In Britain the general term "whiting" seems to have been used for
calcium carbonate powder and hydrated calcium sulphate powder --
gypsum -- and other white mineral powders used as inert fillers.

Powdered calcium carbonate is usually referred to simply as powdered
chalk.  There is no suggestion that gypsum is a modern equivalent for
calcium carbonate.

Anhydrous calcium sulphate is Plaster of Paris and, since it sets once
rehydrated, is definitely not the stuff to use to make gesso.

Calcium sulphate is generally softer, finer and whiter than most
calcium carbonate, which probably explains why it has traditionally
been used for making the best gesso, for sticking on ivories, and also
for polishing them without damage.

Challen pianos were a good British make.  They became famous for one
of their 7-foot designs winning blind, over other famous makes, the
BBC competition for a studio grand in the 1930s, and in 1935 they made
a few of the (then) biggest piano in the world -- an 11' 8" monster
which can be seen and heard on a Pathé newsreel played by Billy Mayerl:
http://www.britishpathe.com/record.php?id=10199 

One comes up for sale here every so often.  I've played one; it was
okay but its tone seems never to have lived up to its bulk!

My beautiful 1912 Broadwood Barless grand naturally(!), like all the
top British makes, has one piece ivories, and they are significantly
thicker than those on my Steinway.

Patrick Handscombe
Wivenhoe, Essex, UK