Saturday, 25 December 2010

The Colour of RLM 02 Grau (Grey)

This may seem somewhat OT for this blog but as part of an ongoing investigation into early Zero colours a series of photospectrometric measurements of samples of the German RLM 02 colour Grau (Grey), sometimes referred to as Grungrau (Green-grey), were recorded from various sources together with difference-calculated comparisons in  Munsell, FS595B and RAL Standards. The results are presented here in the accompanying schematics as a resource, showing the original colour in sRGB format calculated from the the measured L*a*b* values and their closest Munsell, FS595B and RAL equivalents. Difference calculations of DE2000  less than 2.0 = a close match but for completeness I have included those closest Munsell, FS and RAL values even where they exceed 2.0. Also for completeness a table showing the measured L*a*b* and sRGB values from which the chips have been rendered is included here. Two chips are included in the Monogram publication and measurements taken were slightly different so again they have both been included for completeness.

The measurements were taken from the relevant standard and reference source paint chips - not from examples of applied paint - so please bear in mind that distinction which is always emphasised here. When some people enthuse about the paint colour varying - and it did to the extent that the Luftwaffe told their personnel not to worry about it - they can often infer that paint colour standards are useless for modelling purposes. This is to misunderstand the purpose and value of the standards. Rather than pinpointing how each and every aircraft might have been painted they provide a benchmark for the intended average colour to be visualised, which in the absence of documented and accessible samples of extant paint, might otherwise be in a vacuum Even with extant samples of applied paint many authoritative researchers and authors still have to rely on colour photographs, subjective visual comparisons and/or descriptions in order to communicate the colours. Standards can be used to triangulate this information. Again, it may be worth emphasising that the purpose of this blog is not to promote an agenda but to explore and discuss subjects in order to provide a resource by which informed decisions might be made. It is not in competition with anyone and challenges to statements in other forums as being inconsistent with the evidence should not be misconstrued as such. Besides, it's your model, painting or restored aeroplane, not mine!

One extant formula for this paint as a "Rubber Varnish gray N 243" is included in Jerry Crandall's 'Focke-Wulf Fw 190 Dora' Volume 2. It is given as Titanium Oxide and Zinc whites with three yellow pigments Hansa Yellow, Zinc Yellow and Yellow 420 N and, apparently, what may be RLM 66 dark grey (?). If the designation does not mean the latter then the formula appears to be missing a black or grey pigment.

Another RLM 02 formula comes from Herbig Haarhaus A.G. of Koln in their trademark 'Flieglack' series of fire-resistant lacquers. This is for Aviation Lacquer 7115.02 in their Herbeloid series. No less than 73.03 parts of the formula made up a proprietary lacquer binder/solvent consisting of 'vinoflex', butanol, butylacetate, toluol, spirit and ethyl acetate to which the following pigments were added:-

Antimony white (Timonox RS) - 2.94 parts
Chrome green - 0.08 parts
Titanium dioxide (white) - 2.15 parts
Chrome yellow - 0.18 parts
Sechsbrandruss - 0.02 parts
Aluminium bronze - 0.59 parts
Essigather (ethyl acetate) - 16.44 parts
Black paste - 3.47 parts
Red paste - 1.10 parts

Interestingly, ethyl acetate appears twice in the formula, as 17.60 parts of the binder/solvent and in the proprietary 'Essigather' with the pigment as 16.44 parts. The lacquer was intended to be spray-applied with equal parts of thinner. 'Vinoflex' was a polyvinyl chloride made by I.G. Bitterfield. 'Sechsbrandruss' was a proprietary six times refined carbon black pigment. 'Timonox RS' was a proprietary fire-resistant form of lead white. Chrome green is correctly a green mixed from chrome yellow and Prussian blue but was also used, incorrectly, as a generic term for chromium oxide (green). Aluminium bronze was a leafing pigment usually added to improve impermeability to moisture and thereby resist corrosion and gave the paint its earlier description of 'silver-matt grey'. The formula is more complex than expected and the type of pigments used for the black and red pastes unknown.

Note the differing extent of yellow pigmentation in the two formulae which gave the colour its distinct appearance. RLM 02 hobby paints which present an entirely neutral grey or cool grey-green do not capture this slightly "warm" yellowish character. The schematic below shows the measured comparison of three hobby paints to the Kiroff RLM 02 sample. Two of the paints are lighter (as one might expect) but greyer from the introduction of a higher proportion of white pigments

As can be seen, a consistent Munsell equivalent value of 7.5 Y 5/1 has been calculated in this analysis, but although very close it still does not quite convey the true yellowish caste of the original colour. There are no usefully close FS595B comparisons; the closest, 34201, is too yellow. Two RAL values, 7002 Olive grey and 7003 Moss grey, offer useful comparisons, the former probably providing a slightly better average visual comparison than the latter which strays towards a cooler grey-green in character. But the comparisons probably highlight a variance present in the original paint, sometimes warmer sometimes cooler. RAL 7003 is available in hobby paint as Revell 45 'Light Olive' but has not been tested against the samples.

Please bear in mind that all the chips are probably slightly darker than they would have appeared 70 years ago. The type of yellow pigment in use probably ensured that thermally aged browning was minimal but the paint as applied probably appeared slightly lighter and almost certainly brighter.

Image credits: Rendered colour chips and table © Straggler 2010

1 comment:

Ken Glass said...

Hello Nick,

Thanks for this explication. It would seem the LifeColor RLM 02 is the most accurate out there, 'as is' straight from the bottle.

Ken Glass