> So then, perhaps it really was a 'stain' after all
> and not just a flash artifact.
If it is white candle wax then the illuminant (flash) has made a very good job of rendering it as exactly what it is, white candle wax.
Or an unknown white residue.
Anything that appears as white obviously has a higher reflective index and reflects all wavelengths equally than anything that is not white. Which is exactly why it appears as white and we refer to it as white.
Black being identical in that it reflects all wavelengths equally but at a much lower amplitude (intensity) therefore appears as very dark white... Which we call black.
Yes, black and white are the same in terms of spectral reflectance.
Halfway in terms of reflectance (reflected amplitude) is called grey. More specifically 18% grey.
The other localised white hotspots/artifacts are specular highlights which occur when a part of the reflective surface (the wall) that is positioned at exactly the right angle relative to the flash and lens reflect the transmitted light directly back into the camera lens. If you illuminate a surface you will see a hotspot. For example catchlights in a persons eye.
A specular highlight.
In reality there are no white dots in the persons iris.
In the photos the spectral highlights (Hotspots on the wall) are remote from the 'stains'. The 'stains' therefore not being at the optimum position and angle to produce spectral highlights.
The stains appear in the photograph as being white because they are white, High reflective index and reflecting all wavelengths equally.
Something else to consider is orthochromatic or panchromatic photographic emulsion.
Early emulsions were orthochromatic in that they were less sensitive at the red end of the spectrum. Later panchromatic being equally sensitive to all wavelengths.
Orthochromatic emulsions continued to be used in darkrooms (printing), as they are unaffected by red light. "Darkroom Safelight"
Look at a early B&W orthochromatic photo of a red white and blue flag. The red will appear as near black.
A later panchromatic emulsion will render the red as grey.
Airplane's are good because they tend to have red white and blue national insignia. WW1 planes vs. WW2 planes.
Americans were earlier and better at it than Europeans because George Eastman (Mr. Kodak) was an American.
They didn't want to share the technology for commercial and military reasons. (Hollywood/Wars)
Kinda like the wheel ;)
So yes it could very well be a white residue or stain that was once in liquid form as demonstrated by the flow pattern.
I don't know how one would determine the exact chemical nature of the white stain.
An as yet undefined white stain?