; the technique can be useful for determining the original surface coating(s) and any later additions. This is a destructive technique but the sample can be quite small.