GR translation: night-dusted

[alice wellintown]

>> P117.39-40  ..., glassy ambrotypes of her late husband Austin night-dusted
>> inside gilded frames up on the mantel ...
>>
>> What is "night-dusted"?
>
> slightly grayish resulting from the way ambrotypes are made (I think).
>
> Night doesn't really have dust, but making an analogy with snow dust, which
> leaves things with a whitened aspect, night dust leaves things a bit
> grayish.
>
> That's the picture I get.

Another film/photograph detail on the variations in black and white.
And, the flat is, could we say, a studio, the portrait a prop?