Moisture
content and loss on ignition:
The moisture content of raw
materials is a very important consideration as it varies between wide limits in
the same materials owing to such factors as condition of shipping and storage
and the chemical and physical nature of the materials. Water may occur in
materials in tow different ways as water mechanically held (mechanical water),
or as water of constitution. Water of constitution
is water tied up as an integral part of the molecule, as in (Al2O3.3H2O)
or as water of crystallization as in borax (Na2B4O7.10H2O).
This water of constitution is generally constant for any one particular
material. The mechanically held water is the water or moisture on the surface
or between the particles of the material.
To determine the water
mechanically held or, in some cases, some of the water of crystallization, a
weighed sample is placed in a dryer at 110?C until it comes to constant weight. The
difference between the weight of the sample when dry and its original weight is
the weight of the mechanical water contained in it. The percentage moisture may
be based either on the original weight or on the dry weight. The latter is the
better practice.
Example
Suppose a 100 g sample of clay, after
being dried to constant weight at 110?C, weights 87 g.
what is the percentage moisture based on 1) The wet weight? 2)The dry weight?
100 - 87 =13 g of moisture
Percentage moisture (W. W.)= (Grams of
moisture/ Wet weight) X 100
=13/100X100= 13%
Percentage moisture (D. W.)= (Grams of
moisture/ Dry weight) X 100
=
13/87)X100= 14.9%
Many ceramic materials contain other volatile constituents, beside
water, which are driven off only at higher temperatures. Clay for instance, may contain carbonaceous materials,
which decompose, carbon, which oxidizes and carbonates, which decompose along
with sulphides, sulphates, and other salts, which break down under oxidation,
reduction, or heating. The temperatures at which these changes take place vary
in different materials and for different constituents.
The determination of these
volatile constituents is important in the calculation of melted weights of
frits for enamels, glazes etc. The loss on ignition, as this difference is
called, gives an indication of the carbon content of clays.
The loss on ignition is
determined by ignition of all of the volatile constituents. In the laboratory
such a test is made by heating a weighed sample in a porcelain crucible, first
over a dull flame and latter over a blast lamp. The difference between the
original weight of the sample and its weight after ignition is the loss on
ignition, and it represent as a percentage based on unignited sample weight or
on ignited weight.
Percentage loss on ignition = (loss in weight/
weight of original sample)X 100
On heating a sample of clay, the mechanical
water left from drying is first driven off. Near red heat, the water of
constitution is driven off, carbonaceous material decompose, and if there is
sufficient air present, the carbon burns to carbon dioxide.if the sample is not
heated too fast all the carbon burns out, but if it is heated too fast the
carbon becomes entrapped by the fusion of the material and is thereby protected
from further oxidation. The sulphides begin to decompose and oxidize to the
oxides of sulphur, which go off as a gas, at higher temperature the carbonates
decompose into the oxides of the metal and carbon dioxide gas, and at still
higher temperature, under proper conditions, even the sulphates decompose.