Many very well cast piano plates (frames) here in Europe seem not to
have been filled, just primed, before gilding, but the filler of choice
certainly before WW1 seems to have been gesso (pronounced jess-oh),
the same gesso used since time immemorial as the ground for classic
English water-gilding of picture frames, furniture, etc., and sometimes
as a 'stopper' for wood before finishing.

There are as many recipes for gesso as there are gilders, but most
list animal glue (non-crystallising), whiting (hydrated calcium
sulphate powder) and tallow, simmered and stirred until creamy and
applied hot.  Best water gilding practice involves applying as many as
eight coats, but piano plates probably had fewer.  Properly-formulated
gesso will stick to anything, but will crack if allowed to dry too
quickly.  When quite dry the surface is rubbed down to a perfect,
almost polished finish.  Then it can be stained, painted, lacquered,
whatever.

It is possible to colour liquid gesso, black for instance, by stirring
in pigment.  But traditionally, prior to real gold water gilding, two
coats of red, yellow or black 'bole' (bolus), a fine clay suspension,
are applied to the plain gesso, and the surface rubbed smooth again.
Red is used for classic antique gilding, yellow for bright burnished
finishes, and black often under silver leaf.  The colour of the bole is
significant since the leaf is so thin, and also not all classic gilding
is intended to achieve perfect coverage.  I have never seen black gesso
or bole used on a piano plate; I suspect that areas of blackening occur
through a harmless reaction between iron, humidity and calcium
sulphate.

In water gilding the substrate is wetted with a tincture of water,
glue-size and alcohol just before each real gold or silver leaf
is applied.  Areas may be double-leafed for perfect coverage or
durability.  Finally the leaf may be highly burnished into the gesso
with agate-tipped tools until it effectively becomes plate, or left
matt.

This is how Broadwoods double-gilt their plates, if at all, until about
1902.  But for years they had reservations about gilded plates, feeling
that, like veneered furniture, French polish and coasters, they were
just too nouveau-riche.  Even into the 1920s some Broadwood models had
beautiful deep green painted plates and deep crimson stained sound
boards.

They also declined to put the name John Broadwood & Sons on the fall
in brass, and certainly not with a decal, doing the job perfectly and
discreetly in inlaid box or pear wood.  Until 1914 some Broadwood
instruments were still cased in solid, cross-laminated mahogany or
walnut, and they were famous for their stunning 'steel Barless' pianos
in painted or stained 'Arts and Crafts' cases designed by leading
country house architects like Edwin Lutyens.

Patrick Handscombe
Wivenhoe, Essex, UK.