Table 1.
The classification of human skin types pertaining to responses to UVR exposure [70–75].
Constitutive values in brackets refer to the Von Luschan chromatic skin colour classification scale [76]. ‘Exposure’ refers to the amount of time during which UVR exposure would not induce perceptible damage to the skin. RSF is a factor characterizing the protective effect of a sunscreen against the generation of free radicals.
Fig 1.
The location of the Kunene Region (red shaded area) in north-western Namibia (sourced from https://en.wikipedia.org/wiki/Template:Location_map_Namibia/doc).
Fig 2.
The production and application of otjise.
(A) Red ochre powder is obtained by grinding chunks (inset—scale bar is 1cm) between a round upper and a flat lower grindstone, (B) after which it is mixed (at a 1:1 ratio between the palms) with milk-derived clarified butter and (C) applied to the hair, body and ornaments.
Table 2.
Experimental ochre samples subjected to SPF, ED-XRF, XRD, granulometric, colorimetric and UVR analyses.
Table 3.
L*a*b* colourimetric properties of ethnographic and experimental ochre samples with L* representing lightness (100 white / 0 black), a* indicating variations between red (+) and green (-), and b* representing variations between yellow (+) and blue components (-).
The first sample was arbitrarily selected as the reference sample and the Delta E (ΔE) values calculated accordingly.
Fig 3.
Representative grain size distributions for each group.
Table 4.
Results for particle size analyses of ochre samples.
Fig 4.
XRD spectra recorded for ochre samples 5 (Ovinjange fine ground ochre powder), 15 (Napier medium hard red shale chunk) and 19 (Napier soft yellow limonite chunk).
Table 5.
Results of XRD analyses of ethnographic and experimental ochre samples.
Table 6.
Results of ED-XRF analyses of ethnographic and experimental ochre samples.
Table 7.
In vivo SPF and in vitro UVAPF values and critical wavelengths (CW) of ethnographic and experimental ochre powder samples.
Table 8.
SPF ratings of ochre samples 1, 5, 7, 10, 16 and 18, mixed with clarified butter, and obtained by in vivo assessment on three test subjects (A, B and C).
Table 9.
Pearson’s correlation table indicating the relationship between SPF values on the one hand and the granulometric, mineralogical and elemental composition of the pigment samples on the other hand.
Data in bold emphasize prominent positive or negative correlations.
Fig 5.
Ternary diagrams and biplots illustrating the relationship between SPF and grain size composition (A, B), mineral content (C, D) and elemental composition (E, F).
Fig 6.
Ternary diagram (A) and biplot (B) illustrating the relationship between SPF values and ochre colour.