The next level of investigation consists
of probing beneath surfaces using non-destructive methods. Questions
derived from the surface mapping examination and analysis will
help determine which areas to probe. Investigators have perfected
a number of tools and techniques which provide minimal damage
to historic fabric. These include x-rays to penetrate surfaces
in order to see nail types and joining details; boroscopes, fiber
optics and small auto mechanic or dentists' mirrors to look inside
of tight spaces; and ultra violet or infra-red lights to observe
differences in materials and finishes. The most advanced technology
combines the boroscope with video cameras using fiber optic illumination.
In addition to the more common use of infra-red photography, similar
non-destructive techniques used in archeological investigations
include remote sensing and ground-penetrating radar.
Small material samples of wood, plaster,
mortar, or paint can also be taken for laboratory analysis at
this stage of investigation. For instance, a surface examination
of a plaster wall using a raking light may show clear evidence
of patching which corresponds to a shelf design. Were the shelves
original or a later addition? A small sample of plaster from the
patched area is analyzed in the laboratory and matches plaster
already dated to a third period of construction. A probe further
reveals an absence of first period plaster on the wall underneath.
The investigator might conclude from this evidence that the shelves
were an original feature and that the plaster fill dates their
removal and patching to a third period of construction.