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A Low Cost Tester of Glaze Melt Fluidity
A One-speed Lab or Studio Slurry Mixer
A Textbook Cone 6 Matte Glaze With Problems
Adjusting Glaze Expansion by Calculation to Solve Shivering
Alberta Slip, 20 Years of Substitution for Albany Slip
An Overview of Ceramic Stains
Are You in Control of Your Production Process?
Are Your Glazes Food Safe or are They Leachable?
Attack on Glass: Corrosion Attack Mechanisms
Ball Milling Glazes, Bodies, Engobes
Binders for Ceramic Bodies
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
Ceramic Glazes Today
Ceramic Material Nomenclature
Ceramic Tile Clay Body Formulation
Changing Our View of Glazes
Chemistry vs. Matrix Blending to Create Glazes from Native Materials
Concentrate on One Good Glaze
Cone 6 Floating Blue Glaze Recipe
Copper Red Glazes
Crazing and Bacteria: Is There a Hazard?
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Creating a Non-Glaze Ceramic Slip or Engobe
Creating Your Own Budget Glaze
Crystal Glazes: Understanding the Process and Materials
Deflocculants: A Detailed Overview
Demonstrating Glaze Fit Issues to Students
Diagnosing a Casting Problem at a Sanitaryware Plant
Drying Ceramics Without Cracks
Duplicating Albany Slip
Duplicating AP Green Fireclay
Electric Hobby Kilns: What You Need to Know
Fighting the Glaze Dragon
Firing Clay Test Bars
Firing: What Happens to Ceramic Ware in a Firing Kiln
First You See It Then You Don't: Raku Glaze Stability
Fixing a glaze that does not stay in suspension
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
Formulating Your Own Clay Body
G1214M Cone 5-7 20x5 Glossy Base Glaze
G1214W Cone 6 Transparent Base Glaze
G1214Z Cone 6 Matte Base Glaze
G1916M Cone 06-04 Base Glaze
G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
Getting the Glaze Color You Want: Working With Stains
Glaze and Body Pigments and Stains in the Ceramic Tile Industry
Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity, LOI
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
How a Material Chemical Analysis is Done
How desktop INSIGHT Deals With Unity, LOI and Formula Weight
How to Find and Test Your Own Native Clays
How to Liner-Glaze a Mug
I've Always Done It This Way!
Inkjet Decoration of Ceramic Tiles
Is Your Fired Ware Safe?
Leaching Cone 6 Glaze Case Study
Limit Formulas and Target Formulas
Low Budget Testing of the Raw and Fired Properties of a Glaze
Low Fire White Talc Casting Body Recipe
Make Your Own Ball Mill Stand
Making Glaze Testing Cones
Monoporosa or Single Fired Wall Tiles
Organic Matter in Clays: Detailed Overview
Outdoor Weather Resistant Ceramics
Overview of Paper Clay
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified or Granito Tile
Rationalizing Conflicting Opinions About Plasticity
Ravenscrag Slip is Born
Recylcing Scrap Clay
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Single Fire Glazing

Some Keys to Dealing With Firing Cracks
Stoneware Casting Body Recipes
Substituting Cornwall Stone
Super-Refined Terra Sigillata
The Chemistry, Physics and Manufacturing of Glaze Frits
The Effect of Glaze Fit on Fired Ware Strength
The Four Levels on Which to View Ceramic Glazes
The Majolica Earthenware Process
The Physics of Clay Bodies
The Potter's Prayer
The Right Chemistry for a Cone 6 MgO Matte
The Trials of Being the Only Technical Person in the Club
The Whining Stops Here: A Realistic Look at Clay Bodies
Those Unlabelled Bags and Buckets
Tiles and Mosaics for Potters
Toxicity of Firebricks Used in Ovens
Trafficking in Glaze Recipes
Understanding Ceramic Materials
Understanding Ceramic Oxides
Understanding Glaze Slurry Properties
Understanding the Deflocculation Process in Slip Casting
Understanding the Terra Cotta Slip Casting Recipes In North America
Understanding Thermal Expansion in Ceramic Glazes
Unwanted Crystallization in a Cone 6 Glaze
Variegating Glazes
Volcanic Ash
What Determines a Glaze's Firing Temperature?
What is a Mole, Checking Out the Mole
What is the Glaze Dragon?
Where Do I Start?
Why Textbook Glazes Are So Difficult

Soluble Salts in Minerals: Detailed Overview


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. By Nilo Tozzi


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:

  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 sulfur 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 sulfates.

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 decrease 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 enameling 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. These can lead to a local alteration of enamel composition, which therefore becomes more brittle.
  2. The decomposition of calcium and magnesium sulfates 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 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
CaSO42.09Separates into CaO and SO3 between 1000 and 1125C
CaSO4.2H2O2.41Dehydration 140 - 150C
Separates into CaO and SO3 between 1000 and 1125C
CaMg(CO3)20.32In pastes it begins to separate into Cao, MgO and CO2 at approx. 750C
MgCl542.5Complete pyrolysis at 700C
MgCO30.106In pastes it begins to separate into MgO and CO2 at approx. 700C
MgSO4260.0Begins to separate at approx. 750C
Na2CO371.0Melts at 851C
NaCl357.0Complete pyrolysis at 700 C
Na2SO447.6Decomposes between 650 and 700C
K2CO31120.0Melts at 891C
KCl347.0Complete pyrolysis at 700C
K2SO468.5Decomposes 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.

Process for individuating soluble salts in clays.

  1. Extraction of salts from material.
    The finely dry-ground clay (< 100 micron) is dispersed in distilled water for an hour, at ambient temperature.
  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 sulfates 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 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 sulfates without extraction and filtering of the soluble salts.


Individuation of sulfates 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 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.

Individuation of sulfates via dosing.

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


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.


Related Information


Materials Barium Carbonate
Glossary Efflorescence
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
Hazards The Use of Barium in Clay Bodies
Considerations regarding the use of barium carbonate in pottery and structural clay bodies for precipitation of soluble salts.

By Nilo Tozzi

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