Pratchett does make mention, (on page 144 of my hardback anyhow) when Vimes and Sybil try to stand in front of the Troll's head on the embassy wall that:
"'... It's colder up here, Vimes thought. He's quicker on the uptake.* Even Nobby won't play poker with him in the winter. Damn!'...

* Detritus's silicon-based brain ... etc"
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I agree with Hsing about the helmet and wanted to say that after Detritus got the helmet I don't think, in the general text, Pratchett make so much mention of his low intelligence unless he makes a specific point that is required for the storyline ... but i can't really back that point up without going through every book. I just remembered the quote above as it is a nice scene.

So yes after all that I don't really know (anything at all) either.

R.