DOI Page / Citation link:
https://doi.org/10.11588/diglit.7186#0020
6
Introduction.
with it in the very proportion in which it occurs in the composition of clays, we
must admit that these possess some particular texture or molecular arrangement
which the natural circumstances of their formation and, above all, time have
imparted to the constituent parts ; a texture which man has not as yet—so
at least it seems to us—learned how to imitate completely.
Yet although plasticity be an all important condition of this manufacture,
its excess entails grave inconvenience. Objects formed of over-plastic clays are
dried unequally and with difficulty, which renders them liable to lose shape
and crack. Such defects are developed at the stove, and in the furnace they
increase further. Hence, as we mentioned, arises the necessity of introducing
into the composition of the paste powdered quartz, calcareous earths, and sands,
to diminish this excessive plasticity, increase porosity, and thus facilitate the
expulsion of the water; which, although it serves merely to mix the materials
and give them the needful softness, demands weighty consideration and pre-
cautions ; because the wrought portions, however they may sometimes seem
dry, never are absolutely free from water.
We have already observed that the liquid is strongly attracted by the clay,
or, more strictly speaking, so combines with the clay as to require an intense
heat for its total elimination. It has been stated that sandy substances make an
easy channel for the water to pass from molecule to molecule, thus securing to
all parts an equal density. Now, to the end that the desiccation of the largest
pieces may be effected uniformly and without cracking, care must be taken to
diminish the density at the centre ; because, if the outside density did not
exceed the inner, the evaporation of water must naturally proceed more
slowly at the centre, the clays would from the first remain in an unavoidable
state of lumpishness, and a separation of their molecules must ensue.
Rapid drying gives rise to another serious defect: the work becomes coated
superficially with a dry thick crust impervious to internal moisture ; and thus
appears dry when only partially so. The result is that the imprisoned water,
constrained in the furnace to evaporate by the high temperature, acquires an
Introduction.
with it in the very proportion in which it occurs in the composition of clays, we
must admit that these possess some particular texture or molecular arrangement
which the natural circumstances of their formation and, above all, time have
imparted to the constituent parts ; a texture which man has not as yet—so
at least it seems to us—learned how to imitate completely.
Yet although plasticity be an all important condition of this manufacture,
its excess entails grave inconvenience. Objects formed of over-plastic clays are
dried unequally and with difficulty, which renders them liable to lose shape
and crack. Such defects are developed at the stove, and in the furnace they
increase further. Hence, as we mentioned, arises the necessity of introducing
into the composition of the paste powdered quartz, calcareous earths, and sands,
to diminish this excessive plasticity, increase porosity, and thus facilitate the
expulsion of the water; which, although it serves merely to mix the materials
and give them the needful softness, demands weighty consideration and pre-
cautions ; because the wrought portions, however they may sometimes seem
dry, never are absolutely free from water.
We have already observed that the liquid is strongly attracted by the clay,
or, more strictly speaking, so combines with the clay as to require an intense
heat for its total elimination. It has been stated that sandy substances make an
easy channel for the water to pass from molecule to molecule, thus securing to
all parts an equal density. Now, to the end that the desiccation of the largest
pieces may be effected uniformly and without cracking, care must be taken to
diminish the density at the centre ; because, if the outside density did not
exceed the inner, the evaporation of water must naturally proceed more
slowly at the centre, the clays would from the first remain in an unavoidable
state of lumpishness, and a separation of their molecules must ensue.
Rapid drying gives rise to another serious defect: the work becomes coated
superficially with a dry thick crust impervious to internal moisture ; and thus
appears dry when only partially so. The result is that the imprisoned water,
constrained in the furnace to evaporate by the high temperature, acquires an