2007:140: Millette, Turner, Hill
Microscope, Vol 55:1 3-7
“Ghosting” is a term used to describe the dark areas that form for no apparent reason on white or light-colored walls and ceilings. This phenomenon is also referred to as “black soot deposition” (BSD) although the darkening agent is not always soot. One cause of ghosting is thought to be thermophoresis, the movement of particles from a higher temperature region to a lower temperature region. This is what causes soot particles to accumulate on the inside of a chimney (1). Very tiny particles in the air next to a wall or ceiling are attracted to cooler surfaces. Structural building materials (studs, beams and joists) can form a “thermal bridge” and the outlines of the support members can be seen due to the accumulation of tiny particles at the cold spots (2). One theory is that the support materials provide a localized cool spot where a very small amount of moisture condenses. The particles then collect on the condensation producing the grayish shadows. Thermophoresis is sensitive to particle size distribution and only the smallest particles diffuse according to this mechanism.
This article describes the investigation and microscopical analysis results of samples collected with two different sampling media from a residential ceiling that showed dark material generally in lines on the ceiling (Figures 1 and 2). Thermal imaging was used on the ceiling to determine temperature differentials. Particulate samples for microscopical examination were collected of the dark surface areas and sent to the laboratory for analysis.
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