PLASTER OF PARIS
“Though less important than lime, plaster of Paris is nevertheless much
used in building, especially for ceilings, molded chimney-pieces, and
in the filling of cracks and cavities. It is a white powder which is
made into a paste by adding water, prepared a little at a time and only
as fast as needed.”
“I’ve seen them do it,” Emile interposed; “the workman takes a few
handfuls of that powder out of a bag, and then he mixes it with a
little water in his trough with a trowel. He scrapes the paste all
together in his hand and uses it immediately, before making any more.
Why don’t they mix all the plaster at once, as they do with lime when
they make mortar?”
“Plaster is not all prepared beforehand for the reason that it hardens
very quickly, turns to stone, and is then unfit for use. Accordingly,
to have it in a suitable state of softness, it must be prepared at the
moment of using.”
“And what do they make that powder of that turns to stone when it is
mixed with water?”
“Plaster is made from a stone called gypsum, which, always the same as
to its nature, varies much in appearance according to its state of
purity. Sometimes it is a shapeless rock, whitish and more or less
grained; sometimes a fine fibrous mass with a silky luster; or, again,
a substance as transparent as glass and splitting into very thin scales
which show, here and there, the superb colors of the rainbow. Struck by
their beauty, workmen engaged in quarrying gypsum have given the name
of ‘Jesus-stone’ to these brilliant laminæ. Also, from their brilliance
and their cheapness, they are called ‘donkey’s mirrors.’ In ancient
times these beautiful sheets of transparent gypsum were used as
window-panes.
“Impure gypsum, in the form of shapeless rock, is used for ordinary
plaster, while pure gypsum, which comes in glass-like sheets or in
blocks of a silky appearance, is used for fine plaster, as in all sorts
of molding. The stone from which plaster is obtained occurs in
abundance in several departments of France, where it forms hills and
even whole mountains, as for example in the departments of the Seine,
the Mouths of the Rhone, and Vaucluse. For conversion into the usual
plaster of Paris this stone must be subjected to a moderate heat. To
this end it is the practice to build with gypsum blocks a row of small
vaults, and on these vaults to pile fragments of smaller size. Then the
firing is done by burning fagots and brushwood under these vaults.”
“And is it carbonic acid gas this time, too, that is driven out by the
heat, as in the manufacture of lime?” asked Jules.
“No, my friend: gypsum does not contain any carbonic acid gas. It is
made of lime, as in limestone, but united with sulphuric acid, which
heat is powerless to drive out. Besides this it contains water, which
forms a fifth of the total weight of the stone. This water, and nothing
further, escapes under the action of heat. With this expelled the
gypsum is turned to plaster.
“But this latter has a strong tendency to take on again the moisture
parted with in the kiln, and thus to become once more what it was in
the beginning—primitive stone. It is this peculiarity that renders
gypsum suitable for plaster. Moistened in the trough, the powdery
matter quickly incorporates the water that is thus restored to it, and
the whole hardens into a block having the solidity of gypsum that has
not yet passed through the kiln. Lime turns to stone by being permeated
with carbonic acid gas, which restores it to its limestone state.
Plaster becomes stone by absorbing water, which brings it back to the
state of gypsum. The transformation of lime is slow, of plaster very
rapid.
“As soon as it comes from the kiln plaster is ground under vertical
millstones and then sifted. The powder must be kept in a very dry
place, since it contracts moisture easily and then will not harden or
set, as they say, when mixed with water. You will perceive clearly
enough that after being more or loss impregnated with moisture plaster
cannot have the same tendency to absorb the water necessary to change
it into a solid mass; the substance being already somewhat soaked will
not show the same thirst when the time comes for using it. All damp
and, still more, all wet plaster is of no further use.
“Statues, busts, medallions, and various other ornamental objects are
made by casting with fine plaster of Paris. This is prepared from the
purest gypsum, those beautiful transparent scales I told you about a
little while ago. It is heated in ovens similar to those used by
bakers, and cut off from contact with the burning fuel, so as to
preserve its whiteness. The powder, which looks like fine flour, is
mixed with water and reduced to a smooth paste, which is then poured
into molds. When the plaster has set, the mold, which is in several
pieces, all joined together, is taken apart and the finished cast
withdrawn.”