tree calf ( tree marble )
A form of cover decoration consisting of a smooth,
light-colored calfskin treated with chemicals in
such a manner as to represent a tree trunk with
branches. In the usual manner, a dual design
appears on upper and lower covers. The leather is
first paste-washed, and the book is then hung
between two rods which keep the covers flat. The
book is tilted so that it inclines upward towards
its head. In order to bend the boards outwards,
i.e., warp slightly to a concave shape, so that
the solutions will run properly, the insides of
the hoards are not filled in until the decoration
is done. A small amount of water is applied to the
center of both covers to form the trunk, then more
water is thrown on the covers so that it runs down
to the trunk and to a central point at the lower
edge of the hoards. Copperas (a green hydrated
ferrous sulfate) is then sprinkled in fine drops
on the covers, followed by potassium carbonate
(salts of tartar). which causes the chemical
reaction that etches the leather to form a
permanent pattern in shades of gray, ranging from
faint to very dark. Calfskin was used for this
decoration in preference to sheepskin (although
19th century examples of sheepskin tree do exist)
because in addition to being a much superior
leather, it takes a better polish, which suits
this style of decoration admirably.
The spine of the book is protected during the
marbling so that it will not be touched by the
water or chemicals. The entire process calls for
considerable experience and dexterity of
execution, because if the result is to be
effective the copperas and potassium carbonate
must be applied in the correct amounts, as well as
in the proper manner, while the initial water is
still running down the covers; otherwise the
effect will be little more than sprinkling.
Late in the 19th century attempts were made to
produce the tree calf effect with the use of an
engraved block, which was used to print a design
on the covers in black, but the results were
ineffective because the block did not provide the
shading which the genuine method achieved. The
popularity of tree calf began to decline before
the First World War, and by the late 1920s this
once very popular form of decoration had virtually
passed from existence. The first known tree calf
decoration dates from about 1775. See: PLATE V .
(83 , 94 , 236 , 264 )
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