Nieuport's engine cowling

[richard.kiss]

I've found an interesting photo about Nieuport on website of US Library of Congress Prints and Photographs Division.
I cropped the photo to show only the engine cover, the subject of my question:


The caption says "LANGLEY FIELD, VA. FRENCH NIEUPORT PLANE, TYPE 17, WITH GNOME ROTARY ENGINE AND CHAUVIERE PROPELLER".

I would be very interested to know why the engine cover is so dotted?
Reference books say that the Nieuport 17 had a spun aluminium engine cover. As far as I know the metal spinning technique produces a quite perfect, continuous, smooth and shiny cone/dome shaped surface -- at least in modern days. See picture below:


In most Nieuport photos I don't see this mottled surface.
Is this an exceptional engine cover made with a different technology?
Or it's a regular outlook of Nieuport's cowling and we can't see the details on other photos due to the poor photo quality?

(The original photo can be viewed in full size here: https://loc.gov/pictures/resource/hec.09326/)

[Dutch522]

Neat picture! I'm no expert, and someone with more knowledge may well have a definitive answer. That said, I've done some research on Nieuport cowlings and this particular example appears to have been treated with what's referred to as "engine turning" or "jewelling" (https://en.m.wikipedia.org/wiki/Engine_turning) after being imported into the US. It was used on various other period metal panels, some Sopwiths had it and it was closely related to the Fokker "sworling" process. Both, I believe, were employed in corrosion prevention, although the actual degree of protection it provided is somewhat unclear (at least to me).

Dutch

[WD]

Dutch pretty much nailed it there IMHO Richard.

Warren