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 which is of a different colour, varying from black to yellow, and 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 component.
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 centre of the ceramic object up to higher temperatures, when these can cause reduction of the iron.
Normally the size of the black core depends on various factors, such as temperature and firing cycle, moulding technique, porosity of the ceramic object and oven atmosphere.
The black core has no effect on the appearance of enamelled objects if it does not cause bubbles or craters, in fact it increases the mechanic resistance of the objects themselves in that it creates a greater vitrified area on the ceramic object.
In the case of enamelled tiles or porcelain tiles, the black core, despite not damaging the enamel, can cause warpage and thus reduce quality of the final product.In the case of pressed floor tiles or fired with rapid cycles, the phenomenon can prove particularly damaging on enamels, and various methods are used in order to reduce or eliminate it:
Increase of the percentage of non-plastic materials in the composition
Addition of oxidants (MnO2 or nitrates).
Increase of grinding residue.
Diminution of residual humidity.
Diminution of pressing force
Definition of a firing curve so as to increase time spent in the interval 250 – 600°C.
Render oven atmosphere as oxidizing as possible.
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 %
Red firing clays 0,1 - 1,0 %
Ball clays 0,1 - 3,5 %
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)
Tidy method
Infrared absorption and thermogravimetric analysis method (IRA/TG)
Simultaneous thermal analysis method (TG7DTA).
three different analytical methods (5)
Material - Origin - Walkley Peech C% - IRA/TG C% - TG (air) Weight loss %
Kaolin Provins (Francia) -- 0.40 - 0.40 - 0.6
Kaolin
China clay
Illite–kaolin clay Gattinara (Vc, Italy) -- 0.10 - 0.12 - 0.4
Illite–kaolin clay Escalaplano (Ca) -- 1.04 - 0.95 - 0.9
Clay Westerwald (
Clay Westerwald (
Ball clay
Ball clay
Illite–kaolin clay Monte S. Pietro (Italy) -- 0.25 - 0.12 - <0.2
Calcareous clay Codrignano (
Bibliography
(1) E. W. Worrall, C. V. Green, The Organic matter in Ball Clays, Trans Brit. Cer. Soc. 52 p.58).
(2) A. Barba, A. Moreno, F. Negre, A. B,asco, Oxidation of black cores in firing, Tile and Brick Int. 6 (1990) p. 17.
(3) X. Elias, The formation and consequences of black core in ceramica ware, Interceram 3 (1980) p. 380.
(4) H. M. M. Diz, B. Rand, I. B. Inwang, The effect of organic matter and electrolyte on the rheological behaviour of ball clays, Br. Ceram Trans. 89 (1990) p. 124.
(5) A. Barba, F. Negre, M. J. Ortis, A Escardino, Oxidation of black core during the firing of ceramic ware –3. Influence of the thickness of the piece and the composition of the black core, Br. Ceram. Trans. 91 (1992) p. 36.
(6) M. Raimondo, P. Damasino, M. Dondi, Determinazione quantitativa del carbonio organico nei materiali argillosi per uso ceramico: un confronto fra tre diversi metodi analitici, Ceramurgia 3 (1999) p. 179.
This is an analytical procedure which allows for the quantitative evaluation of organic substance content in an argillaceous material via chemical oxidation.
Principle
This method provides the percentage of organic carbon present in the material or the total percentage of organic substances, using a suitable correction factor.
The organic substances are oxidated using potassium bichromate in an acid environment, with concentrated sulphuric acid, at the temperature that the mixture reaches during the fast dilution of the acid. After a pre-established time, the excess bichromate that has not reacted is identified by dosing with a solution of Fe(2).
Apparatus and reactants
§ 25 ml flask (divisions of 0,05 ml)
§ Potassium bichromate solution 1,000 N. Dissolve 49,035 g of K2Cr2O7, dried at 105°C, in
one litre of distilled water.
§ Fe(2) solution 0,5 N. Pour 600 ml of distilled water and 15 ml of concentrated sulphuric
acid into a 1 litre flask. Add 200 g of Mohr salt [Fe(NH4)2(SO4)2• 6H2O] and bring up to
volume. The title of the solution is not stable and should be examined for each series of
analysis.
§ Sulphuric diphenylamine solution (0,5 g in 50 ml concentrated sulphuric acid).
Procedure
Place a quantity of sample sieved at 150 mm in a 500 ml flask.
Quantities:
§ 0,500 g for samples containing more than 3% of organic substances
§ 1,000 g for samples containing between 1 and 3% of organic substances
§ 2,000 g for samples containing less than 1% 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
sulphuric acid, letting is 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 colour 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.
Calculation
Organic carbon % = 10 · (1 – T/S) · (0,39/P)
Where: P = weight of sample
T = ml of Fe(2) solution used for dosing.
S = ml of Fe(2) solution used for the blank test.
If we presume that each equivalent of carbon is 77% oxidated, 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.
Notes
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, then in very low quantities. Iron (2) oxide can be oxidated by air exposure during drying whereas the interference of chlorides, which are normally present in quantities inferior to 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%).
(1) Methods of Soils Analysis (Part 2), Soil Science Society of
Analytical instrumental procedure allowing for the quantitative evaluation of organic substance content in an argillaceous material.
The quantity of total CO2 developed from a sample is measured and subjected to a combustion process, thereby 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 detracted from the measurement via individuation through thermogravimetric analysis or calcimetry (2).
Thermogravimetric analysis for the individuation of carbon in carbonates can be carried out either exposed to air or in a carbon dioxide environment, in order to increase the characteristic temperatures of calcite and dolomite decomposition and in order to reduce interference attributable to deoxydrilation of the argillaceous materials.
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 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 cays, 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.
(1) W. Gruner,
(2) B. Fabbri, P. Gazzi, G. G. Zuffa, La determinazione della componente carbonatica delle rocce, La Ceramica 3 (1974) p. 13.
Instrumental 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 when 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 esothermic 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.
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