DOI Page / Citation link:
https://doi.org/10.11588/diglit.7186#0019
Introduction.
5
action and shocks, the other of less tenacity, but in grain and tone more
resembling the ancient terra- cottas, and less costly than the former.
To produce the first sort, which in baking turns out almost ash-coloured,
he employs a mixture of whitish clays of somewhat tough consistency, which
are found in the neighbourhood of Maggiora, mixed with finely-powdered
quartz.
The second sort, at present most in vogue, is composed of two parts of
the above-named clays of Maggiora; four parts of rich and very ferruginous
earth of a deep red hue, found in the territory of Lesmo; four parts of tougher
earth, derived from Trezzo; and two parts of a sandy lime of finest grain,
gathered in the commune of Ronco, which, as a disjunctive element, increases
the porosity, and prevents the excessive cohesion of the pieces.
Plasticity and homogeneity of ingredients are the two conditions essential
to the composition of any ceramic paste. Plasticity is the property of clays
which contain only aluminum, silica, and a little oxide of iron ; and depends
in great measure on the water contained in the clay, which cannot be eva-
porated even by the drying power of 100 degrees.1
Clays, regarded even as hydrated silicates of aluminum, were at first sup-
posed to owe their elasticity to the aluminum alone. But since this substance,
whether already reduced to a gelatinous state, or kneaded long while in water,
never furnishes a tenacious and plastic paste however pure silica may be mixed
1 " All clays as they occur in nature are essentially for example, is dehydrated clay; and it may absorb a con-
bydrated silicates of alumina; and upon the presence of siderable quantity of water without regaining the slight-
the water of combination depends their fictile or plastic est amount of plasticity.
property, that is, their capability of being moulded into If we compare different clays together in respect to
vessels when mixed with water and kneaded to a pasty elementary composition, we find the relation between the
consistency. All clays contain hygroscopic water, which silica and alumina to be extremely variable ; and, accord-
may be expelled at ioo° C. without lessening their plas- ingly, the formula; which have been proposed to express
ticity. When, however, clay is heated to redness, it their rational constitution are very discordant, This is
loses not only its hygroscopic water, but also its water in great measure to be explained by the fact that, in many
of combination, and, as a consequence, it ceases to be clays, a large proportion of silica exists uncombined either
plastic. In this dehydrated state it cannot again directly as sand, or in a much finer state of division. The gritti-
combine with water and.regain its plasticity. It may, ness of a clay is due to the presence of sand." Dr. Percy's
indeed, absorb water with avidity; but not the smallest 'Metallurgy and Fire Clays,' vol. i. p. 208.
degree of plasticity is thereby restored. Pounded brick,
5
action and shocks, the other of less tenacity, but in grain and tone more
resembling the ancient terra- cottas, and less costly than the former.
To produce the first sort, which in baking turns out almost ash-coloured,
he employs a mixture of whitish clays of somewhat tough consistency, which
are found in the neighbourhood of Maggiora, mixed with finely-powdered
quartz.
The second sort, at present most in vogue, is composed of two parts of
the above-named clays of Maggiora; four parts of rich and very ferruginous
earth of a deep red hue, found in the territory of Lesmo; four parts of tougher
earth, derived from Trezzo; and two parts of a sandy lime of finest grain,
gathered in the commune of Ronco, which, as a disjunctive element, increases
the porosity, and prevents the excessive cohesion of the pieces.
Plasticity and homogeneity of ingredients are the two conditions essential
to the composition of any ceramic paste. Plasticity is the property of clays
which contain only aluminum, silica, and a little oxide of iron ; and depends
in great measure on the water contained in the clay, which cannot be eva-
porated even by the drying power of 100 degrees.1
Clays, regarded even as hydrated silicates of aluminum, were at first sup-
posed to owe their elasticity to the aluminum alone. But since this substance,
whether already reduced to a gelatinous state, or kneaded long while in water,
never furnishes a tenacious and plastic paste however pure silica may be mixed
1 " All clays as they occur in nature are essentially for example, is dehydrated clay; and it may absorb a con-
bydrated silicates of alumina; and upon the presence of siderable quantity of water without regaining the slight-
the water of combination depends their fictile or plastic est amount of plasticity.
property, that is, their capability of being moulded into If we compare different clays together in respect to
vessels when mixed with water and kneaded to a pasty elementary composition, we find the relation between the
consistency. All clays contain hygroscopic water, which silica and alumina to be extremely variable ; and, accord-
may be expelled at ioo° C. without lessening their plas- ingly, the formula; which have been proposed to express
ticity. When, however, clay is heated to redness, it their rational constitution are very discordant, This is
loses not only its hygroscopic water, but also its water in great measure to be explained by the fact that, in many
of combination, and, as a consequence, it ceases to be clays, a large proportion of silica exists uncombined either
plastic. In this dehydrated state it cannot again directly as sand, or in a much finer state of division. The gritti-
combine with water and.regain its plasticity. It may, ness of a clay is due to the presence of sand." Dr. Percy's
indeed, absorb water with avidity; but not the smallest 'Metallurgy and Fire Clays,' vol. i. p. 208.
degree of plasticity is thereby restored. Pounded brick,