SOLUBLE SALTS IN MINERALS

In non-enamelled products such as bricks, tiles or extruded products in general, the presence of soluble salts is visible as efflorescences, that is to say as whitish salt deposits on the external surface of dried or baked products. The salts reach the surface after water evaporation and are deposited in the areas of greatest evaporation.
In the case of pressed floor tiles, soluble salts are deposited mainly along the edges andmay cause bubbles or local alterations of the enamel which lower the quality of the product.
This phenomenon can have a number of causes:

1. Salts that are naturally present in the argillaceous materials.

2. Salts present in the water used for grinding the paste, or for the enamel.

3. Salts that form during firing due to pyrite pyrolysis.

4. Salts that form on the pieces during firing owing to the presence of sulphur in the fuel used for the oven.

The phenomenon is perceptible to the eye when the total concentration of soluble salts is higher than 0.5% and, in general, the salts present are sodium, potassium, calcium and magnesium sulphates.
The formation of superficial salt deposits is affected by drying conditions and the following can be observed:

1. The phenomenon intensifies with an increase in ambient temperature.

2. The phenomenon intensifies with diminution of relative humidity.

Enamel alteration along the edges can be particularly intense in the double firing process and can be attributed to two causes:

1. Water absorbed by the ceramic object during the enamelling process carries back into solution any salts that may still be present after firing and the evaporation of these salts along the edges of the object leaves a local deposit of sodium, potassium, calcium and magnesium salts, which lead to a local alteration of enamel composition, which therefore becomes more brittle.

2. The decomposition of calcium and magnesium sulphates can be accelerated when the enamel has already passed softening temperature, due to the reaction of molten glass with these salts, causing the emission of gas which in turn leads to the formation of surface bubbles and craters.

In the case of porcelain gres, the concentration of salts along the edges leads to glassier and therefore also shinier areas.
The best solution for this phenomenon is to replace the materials affected by the presence of soluble salts, or, with sometimes unsatisfying results, to accelerate the heating process during drying in order to prevent water evaporation in particular points, thereby preventing the accumulation of salts. In the double firing process a drying agent can be useful at the end of the enamelling process.
In some cases, especially when the salts are present in water used for grinding, it can be useful to add a small percentage of barium carbonate to the paste, during grinding, in order to precipitate the sulphates.
The most common soluble salts present in materials used for ceramics are:

§ Calcium, magnesium sodium, potassium and aluminium sulphates.

§ Double sulphates of sodium-aluminium and potassium-aluminium.

§ Sodium and potassium carbonates.

§ Sodium and potassium chlorides

Other compounds have such low solubility that, for practical reasons, they can be

considered insoluble.

Highly soluble salts present in argillaceous materials

Salt Solubility g/l Decomposition temperature

CaCo3 0,015 In pastes it begins to separate into Ca and Co2 at approx. 800 C

CaSO4 2,09 Separates into CaO and SO3 between 1000 -1125 C

CaSO4 2H2O 2,41 Dehydration 140 – 150°C

Separates into CaO and SO3 between 1000 – 1125 C

CaMg(CO3)2 0,32 In pastes it begins to separate into Cao, MgO and Co2 at approx. 750 C

MgCl 542,5 Complete pyrolysis at 700 C

MgCO3 0,106 In pastes it begins to separate into MgO and Co2 at approx. 700 C

MgSO4 260,0 Begins to separate at approx. 750 C

Na2CO3 71,0 Melts at 851 C

NaCl 357,0 Complete pyrolysis at 700 C

Na2SO4 47,6 Decomposes between 650 and 700°C

K2CO3 1120,0 Melts at 891 C

KCl 347,0 Complete pyrolysis at 700 C

K2SO4 68,5 Decomposes between 500 and 600 C

Calcium sulphate is a very common soluble salt in argillaceous material and it can be seen on the above table that it is also the most harmful salt in enamelled products because its decomposition temperature, and resulting emission of gas, is to be found in the temperature interval at which most enamels sinter and mature.

Process for individuating soluble salts in clays.

1. Extraction of salts from material.

The finely dry-ground clay (<>

2. Separation of water containing salts.

The slip thus obtained is vacuum-filtered with 0.45 micron filters, or centrifuged, in order to separate out the water in which the salts are dissolved. The solution is brought to a known volume.

3. Chemical analysis of water.

The desired ions can be individuated within the solution (usually chlorides, sulphates, calcium, magnesium, sodium and potassium).
Various techniques can be used: colorimetry, atomic absorption spectrometer (AAS), plasma spectrometer (ICP), gravimetric analysis (precipitation of sulphates with barium chloride or of chlorides with Silver nitrate) or via dosing with EDTA.
Finally, the total quantity of soluble salts can be determined via gravimetric analysis by weighing the residue after evaporation of a known volume of water.

Quick definition of the presence of sulphate ions.

Sulphates are generally present as calcium sulphate and this element can be present also as a carbonate or as an exchangeable cation.
The addition of sodium silicate as deflocculant can cause the following reactions:

1. Ca (Clay-OH)₂ + Na₂SiO₃ = Na clay-OH + CaSiO₃

2. CaCO₃ + Na₂SiO₃ = Na₂CO₃ + CaSiO₃

3. CaSO₄ + Na₂SiO₃ = Na₂SO₄ + CaSiO₃

Reaction 1 provokes an increase in deflocculation and calcium is precipitated as an insoluble salt (CaSiO3).
Reaction 2 causes the formation of a deflocculant salt (Na2CO3) and calcium is precipitated as an insoluble salt (CaSiO3).
Reaction 3 causes the formation of an insoluble salt (CaSiO3) and a soluble salt (Na2SO4) which acts as flocculant.
It is possible to individuate the presence of sulphates without extraction and filtering of the soluble salts.
Method:

§ Prepare a suspension 1:1 of clay with distilled water. For example 500 g of clay in 500 ml of water.

§ Add sodium silicate until sufficient fluidity is obtained and shake for 10 minutes.

§ Measure viscosity. Add BaCO3 in small doses. For example 100 mg at a time per 500 g

of clay.

§ Mix for 15 minutes each time and measure viscosity.

If viscosity increases, no sulphate ions are present.
If viscosity decreases, sulphate ions are present.

Individuation of sulphates via precipitation.

The analysis consists in slow addition of a diluted solution of barium chloride to the heated and slightly acidified solution containing the salts.
The following reaction takes place:

Ba₂+ + SO₄= = BaSO₄

BaSo4 is a low solubility salt (circa 3mg/l) and is separated by filtration, washed with water, calcined at approx. 800°C and then weighed.
The low solubility of barium sulphate is further lowered in presence of a slight excess of Ba2+, but slightly increased in presence of H+ ions, via the following reaction:

H⁺ + SO4⁼ = HSO₄^-

Despite this, precipitation is carried out in a slightly acidic solution in order to reduce contamination of the precipitate and in order to encourage the formation of larger and more easily filtered crystals. In an acidic environment, moreover, the formation of carbonate, chromate and barium phosphate can be avoided.
BaSO4 tends to co-precipitate other salts which may be present, such as Ba(NO3)2 and Ba(ClO3)2 by forming mixed crystals. Chlorates and nitrates, if present, need therefore to be removed beforehand.
Ions such as Ca²⁺, Al³⁺, Cr³⁺ and Fe³⁺ also interfere by co-precipitating BaSO₄ isomorphous sulphates.
One should remove these ions first, and also use highly diluted solutions.
During calcination, the carbon derived from partial combustion of the filter paper can reduce the sulphate to sulphur at temperatures below 600°C, according to the following reaction:

BaSO₄ + 4C = BaS + 4CO

Combustion of the filter paper should be avoided.
Calcination takes place at approx. 900 C, keeping the crucible in a tilted position so as to guarantee good air circulation, thereby oxidising into sulphate any sulphur that may be present. At higher temperatures, sulphate cracking takes place.
If the calcination residue is of a greyish colour, this indicates the presence of carbon, in which case it should be left to cool before adding 1 or 2 drops of concentrated sulphuric acid, before repeating calcination, so as to transform any sulphur into sulphate, according to the following reaction:

BaS + H₂SO₄ = BaSO₄ + H₂S

This method is also useful for individuating barium and other cations such as Pb2+ and Sr2+. These two cations have higher solubility, which can be reduced in hydro-alcoholic solution.

Individuation of sulphates via dosing.

Sulphates can be individuated via dosing, with a standard solution of BaCl2, using tetrahydroxyquinone as internal indicator.
Method

§ Transfer 25ml of solution into a 250ml flask and create slight acidity using a N/100

solution of chloric acid with phenolphthalein as indicator.

§ Add 25ml of isopropylic alcohol, which aids quick precipitation during dosing.

§ Add 0.2 g of tetrahydroxyquinone. The colour should tend towards yellow.

§ Dose with a standard solution of BaCl2 until the colour tends towards pink.

§ Calculate the percentage of sulphates (SO4=) in the water using the ratio 208,27 (BaCl2): 96 (SO4--).

Individuation of chlorides.

The classic method is dosing using Mohr salt.
The solution is dosed with a solution of Silver nitrate, in the presence of potassium chromate as indicator.
The method is based on the relative solubilities of silver chloride and silver chromate. Silver chloride is white, whereas Silver chromate is bright red, and the final stage of dosing takes place when a slight excess of silver nitrate leads to the formation of chromate.
Method:

Transfer 50ml of solution into a 250ml flask and add 1ml of potassium chromate solution at 5%.

Dose with a N/50 solution of silver nitrate until the red colour disappears.

Calculate the percentage of chlorides using the ratio 170 (AgNO₃) : 35,5 (Cl^-).

ORGANIC MATTER IN CLAYS

Clays always contain organic material of various type and origin. 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 to be found in the form of lignite, in grains of variable dimension 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 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).

Organic substances in some argillaceous materials according to
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 Cornwall (U.K.) -- <0,10 style="">0,13 - <0.2
China clay S. Severa (Rome, Italy) -- 0.12 - 0.10 - <0.2
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 (Germany) -- 0,10 - 0,15 - <0.2
Clay Westerwald (Germany) -- 0.30 - 0.14 - <0.2
Ball clay Devon (U.K.) -- 2.98 - 2.93 - 3.2
Ball clay Devon (U.K. ) -- 2.10 - 1.95 - 2.4
Illite–kaolin clay Monte S. Pietro (Italy) -- 0.25 - 0.12 - <0.2
Calcareous clay Codrignano (Ra, Italy) -- 0.70 - 1.70 - 0.6

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.

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.

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%).

Bibliography

(1) Methods of Soils Analysis (Part 2), Soil Science Society of America, 1982

Infrared absorption and thermogravimetric analysis method (Ira/TG).

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.

Bibliography

(1) W. Gruner, E. Grallath, Improvements in the combustion method for the determination of low carbon contents in steel, Steel Research 66 (1995) p. 455.
(2) B. Fabbri, P. Gazzi, G. G. Zuffa, La determinazione della componente carbonatica delle rocce, La Ceramica 3 (1974) p. 13.

Simultaneous thermal analysis method (TG7DTA).

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%.

Remark

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.

DEFLOCCULANTS FOR CERAMIC

Deflocculation and flocculation

The particles of an argillaceous material, when suspended in water, behave in two entirely different ways, as the electrostatic charges present on their surface may cause both attraction and repulsion. Normally, in an acid environment, the particles of an argillaceous material are attracted to each other, and this state is called “flocculation”, whereas in an alkaline environment the particles repulse each other and this state is called “deflocculation”.
In the state of deflocculation, the charges on the particles have been neutralised, with the addition of deflocculants, and the particles remain in suspension as single units, with a consequent reduction in viscosity. In the state of flocculation, the particles form three-dimensional groups or structures, due to electrostatic attraction, with a consequent
increase in viscosity.

Deflocculants

The term “deflocculant” denotes a substance which, when added to scattered particles in suspension, causes a reduction in apparent viscosity.
Deflocculants are substances which prevent flocculation by increasing zero potential and therefore the repulsive forces between particles.
The mechanisms by which deflocculants act can be schematised as follows:

1. A shift of pH towards basic values, by addition of bases or by hydrolysis.
2. Substitution of flocculant cations, present in the double layer of clays, with alkaline cations.
3. Increase of negative charge on argillaceous particles by adsorption of anions with elevated electric field.
4. Addition of a protective colloid.
5. Elimination of flocculant ions which might be present in solution, via precipitation or formation of coordination complexes. For example via the following reactions:

o BaCO₃ + SO₄^2- = BaSO₄ + CO₃^2-

o Na₂CO₃ + Ca²⁺ = CaCO₃ + 2Na⁺

o BaCO₃ + Ca²⁺ = CaCO₃ + Ba²⁺

Normally deflocculants act via a combination of the above-mentioned mechanisms and can be of either organic or inorganic nature.

Main deflocculants

Organic

Humic acids and derivatives
Alkaline lignosulfonates
Tannin compounds
Polyacrylates and acrylic derivatives
Polycarbonates
Sodium citrate
Gum Arabic
Low viscosity Na-CMC

Inorganic

Sodium and potassium carbonates
Sodium and potassium hydroxides
Sodium silicates
Phosphates and polyphosphates
Sodium and ammonium oxalates

Graphs of viscosity/deflocculant percentage shows that addition of a deflocculating substance causes viscosity reduction to a point at which the forces of attraction are neutralised. At this point, called “full deflocculation”, viscosity reaches its minimum value and subsequent additions of deflocculants only have an adverse effect.
The position of the point of minimum viscosity is affected by slip density.
The most efficient products having deflocculant action for uses in ceramics are sodium silicate, polyphosphates (pyro – tripoly – tetrapoly – etc.) and organic sodium and ammonium polyelectrolytes.
No single product acts according to all of the mechanisms described above, therefore a mixture of various compounds is usually used, whose combined action is often superior to the sum of their single actions.
For tile pastes, the following products are most commonly used

Liquid or solid mixtures of sodium silicates, polyphosphates and polyacrylates
Sodium tripolyphosphate
Liquid or solid sodium silicates

Some clays can be easily deflocculated using compounds which raise pH, such as sodium silicate or sodium carbonate, as these contain organic material which can react in the presence of an alkaline environment, forming deflocculant compounds.

Bibliography

P. Prampolini, Ceramica Informazione, 311, 1992, pag.88.

MOST COMMON DEFLOCCULANTS

Sodium carbonate

This compound is commonly called “soda” and has the formula Na2CO3 or Na2CO3×10H2O depending, respectively, on whether it is anhydrous or hydrous.
The deflocculating action is carried out by an increase in pH, but the carbonate ion, before hydrolysis, can react with calcium ions that may be present in the solution, thereby forming CaCO3 which is insoluble and therefore a flocculating element is removed from the suspension.
This carbonate is often used in combination with a silicate, and the resulting mixture, whose exact proportions have to be arrived at via experimentation, is the traditional fluidifier for fine tableware, porcelain and sanitary fixtures.

Sodium silicate

This is the main deflocculant used for the preparation of pastes for casting or for refractory plastics. The ratio of SiO2 to Na2O can vary from 3.75:1 to 1:1 and is available in liquid or solid form.
Sodium silicate increases the pH of the suspension, due to hydrolysis, whereas the silicon
separates out in the form of colloidal silica which also performs a role as protective colloid, according to the following reaction:

Na₂O nSiO₂ + H2O = nSiO₂ + 2Na+ + 2OH

When used alone, the percentage in pastes varies between 0,3 and 0,7%.

Alkaline lignosulfonates

These compounds are water-soluble by-products from the manufacture of cellulose using the bisulphite method. Their molecular mass varies between 200 and 100,000, but the most common types have a molecular mass around 4000 and contain monomers on which 8 –SO3^¯ functional groups can be found, associated with benzene rings. They can also act as binding agents for flocculant cations, but their deflocculant action is carried out by the functional groups already mentioned.
They are anionic polyelectrolytes which are strongly hydrolysed even at pH’s below 5 and can be absorbed by argillaceous particles up to a pH of 10. Sure enough, dissociation in sulfonate groups - SO3Na ^– is considerably stronger than in carboxylic or phenolic groups associated with other polymers of natural origin or resulting from synthesis, such as humates.

Polyphosphates

Alkaline polyphosphates (normally from sodium or ammonium) are dissociated in solution and the anions are absorbed onto the clay particles, generating a strongly negative potential. Moreover they are able to capture polyvalent flocculant cations, such as calcium and magnesium, associated with water and soluble salts.
Polyphosphates evolve slowly, by hydrolysis, and are transformed into orthophosphates, thus reducing their deflocculant power with the aging of suspensions.
The main sodium salts used as deflocculants are listed on the following table.

Tripolyphosphate Na₅P₃O₁₀ Solub. 2% - 140 g/l per 25 C

Pyrophosphate Na₄P₂O₇ Solub. 5%

Tetraphosphate Na₆P₄O₁₀ Solub. High

Esametaphosphate (NaPO₃)₆ Solub. Unlimited

Tripolyphosphate

This is a sodium phosphate triple-polymerised so as to form a single molecule with a chain structure.Its deflocculant power is shown by an increase in negative charge on the surface of the
clay particles, via adsorption of the phosphoric anion, and therefore by an increase of zero potential which causes repulsion between the particles.
It also forms insoluble compounds with flocculant anions, removing them from the dispersive vehicle and preventing their action. In particular, the tripolyphosphate anion forms complex and highly stable anions with calcium, of type (CaP6O18)4- and (Ca2P6O18)2-.
It hydrolyses in water, increasing pH up to 9-10 depending on its concentration.
Products on the market are often a mixture of different salts, mainly anhydrous and hydrous tripolyphosphates with pyrophosphate, metaphosphate and orthophosphate; in some cases there may be residues of reactants used in preparation of the product, such as monosodic phosphate (NaH2PO4) and bisodium phosphate (Na2HPO4). The content of phosphates other than tripolyphosphate must be minimum as these reduce the deflocculant capability of the product.
Tripolyphosphate also exists in two crystalline forms with different speeds of dissolution in water.

Esametaphosphate

As for tripolyphosphate, its deflocculant power is shown by an increase in negative charge on the surface of the clay particles, via adsorption of the phosphoric anion, and therefore by an increase of zero potential which causes repulsion between the particles.
It also forms insoluble compounds with flocculant anions, removing them from the dispersive vehicle and preventing their action. In particular, the tripolyphosphate anion forms complex and highly stable anions with calcium, of type (CaP6O18)4- and (Ca2P6O18)2-.

Alkaline polyacrylates (Na-NH₄)

These are polymers with a molecular mass varying between 1000 and 20,000.
They are effective deflocculants above pH 5 for the dissociation of carboxylic groups and for the absorption of polymeric anions on clay particles.
They are highly stable polymers over time and also under variation of temperature.
They do not interact with plaster moulds and can also be used for hot casting.
They have been used in the traditional ceramics sector since the 1970’s.
Polyacrylic acid is obtained from polymerisation of acrylic acid, and after neutralisation with soda or ammonium, sodium and ammonium polyacrylates are obtained.
The process allows for adjustment of chain length and it is therefore possible to obtain a broad range of molecular weights, whose value depends on the properties of the product.
Polyacrylates with a molecular weight between 1000 and 10,000 are energetic fluidifiers, whereas those with a weight higher than 10,000 increase viscosity in suspensions.
Chains are less rigid and complex than those of CMC and thus products with low molecular weight cause little water retention.
Polyacrylates reduce interactive forces between particles, attaching themselves to those areas of the particles whose charge is responsible for the formation of three-dimensional structures.
Polyacrylates act more strongly than polyphosphates in reducing tixotropy and yield point, and, like them, are strong sequestrators of polyvalent ions.
In case of excessive dosage, yield point can be reduced to zero, in which case sedimentation may occur.

polymers with low molecular weight of natural origin or from synthesis

The functional groups of those polymers responsible for electrostatic interaction between molecules absorbed on clay particles are essentially: –COOH; –SO₃H; –SO₄H; –OH; –NH₂–R–COOH.

One can list gum arabic, alginates and low molecular weight and low viscosity

carboxymethylcellulose, starch derivatives, and vegetal gum and protein derivatives.

Barium carbonate, BaCO₃

This is not a true deflocculant, but it aids deflocculation by precipitating the sulphate ion which prevents the process, and is often associated with calcium, magnesium and iron which are flocculant ions.
The anion SO₄ ^2- is easily absorbed by clay particles, also in substitution of hydroxyls, and prevents reduction of zero potential.
Barium carbonate is used, even if not very soluble, as it precipitates the sulphate ion easily, whereas barium chloride is soluble but would carry barium ions into the solution which in excess would act as flocculants.

BaCO₃ must be added before the deflocculants; quantity varies between 0.02 and 0.1%.

Bibliography

D. Chiavacci, Ceramica Informazione, 355, 1995, p. 593