If you deal with white minerals and fillers you have probably pondered this question at some time or other. Comparing the whiteness of one white powder with another is not so easy. Especially if you only have the data sheets to go on!
Measuring the whiteness of minerals is a complex subject. What we need are a few simple guidelines that are easy to use.
Let’s look at the simplest method first. If you have samples of the two minerals, you can do a comparison by eye. I suggest you pour out some powder on to a plain dark background. Press some flat with your finger. Wet a corner of it, then compare the two. Which is whiter? Is it greyish or yellowish?
The lighting in the room can affect this method a lot, so it is only a very rough test.
Another plan is to keep a standard and compare your new sample by eye to your standard – a quick and easy way to check how different batches compare with each other.
I am always impressed how good paint chemists are at this. They make up a paint sample with the standard and one with the new filler, and then they do a drawdown. This compares two films of paint against a black and white striped background and is a quick way to check whiteness as well as hiding power (opacity).
A more accurate approach is to measure the whiteness with a spectrophotometer. There are several types and makes of these; the one usually used to test kaolin is the Elrepho spectrophotometer.
The measurements you get from a spectrophotometer are usually taken at different wavelengths. A list of these reads something like this: X, Y, Z, L*, a*, b*, x, y, Rx, Ry, Rz, R457, CIE Whiteness/Tint-D65 resp. C, Fluorescence, ISO-Brightness, D65-Brightness, Yellowness, Opacity, Transparency, K/N-value, IE, Residual ink, DW, pe, s, k, sR457, kR457, Metamerism, ΔE.
So which of these parameters should be used for comparing whiteness? More than one from this list can be used – the confusion arises because we don’t all use the same one! It doesn’t matter so much which one we use, but for comparing apples with apples, we should use the same parameter for the comparison.
The paper industry has standardised on ISO-Brightness, which is the same as the R457 Reflectance measurement, measured at 457nm. So if you find one mineral’s data sheet reports ISO-Brightness and another says Reflectance R457, you can use these as one and the same and easily compare one mineral to another.
Whiteness Ry is the other common parameter used as a measure of whiteness. You will often find this parameter on talc and calcium carbonate data sheets. However it gives a different (usually higher) result to the R457 measurement.
Some mineral suppliers however, just report the Y-value as a whiteness measurement.
So read the data sheet carefully. If you need a different parameter to make a particular comparison, ask the supplier for it – he has probably measured it but has just not reported it on his data sheet.
If you are given the L*, a* and b* values (sometimes called CIELAB colour space) these can tell you a bit more about the colour of that mineral than just whiteness.
These values were designed to approximate human vision. The L component closely matches human perception of lightness and is given on a scale of 0 to 100.
The b-value gives a reading between -1 and +1 on the b-axis or the yellow/blue scale. The a-value is similarly a measure of greenness or redness as shown on this schematic:
Look out for a high b-value. This means the mineral is yellowish. Similarly, a positive a-value indicates reddishness.
Hope this helps you. If you are in the Northern hemisphere, you are probably looking a bit blue-ish yourself.