On Jun 23, 7:09 am, Nick wrote:
Hi,

Got up bright and early this morning to find some fur/fluff on the
surface of some plastering that was done earlier in the week for a
hole in the wall fire. I've no idea what it is (hopefully not mould!),
and would welcome any advice as I've never seen this on any other
plastering.

There was already a large opening for an existing fireplace, so that
was bricked up where necessary and the brickwork plastered. The
plaster on the new brickwork has lightened in colour as it should and
seems fine. Some of the existing brickwork around the hole was also
exposed and needed to be filled, and it's the plaster onto the
existing brickwork that hasn't lightened in colour in the same way and
that now has the white fluff. The depth from wall surface to existing
brickwork was a good 10 to 20 mm at least, and I'm not sure whether
the fitter used plasterboard to pack out the bulk of this depth before
plastering or whether it's filled totally with plaster. I suspect the
latter, and given the depth, I could imagine that this may not have
dried yet hence not going light in colour.

Any ideas on what the fluff is are most welcome, and also whether
anything needs to be done about it other than getting it off somehow.

Cheers
Nick

do a google search using the words "Efflorescence" and "plaster" and
see what you get....

http://www.oldhouseweb.com/stories/Detailed/723.shtml

this is the most likely explaination.
cheers
www.diypaint.co.uk