Table 7 Measurements carried out on cubes before and during exposure trial

Photography

Digital colour photographs, of each exposed face

Low tech, low cost; comparison will be visual

Average surface roughness (Ra)

 Diavite DH-6 or similar. Industry standard settings used = Lt 4.80 mm trace (sample length) and Lc 0.8 cut off filter (does not measure wavelengths >0.8 mm)

The profile of the surface is measured using a stylus, magnified through software and quantified as roughness average (Ra) in µm, accurate to 0.01 mm

Average from 10 profiles on each exposed face

Highlights any changes in surface characteristics, e.g. smoothing or roughening

Standard deviation in the Ra measurements can be used to indicate homogeneity of the surface

Colour meter

 Ultra Scan Pro USP1577 Hunter Lab. Mode #3 RSEX or similar

Values for brightness (L*) redness (a+) and yellowness (b+) are taken. Average values are calculated from the five points by the Ultrascan

Colorimetry has successfully been used as a measure of biomass on stone [10, 43]. Visual examination must accompany this assessment

Callipers

 Vernier callipers

Digital Vernier callipers. Measurements taken in three dimensions (width, depth and height). Three measurements taken in each case

Measurements accurate to ± 0.1 mm; quantifiable but of low accuracy

Weight

 Digital laboratory scales (measure to 0.00 g)

Demounted stones (including internally fixed nut) weighed in grams. Stones must be dry before weighing

The requirement for bringing the stones to a standard RH (approx 50–60 %) can delay this method of assessment for several weeks after demounting

3D profile scanning

 Renishaw Cyclone Series 2 SP600M machine or similar. Used Tracecut programme

Profiling done in Z plane in increments of 5–10 mm (i.e. profiles taken at 45, 40, 35, 30, 20 and 10 mm from base). Profiles stored as DXF lines and arcs in CAD

The CAD software will compare profiles over time, quantify change and can produce visual overlays that show the progression of loss