Key points for selecting feldspar powder
To prepare this report, the Minerallum team reviewed and compared scientific papers, university research and institutional references concerning the use of feldspar in ceramic manufacturing. We brought together findings on chemical composition, mineralogy, particle-size distribution and firing behaviour to identify the criteria that genuinely matter when selecting feldspar powder.
Our analysis leads to a clear conclusion:
Feldspar powder should not be selected only by its commercial name or price per tonne.
To compare two products, we examine at least:
- whether the material is sodium-, potassium- or mixed-feldspar;
- chemical composition;
- mineral phases;
- particle-size distribution;
- moisture and loss on ignition;
- colour before and after firing;
- behaviour in the slip or dry mix;
- densification in the actual firing cycle;
- lot-to-lot consistency;
- documentation and delivered cost.
Feldspar is used mainly in glassmaking and ceramics. In a ceramic body, it acts as a flux: alkali oxides promote the formation of a glassy phase during firing and contribute to densification [1–3].
| Purchasing decision | Data to examine | Industrial objective |
|---|---|---|
| Select the feldspar family | Na₂O, K₂O, CaO and mineralogy | Adjust fusibility and the firing window |
| Control colour | Fe₂O₃, TiO₂ and fired test | Maintain the required whiteness or shade |
| Define fineness | Residue, D10, D50, D90/D97 | Control milling, rheology and firing reaction |
| Compare lots | XRF, XRD, moisture and particle size | Limit production drift |
| Validate the application | Test pieces in the real recipe | Measure shrinkage, absorption, strength and deformation |
| Compare offers | Dry mass and delivered cost | Evaluate the actual industrial cost |
Why is feldspar used in ceramics?
Feldspars are a family of aluminosilicates containing mainly potassium, sodium or calcium. Their industrial value comes not only from their abundance but also from their chemical and thermal contribution to ceramic formulations [1,3].
A traditional ceramic recipe normally combines several functions:
- clays and kaolins provide plasticity and phase precursors;
- quartz contributes to the structure and affects shrinkage;
- feldspar supplies a fluxing fraction;
- other raw materials adjust colour, fusibility, expansion or final properties.
As temperature rises, feldspar participates in the formation of a liquid or glassy phase. This phase helps bring particles together, reduce porosity and consolidate the microstructure.
The result nevertheless depends on the entire system:
- body composition;
- flux-to-filler ratio;
- particle size;
- preparation method;
- forming pressure;
- heating rate;
- peak temperature;
- soaking time;
- firing atmosphere;
- cooling rate.
The European BREF for ceramic manufacturing shows the range of affected processes: wall and floor tiles, sanitaryware, tableware, refractories, technical ceramics and other fired products [2]. The same powder cannot therefore be qualified as “ideal for ceramics” without identifying the application.
Sodium, potassium or mixed feldspar
Sodium feldspar
Sodium feldspar is associated with a higher Na₂O content and with albite as a major or significant phase.
In the comparative study by Das and Dana, the sodium-rich feldspar body achieved full vitrification at a lower temperature than the potassium-rich body in the tested formulations [5].
A separate sanitaryware study showed that, for the investigated cycle, the combination of sodium feldspar with the smallest particle size produced water absorption below 0.5% and a large glassy-phase content [6].
We take from these studies that sodium feldspar can be valuable where the formulation requires rapid densification or strong fluxing action. This tendency must be considered together with liquid-phase viscosity and deformation risk.
Potassium feldspar
Potassium feldspar is associated with a higher K₂O content and phases such as orthoclase or microcline.
It can provide different melting development and viscosity. In some formulations, this difference can help widen the firing window or limit an excessively fluid liquid phase.
The word “potassium” is still not enough. Two materials can contain different proportions of microcline, orthoclase, albite, quartz, mica and other phases.
Mixed feldspar
Mixed feldspar or a combination of sodium and potassium feldspars can be used to adjust:
- the start of densification;
- liquid-phase volume;
- glass viscosity;
- shrinkage;
- water absorption;
- mechanical strength;
- dimensional stability.
A study published by the Journal of the Ceramic Society of Japan obtained, in its experimental porcelain-tile formulation, water absorption of 0.03% and flexural strength of 78.3–85.8 MPa for a K₂O–Na₂O system fired between 1170 and 1190°C [9].
Those results belong to the specific recipe, raw materials and firing cycle. Their value for purchasers is to show that the balance between fluxes can matter as much as total alkali content.
Chemical composition: parameters to request
Bulk chemical composition is commonly obtained by X-ray fluorescence.
The main parameters monitored in feldspar powder include:
| Parameter | Purpose in the assessment |
|---|---|
| SiO₂ | Contributes to the silicate network and total body balance |
| Al₂O₃ | Contributes to structure and glass-phase viscosity |
| Na₂O | Alkali oxide associated with sodium fluxing action |
| K₂O | Alkali oxide associated with potassium fluxing action |
| CaO and MgO | Can alter phase equilibria and fusibility |
| Fe₂O₃ | Important for colour and selected firing reactions |
| TiO₂ | Monitored where high whiteness is required |
| Loss on ignition | Indicates mass change from volatile or decomposing constituents |
| Moisture | Affects purchased dry mass and storage |
Alkalis should not be added together without analysis
A value such as “K₂O + Na₂O = 10%” does not fully describe behaviour.
The following also matter:
- Na₂O/K₂O ratio;
- phases carrying those oxides;
- calcium and magnesium;
- free quartz;
- mica and accessory minerals;
- product fineness;
- firing cycle.
Feldspars with a similar total alkali content can behave differently because their mineralogy and the eutectics formed are not identical.
Iron, titanium and fired colour
Iron and titanium control is particularly important for white bodies.
Final colour also depends on:
- iron oxidation state;
- kiln atmosphere;
- other raw materials;
- temperature and soaking time;
- glassy phase;
- piece thickness and geometry.
We therefore recommend combining XRF with a comparative firing test and, where colour is critical, an instrumental L*, a*, b* measurement.
Mineralogical composition: the often-missed parameter
XRF shows how much of each element or oxide is present. It does not automatically show the crystalline phases in which they occur.
X-ray diffraction can identify phases including:
- albite;
- microcline;
- orthoclase;
- quartz;
- mica;
- kaolinite or other clay minerals;
- calcite or dolomite;
- accessory phases.
The Moroccan study by Arib et al. is particularly relevant to the Moroccan ceramic industry. Two feldspars mined in Morocco were compared with a commercial feldspar through grindability, chemical and mineralogical composition, thermal behaviour and ceramic-product properties [4].
The authors concluded that, beyond suitable grindability, the mineralogical composition of the feldspar source was decisive for the technological and mechanical properties of the sintered products.
Our reading of this study leads to a practical rule:
Feldspar should not be accepted only because its average chemistry resembles that of the current product.
Substitution should also compare mineralogy and behaviour in the body.
Particle size: why D50 is not enough
An industrial powder has a particle-size distribution, not one particle size.
According to the process, the specification can include:
- residue on 45 µm, 63 µm or another sieve;
- fraction passing a limit;
- D10;
- D50;
- D90 or D97;
- full particle-size curve;
- specific surface area;
- agglomerate content.
Effect on slip behaviour
Phelps and Dennis showed that specific-surface variations arising from different particle-size distributions could be a major source of variation in feldspar-containing slips [7].
At the same average fineness, a larger ultrafine fraction can change:
- viscosity;
- water demand;
- deflocculant demand;
- suspension stability;
- casting rate;
- drying.
Two products with the same D50 are therefore not necessarily equivalent.
Effect during firing
Reducing particle size generally increases the surface available for reaction. A study on semi-transparent porcelain observed that reducing quartz and potassium-feldspar particle size altered the liquid phase and increased densification in the investigated system [8].
The sanitaryware study discussed above also compared 45 and 75 µm feldspar and observed an interaction between flux type, particle size and firing cycle [6].
The finest powder is not automatically the most economical
Finer grinding can:
- increase energy consumption;
- accelerate equipment wear;
- increase specific surface area;
- alter rheology;
- require tighter control of fines;
- generate more dust during handling.
The correct fineness is the one that provides the target behaviour with stable processing and controlled overall cost.
Behaviour during firing
Firing a ceramic body is not equivalent to melting feldspar in isolation.
Several transformations overlap during the cycle:
- residual-water removal;
- dehydroxylation of clay minerals;
- quartz transformation;
- intermediate phase formation;
- partial dissolution of quartz and feldspar;
- mullite formation;
- glassy-phase evolution;
- pore closure;
- shrinkage;
- possible bloating or deformation.
A kinetic study of an industrial spray-dried porcelain-stoneware body followed densification, mullite precipitation and feldspar and quartz dissolution [10]. It confirms that the outcome depends on temperature and time.
Vitrification window
A vitrification curve generally relates temperature to:
- water absorption;
- shrinkage;
- apparent density;
- sometimes mechanical strength.
A sufficiently broad window supports industrial stability. A liquid phase that forms too early or becomes too fluid can increase pyroplastic deformation. Insufficient fluxing action can leave water absorption too high.
Fast firing
Industrial tile cycles impose different kinetics from long firing cycles.
The powder should therefore be assessed with:
- actual heating rate;
- peak temperature;
- time in the maturation zone;
- cooling;
- piece thickness;
- compaction pressure;
- full body composition.
A slow laboratory test can characterise the raw material, but industrial acceptance benefits from reproducing the plant cycle as closely as practical.
Criteria by ceramic application
| Application | Feldspar priorities | Useful tests |
|---|---|---|
| Porcelain stoneware | Fast densification, low absorption, dimensional stability | Vitrification curve, shrinkage, absorption, deformation |
| Porous tiles | Maturation temperature, porosity and glaze compatibility | Absorption, shrinkage, strength and body–glaze fit |
| Sanitaryware | Slip rheology, casting, whiteness and vitrification | Viscosity, thixotropy, casting rate, absorption and deformation |
| Porcelain and tableware | Whiteness, translucency, strength and thermal stability | Lab* colour, expansion, strength and porosity |
| Glazes | Fusibility, viscosity, surface tension and colour | Fusion button, substrate test, gloss and defects |
| Technical ceramics | Purity and strict phase control | XRF, XRD, particle size and functional properties |
The same feldspar can be useful in a tile body and unsuitable for white porcelain. Selection starts with the finished product, not the mineral name.
Control programme for a new lot
1. Identification and traceability
The file should connect:
- commercial name;
- origin;
- lot;
- production date;
- packaging;
- sampling date;
- sampling method;
- laboratory.
2. Physical controls
According to the application, define:
- moisture;
- bulk density;
- sieve residue;
- particle-size distribution;
- D10, D50 and D90/D97;
- powder whiteness or colour;
- flowability where needed.
3. Chemical and mineralogical controls
The programme can include:
- XRF;
- XRD;
- loss on ignition;
- iron;
- titanium;
- alkalis;
- calcium and magnesium;
- accessory-phase identification.
4. Technological tests
Test pieces should follow a documented method:
- same reference recipe;
- same water content;
- same milling or dispersion;
- same pressure or casting method;
- same drying;
- same firing cycle;
- same dimensions.
Possible measurements include:
- drying shrinkage;
- firing shrinkage;
- water absorption;
- density;
- porosity;
- flexural strength;
- fired colour;
- thermal expansion;
- pyroplastic deformation;
- defect observation.
5. Comparison with the reference
A new feldspar is easier to interpret when it is compared with the industrial reference product under identical conditions.
The result sheet can distinguish:
- improvement;
- equivalence;
- difference manageable by adjustment;
- process incompatibility.
Building a usable specification
A useful specification should not state only “200-mesh feldspar powder”.
Product identity
Specify:
- sodium, potassium or mixed feldspar;
- application;
- approved origin or source;
- appearance and colour;
- delivery form.
Composition
Define as required:
- SiO₂;
- Al₂O₃;
- Na₂O;
- K₂O;
- Na₂O + K₂O;
- maximum Fe₂O₃;
- maximum TiO₂;
- CaO and MgO;
- loss on ignition;
- moisture.
Mineralogy
Request, where critical:
- albite;
- microcline;
- orthoclase;
- quartz;
- mica;
- carbonates;
- other phases.
Particle size
Choose a measurable basis:
- maximum residue;
- D50;
- D90 or D97;
- full curve;
- test method;
- dispersion method;
- lot-to-lot tolerance.
Documentation
Plan for:
- technical data sheet;
- certificate of analysis by lot;
- XRF report;
- XRD report;
- particle-size curve;
- sample;
- retained reference lot;
- origin declaration;
- logistics documents.
Logistics
Specify:
- quantity;
- bags, big bags or bulk;
- unit mass;
- pallets;
- moisture protection;
- destination;
- Incoterm;
- schedule;
- split deliveries.
Comparing the actual industrial cost
The price per tonne must be placed in the process context.
Dry basis
For a material containing moisture:
Dry mass = delivered mass × (1 − moisture rate)
One tonne at 2% moisture contains 980 kg of dry material. Economic comparison should use the same basis.
Production performance
A higher-priced feldspar can become more economical when it helps:
- reduce firing temperature;
- shorten the cycle;
- lower water absorption;
- stabilise shrinkage;
- reduce defects;
- lower reject rate;
- improve strength;
- reduce another expensive component.
Conversely, a cheaper powder can increase:
- energy use;
- deflocculant demand;
- milling time;
- colour variation;
- deformation;
- sorting and rejects.
The useful calculation is therefore:
Industrial cost = delivered material + preparation + energy + additives + losses + non-quality
Contribution of Moroccan research
Research conducted in Morocco provides a particularly useful lesson for domestic and international purchasers.
The study by Arib et al. connected university, applied laboratory and ceramic-industry expertise. It evaluated two Moroccan sources against a commercial product [4].
The results show that assessment should combine:
- grindability;
- chemical composition;
- mineralogy;
- thermal behaviour;
- vitrification curve;
- mechanical properties.
This approach matches Minerallum’s view: connect raw-material characterisation with the real industrial requirement.
Our team does not assign universal performance to feldspar solely from its origin. We collect the relevant data, compare published research and structure the tests needed to understand its behaviour in a defined application.
Procedure before changing supplier
Step 1 - Define the reference
Retain:
- current product data sheet;
- recent lot results;
- particle-size curve;
- reference test pieces;
- cycle parameters.
Step 2 - Sample the new product
Sampling should represent the lot and packaging.
Step 3 - Compare analyses
Review:
- chemistry;
- mineralogy;
- particle size;
- moisture;
- colour.
Step 4 - Run laboratory tests
Use the same recipe and conditions as the reference.
Step 5 - Pilot trial
Move to a scale that reveals:
- preparation;
- rheology;
- spray drying or granulation;
- pressing or casting;
- drying;
- firing;
- sorting.
Step 6 - Monitored industrial lot
Document the parameters and compare results with production history.
Step 7 - Final specification
Set limits that are genuinely correlated with performance.
Conclusion: buy behaviour, not only powder
Our study shows that feldspar powder for ceramics should be defined at four levels.
The raw material
- sodium, potassium or mixed character;
- chemical composition;
- mineral phases;
- moisture;
- loss on ignition.
Particle size
- residue;
- D10;
- D50;
- D90 or D97;
- ultrafine fraction;
- lot-to-lot consistency.
The process
- milling;
- slip preparation;
- forming;
- drying;
- firing cycle;
- cooling.
The finished product
- shrinkage;
- absorption;
- density;
- strength;
- colour;
- expansion;
- deformation;
- defects.
The reviewed publications converge on one point: feldspar source, Na₂O/K₂O balance, particle size and firing cycle interact.
A well-structured enquiry therefore does not stop at:
“We need feldspar powder.”
It states:
“Here is our ceramic product, formulation, firing cycle, chemical and particle-size limits, required tests, volume and destination.”
With these elements, Minerallum can review the requirement, organise the comparison of characteristics and support the preparation of a documented supply.