Three different measurements
| Measure | Typical use | How it is measured | Note |
|---|---|---|---|
| ISO brightness (R457) | Paper filler and coating kaolin; pigments | Diffuse blue reflectance at 457 nm on a pressed powder pad, ISO 2470 method | One wavelength only, so it responds strongly to yellowness |
| Whiteness / colour (L*, a*, b*) | Paint, plastics, rubber fillers | Spectrophotometer on a pressed sample; whiteness indices are calculated from the colour values | b* shows yellowness; a* shows red–green tone |
| Fired whiteness | Ceramic kaolin | A test piece is fired at an agreed temperature (for example within 1250–1300 °C) and its colour is measured | Depends on Fe₂O₃, TiO₂ and firing conditions |
| Calcined brightness | Calcined grades | Same instrument method, on calcined powder | Calcination typically raises brightness |
Why iron and titanium lower whiteness
Ferric iron absorbs blue light, so iron-stained kaolin looks yellow or cream, and a 457 nm reading drops quickly. Titanium minerals such as anatase, especially when they carry some iron, give a grey or yellow cast. Organic matter makes the clay look greyer. After firing, iron and titanium together determine the fired colour, so a ceramic kaolin needs both Fe₂O₃ and TiO₂ low, not only the unfired whiteness high. This is why iron removal is the main lever for whiteness.
Other factors that move the reading
- Particle size. Finer particles scatter more light, so a finer fraction of the same clay reads brighter.
- Sample preparation. Moisture, how the powder is pressed and the surface of the sample all change the result.
- Instrument and geometry. Different instruments, illuminants or UV settings give different numbers for the same sample.
- Calcination. Heating removes organic matter and combined water and creates fine voids that scatter light — see hydrous vs calcined kaolin.
What to specify by industry
- Paper: ISO brightness together with % < 2 µm and viscosity.
- Paint, rubber, plastics: dry-powder whiteness and D50, plus oil absorption.
- Ceramics: fired whiteness at your firing temperature, plus Fe₂O₃ and TiO₂.
- Fiberglass and catalysts: usually Fe₂O₃ limits matter more than whiteness itself.
Our grade pages list typical reference values for each: paper, fillers, ceramics. These are typical references, not guarantees; final values are agreed per order.
Comparing results between suppliers
Ask for the test method, instrument, illuminant, sample preparation and moisture basis behind every whiteness figure. Two numbers from different methods are not comparable. The most reliable approach is to test samples from each supplier on your own instrument, using your own sample preparation, before agreeing an acceptance limit. Our specification sheet guide lists the usual methods.
Frequently asked questions
What is the difference between brightness and whiteness?
Brightness (ISO 2470, R457) is a single-wavelength blue reflectance used mainly in paper. Whiteness is calculated from full colour measurements and also takes yellow or other tints into account. A sample can have high brightness and still look slightly tinted.
Why is fired whiteness different from the unfired whiteness of kaolin?
Heating changes the iron and titanium compounds in the clay, and the firing atmosphere and temperature affect the colour. Ceramic users therefore specify fired whiteness on a test piece, not only the powder reading.
How can kaolin whiteness be improved?
By selecting better ore, removing iron- and titanium-bearing minerals with magnetic separation, bleaching fine iron staining, classifying to a finer particle size and, for some grades, calcining.
Related guides
Need a sample or a quote?
Free samples available on request. Build your spec in two minutes and send it via WhatsApp or email.
Published: · Updated: