Gloss black piano finish only became popular in Britain in the late
19th century with the influx of instruments and concert pianists from
Germany, where respectable, formal black was admired.  Before this even
platform concert grands were here usually in rosewood, walnut or
mahogany.

Until WW2 spray finishing of piano cases and plates was virtually
unknown in Britain, and everything was done by hand.  Nevertheless
British makers produced immaculate gloss black when required.

As other contributors have pointed out, for gloss black especially, the
surface to be finished must be perfect.  This was traditionally achieved
here by a generous layer of "stopping", the gesso-like spackle, and
lots of elbow grease.  Sometimes the wood was stained black initially,
but the stopping was stained black after drying, followed by a final
smoothing with finest "crocus paper".  Note that a perfect finish does
not mean mechanically flat: there's an almost imperceptible ripple and
shimmer to hand finishes that elevates them above machine-made
furniture.

At the finest cabinet makers spirit stain has always been disliked and
water stain invariably used because the final colour can be closely
controlled, especially with fine veneers and light finishes such as
birdseye maple and satinwood.  This applied equally to black finishes
-- there's an infinite number of blacks!  But the basic ingredient was
always carbon black, and I think it was this, rather than aniline
spirit black, which allowed the immaculate see-through "jet" finish so
unlike the ghastly black polyester skin of today.

Of course the deep gloss was produced perfectly conventionally by hand
French polishing; again, lots of coats of purest shellac and lots of
rubbing.  Before polishing out the final coat the best makers sometimes
"burned" it with hydrochloric acid to just the right degree, stopping
the reaction with sprinkled whiting and rubbing with the palm of the
hand; appalling to contemplate nowadays, but with what superb results.
I have never watched this being done, but have seen some gnarled hands!

One last point: if you look at a great make of piano with a beautiful
original finish, you will never see that awful goo in the joint lines
and mouldings that many repolished instruments show.  Quality polishing
means the finishing of some parts _before_ (re)assembly and keeping the
polish on the mouldings "sharp".

Patrick Handscombe
Wivenhoe, Essex, UK.