"Chalk" is a term somewhat loosely used to denote the soft white
limestone--the "_Creta Scriptoria_"--that is cousin to Marl on one side
and to Ragstone on the other.
In its purest form chalk consists of over 95 per cent. of carbonate of
lime in the form of fine granular particles held together by a
calcareous cement, its organic origin being clearly traced in the
remains of the minute sea creatures with which it abounds.
Hewn blocks of chalk have been used for walling and vaulting from
immemorial times, and, where not exposed to direct erosion by the
weather, remain to this day as clean-cut as when they were first
quarried and a very great deal harder.
The filling in of the great vaults at Salisbury Cathedral and in the
Bishop's Palace are of chalk, whilst innumerable lesser buildings of
more or less antiquity still remain to us as monuments to the excellence
and durability of this stone.
Chalk, too, was often used in combination with flint or brick to build
the engaging chequer-work walls that embellish so many downland
villages.
At Medmenham there are cottages both old and new of hewn rock chalk, and
both the Berks and Bucks banks of the Thames have many buildings to show
of this beautiful material.
Amongst present-day architects Sir Edwin Lutyens was the first to give
hewn chalk an opportunity of showing its quality in serious
architecture, Marsh Court in Hampshire being an instance of more than
local celebrity.
In the great walls at the Bishop of Winchester's palace, Farnham Castle
in Surrey, the old builders appear to have used bricks, limestone and
chalk proper, according as the several materials were delivered, quite
indifferently, and with results altogether delightful.
Not all chalk is suitable for building, that near the surface being
often far gone in decay and much too friable for such a purpose.
Even when apparently sound blocks have been gotten they are not
infrequently found to be crossed in all directions by planes of weakness
along which they are apt to fall to pieces in the handling.
From this cause the "waste" is sometimes considerable.
The well-known building "stones" from the quarries of Beer, Sutton, and
Tottenhoe in Devonshire are really chalk, but in a form not readily
distinguishable from ordinary free-stone.
The longer that chalk blocks are kept to dry before building-in the
better, and the sun and wind of at least a year should be allowed free
play upon them to dry out their natural sap and render them
"frost-proof."
During the drying-out process the chalk should, if possible, be
protected from the rain.
For years after being built into the walls of a house, chalk will
continue to dry and harden.
But it is essentially a somewhat porous material, and will quickly
revenge itself on those neglecting its just demands for a sound roof and
a proper damp-course.
In exposed situations new chalk walling is liable to allow the
penetration of moisture under the pressure of the wind unless a cavity
is provided or unless the surface is treated with a silicate or other
"vitrifying" fluid.
Chalk, however, has one shining virtue in common with its great
antithesis--it improves mightily with keeping.
Chalk walls sometimes have youthful vices in the way of porosity that
entirely disappear with advancing years through the closing up of the
surface pores, which eventually makes a cavity and inner lining
superfluous.