Lime - use - history - transformation - the different types of lime.
Production of lime - finishing products based on lime
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The Lime
Acient like the egyption, the greek and romans; the architets of Rinascimento, Andrea Palladio,
describe them thoroughly:
..."You know that it's called "fresco" because it's painted
on plaster of lime and sand, fresh that is still damp and soft not dryed
and rigid. When spreading the paint and creating the form, it's necessary
quickness and capacity because the lime plaster absorbs rapidly the spreaded
colors. It drys in a few hours becoming hard, forming, in this way, a
coating of coloured plaster.
It's important to be very precise when spreading the paint with the brush.
Only few and secure brushing are needed, because if passed more times
on the same point to correct the colour, the lime plaster is not able
to absorb all this paint and once the plaster is dry the paint can flake
and fall"...
Taken from "Impossible Interview" - disguised
as Gian Battista Tiepolo, Piero de Fina - painter and art experts.
Lime is obtained by heating a limestone, composed essentially of carbonate
of calcium, at a
relative high temperature. The limestone is reduced in pieces (if not
already small, as example river stones), put into ovens and calcinated
at a temperature between 700-800° C, obtainting this way the calcination
of limestone in unslaked lime (oxide of calcium CaO).
Calcination of limestone
The unslaked lime is a porous mass, eager of
water, able to hydrate spontaneously if exposed to air and according to
the quantity of impureness contained in the limestone. Its colour goes
from white to light yellow.
For a long time two types of ovens were used to calcinate the limestones:
a discontinue type of oven and an rotative production oven. Today only
continuous ovens are used.
Photo of the 1930 furnace of the family De Toni (actualy not in use).
Transformation of limestones
The reaction develops high temperature and increased the volume of about 10% to the high temperature that can be produced instantaneously (over 150°C), the stones grow and crack and the water is transformed immediately into vapour. For this reason more water is needed, little by little, and carefully mixed to the mass
Types of lime
It's obtained when transforming limestones, with a quantity of water more or less equal to that of lime.
The hydrated compound results to be a soft powder . It's sold in paper sacks, like cement, and it must be kept in a dry place. This hydrated lime powder is less sensible to froast respect to the slaked lime putty.
Slaked lime putty (Grassello)
This is obtained when hyrating lime, with a quantity
of water higher than with lime (ratio 3:1). It becomes, in this way, a
plastic fatty mass called "slaked lime putty" (grassello).
In the past, rarely at these days, lime was transformed in a ditch where
it was kept under water for many months (seasoning). It's important to
remember that an insufficient calcination of the stones causes serious
problems to the mortar: in fact, the grumes can swell and crumble after
use with negative results. "Grassello" is sold ready to use
in plastic sacks
Grip and setting of lime
Technically speaking the main characteristics
of lime is to produce substances that, mixed appropriately, are easy to
work and to be mould when damp, and to be transformed, when dry, in a
water-resistent product. At the same time must be be provided with good
mechanic qualities and an excellent stability in time.
This characteristic is based on a chemical reation between slaked lime
and carbon dioxide contained in the air: while the mixture is drying,
water evaporates and forms the carbonate of lime. This is called "carbonating"
and means that the lime returnes to the identical composition of the original
stones but with crystals much more smaller than the originals.
The reaction of carbonating is very slow due to the low concentration
of carbon dioxide of air, therefore the complete transformation of hydrate
of lime in carbonate can be years long.
This lime is called "Aerea" because it's necessary the contact
with air in order that gripping occurs. The loss of water causes a reduction
in volume, for this reason the slaked lime is never used alone but always
with other substances to avoid cracking.
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