There are a wide range of soluble materials that can be in clay, this article enumerates them, provides procedures on identifying and measuring them and outlines what to do about the problem.
In non-enameled products such as bricks, tiles or extruded products in general, the presence of soluble salts is visible as efflorescence, 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 and may cause bubbles or local alterations of the enamel which lower the quality of the product.
This phenomenon can have a number of causes:
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 sulfates.
The formation of superficial salt deposits is affected by drying conditions and the following can be observed:
Enamel alteration along the edges can be particularly intense in the double firing process and can be attributed to two causes:
In the case of porcelain gres, the concentration of salts along the edges leads to glassier and therefore also glossier 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 enameling 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 sulfates.
The most common soluble salts present in materials used for ceramics are:
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. 800C|
|CaSO4||2.09||Separates into CaO and SO3 between 1000 and 1125C|
|CaSO4.2H2O||2.41||Dehydration 140 - 150C|
Separates into CaO and SO3 between 1000 and 1125C
|CaMg(CO3)2||0.32||In pastes it begins to separate into Cao, MgO and CO2 at approx. 750C|
|MgCl||542.5||Complete pyrolysis at 700C|
|MgCO3||0.106||In pastes it begins to separate into MgO and CO2 at approx. 700C|
|MgSO4||260.0||Begins to separate at approx. 750C|
|Na2CO3||71.0||Melts at 851C|
|NaCl||357.0||Complete pyrolysis at 700 C|
|Na2SO4||47.6||Decomposes between 650 and 700C|
|K2CO3||1120.0||Melts at 891C|
|KCl||347.0||Complete pyrolysis at 700C|
|K2SO4||68.5||Decomposes between 500 and 600C|
Calcium sulphate is a very common soluble salt in argillaceous materials and it can be seen in the above table that it is also the most harmful salt in enameled products because its decomposition temperature, and resulting emission of gas, occurs at the temperature interval at which most enamels sinter and mature.
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 deflocculant can cause the following reactions:
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 sulfates without extraction and filtering of the soluble salts.
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 sulfate 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 oxidizing into sulfate any sulfur that may be present. At higher temperatures, sulfate cracking takes place.
If the calcination residue is 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 sulfuric acid, before repeating calcination, so as to transform any sulfur into sulfate, 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.
Sulfates can be individuated via dosing, with a standard solution of BaCl2, using tetrahydroxyquinone as internal indicator.
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.
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