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Organic Matter in Clays: Detailed Overview
Section: Ceramic Tile, Subsection: General
A detailed look at what materials contain organics, what its effects are in firing (e.g. black core), what to do to deal with the problem and how to measure the amount of organics in a clay material.
Clays always contain organic material of various types and origins. In clays dating from more recent eras we can find lignin and humic acids, in colloidal form and with notable ionic exchange properties due to the functional groups -CH e –COOH present in their molecules. In clays of older eras, carbonaceous and bituminous substances are more frequent, with few functional groups capable of influencing colloidal and ionic exchange properties. Generally the calcareous material is found in the form of lignite, in grains of variable dimensions that form agglomerates or layers, or in the form of colloidal particles clinging to the crystals of argillaceous material. In so-called “ball clays” the material in colloidal form can also be composed of humic acids which facilitate the deflocculation process.
Combustion of organic substances occurs between 300 and 600°C and they decompose entirely if the quantity of oxygen is sufficient for complete reaction development.
During the firing process of ceramic parts, the organic substances present in the clays can cause the development of a central area (in the ceramic object) of a different color, varying from black to yellow. This is known as the “black core”. This phenomenon is due to the thermal decomposition of the organic material and to oxidation-reduction reactions of the inorganic components. (1)(2).
Basically, whenever the quantity of organic substances is higher than a certain value or whenever low permeability of the ceramic object does not permit complete combustion due to lack of oxygen, carbon remains in the center of the matrix up to higher temperatures (where these can cause reduction of the iron). The size of the black core depends on various factors, such as temperature and firing cycle, forming method, porosity of the ceramic object and oven atmosphere (3)(4).
The black core has no effect on the appearance of enameled objects if it does not cause bubbles or craters. In fact, it increases the mechanic strength in that it creates a greater vitrified cross section in the ceramic object. However, in the case of enameled tiles or porcelain tiles, the black core, despite not damaging the enamel, can cause warpage (and quality reduction in the final product). In the case of pressed floor tiles or those fired with rapid cycles, the phenomenon can prove particularly damaging to enamels, and various methods are used in order to reduce or eliminate it:
The content of organic carbon in clays for ceramics can be identified and this is particularly important if the transformations that take place in these substances during the production cycle are to be studied, as well as their influence on the properties of intermediate and finished products.
Normally the values found are in the following range:
Light firing clays 0.1 - 0.5 %
The analytical techniques most commonly used in the ceramics sector for quantitative determination of organic fractions are the following (see also description below):
Walkley-Peech method (chemical oxidation)
This is an analytical procedure which allows for the quantitative evaluation of organic substance content in an argillaceous material via chemical oxidation.
This method provides the percentage of organic carbon present in the material or the total percentage of organic substances, using a suitable correction factor (1).
Apparatus and reactants
Place a quantity of sample sieved at 150µm in a 500 ml flask.
500 g for samples containing more than 3% of organic substances
The quantity is calculated so as to have at least 3 ml of unreacted bichromate after initial oxidation.
Add 10 ml of the potassium bichromate at 1.0 N. Shake and add 20 ml of concentrated sulfuric acid, letting it flow down the sides. Shake and leave to settle for 30 minutes. Add 200 ml of distilled water.
At this point one proceeds to dosing of the excess bichromate by adding 5 ml of phosphoric acid at 60%, 0.5 ml of diphenilamine indicator and finally the Fe(2) solution, until the color turns from blue to green.
At the same time a blank test is carried out with 10 ml of bichromate, 20 ml of sulphuric acid and 200 ml of distilled water.
Organic carbon % = 10 • (1 – T/S) • (0.39/P)
If we presume that each equivalent of carbon is 77% oxidized, then the quantity of oxidated carbon is given by: 10 • (1 – T/S) • (0.003/0.77).
In order to obtain the percentage of organic substances we must multiply the percentage of organic carbon by the empirical factor of 1.72.
The percentage of organic substances as determined above could be higher than the actual substances present due to interference by reducing oxides, such as manganese, and ferrous or chloride compounds.
Generally if manganese oxides are present they exist in very low concentrations. Iron (2) oxide can be oxidized by air exposure during drying whereas the interference of chlorides, which are normally present in quantities less than 0.2%, can be eliminated by adding a few mercury chloride crystals to the flask (before adding reactants).
The detection limit of this method is approx. 0.1% with good consistency (0.05%).
Infrared absorption and thermogravimetric analysis method (Ira/TG).
Analytical instrument procedure enabling the quantitative evaluation of organic substance content in an argillaceous material. (1).
The quantity of total CO2 developed from a sample is assessed by subjecting it to a combustion process and measuring the intensity of the infrared absorption bandwidth. The instrument (LECO CS-225) is calibrated (ASTM E 1019) with a reference standard at a known CO2 value.
The instrument individuates total carbon content, i.e. also that present in carbonates (which must therefore be subtracted from the measurement via individuation through thermogravimetric analysis or calcimetry (2).
Thermogravimetric analysis for the individuation of carbon in carbonates can be carried out in either exposure to air or in a carbon dioxide environment, in order to increase the characteristic temperatures of calcite and dolomite decomposition and to reduce interference attributable to deoxydrilation of the argillaceous materials (decomposition reactions are normally in the range 650-800 C but in a CO2 atmosphere they are pushed to higher temperatures where we don't have any reaction of water elimination from structures of clays).
Individuation is carried out on a sample quantity of 0.1 g and the results are expressed as a percentage on the weight of the sample. The detection limit is less than 0.1% of total C (total carbon in the clay including carbonates) and the consistency of data varies between 0.05 and 0.1% of total °C. With the calcimetric method the detection limits (0.2%) and consistency of the method (0.2 – 0.3%) are increased.
In the case of calcareous clays, it is also necessary to carry out a thermogravimetric test, with consequent uncertainties in the interpretation of the TG curve in order to find the percentage of carbonates, rendering the method slower and less precise than the Walkley-Peech method.
Simultaneous thermal analysis method (TG7DTA).
Instrument analytical procedure allowing only for a semi-quantative evaluation of organic substance content in an argillaceous material, as it is less sensitive and accurate that the previous two methods.
Using thermogravimetric analysis, the variations in mass of an argillaceous material are identified as it is subjected to a controlled temperature gradient.
The combustion of organic substances occurs in the interval 200 – 500°C and is associated with an exothermic effect on the DTA curve.
At the same temperature interval, weight loss and endothermic effects occur, due to dehydrating reactions in Fe, Al and Mn hydroxides which may be present.
In order to eliminate interference, thermogravimetric analysis in nitrogen atmosphere can be carried out in order to define weight loss due to deoxydrilation reactions of the previous elements (which is subtracted from total weight loss in the same thermal interval).
The two analyses are carried out with approx. 10 mg of sample at the thermal interval 100 – 500°C with a heat increase of 10°C/min. Detection limits and consistency of this method are influenced by the difficulty in interpreting the TG curves; uncertainty amounts to approx. 0.2 – 0.3% of total organic substance weight.
Through weight loss in air between 100 and 500°C, as shown in the previous table, it is possible only to obtain a semi-quantitative estimation of organic carbon if this is higher than 0.5%.
According to F. Q. Al Khalissi e W. E: Worral (Trans. Brit. Ceram. Soc., 8,1982,pag.145) organic substances can be completely removed by treating the ground clay with water oxygenated at 30% vol. and heated for several hours at approx. 80 C.
By Nilo Tozzi
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