Measuring Pressed Powder Plaques Using the LabScan XE

The color of powders that are used in manufacturing, such as limestone or raw pigment powders, is often measured to evaluate how the manufacturing process is proceeding or to estimate the color of the final product. In other cases, a powder may be the final product itself, such as cosmetic facial powder or baking powder. Color measurement of these types of products is often necessary to ensure lot-to-lot color consistency before they are shipped to the end user.

A HunterLab LabScan XE 0/45 spectrophotometer in the port-down orientation with the 1-inch area of view can be used to measure the reflectance of powders pressed into a smooth plaque. This is the most common method advocated by HunterLab for the measurement of finely-ground material.

The LabScan XE

The Application

Powders have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.

• Powders come in the form of fine particulates — not a solid sample — and must be pressed and smoothed in order to be effectively made a solid.

• Powders are translucent — not opaque. Using a sufficient and constant sample thickness (at least 5 mm) will make the powder effectively opaque.

• Powders may be slightly fluorescent, which means that they will be sensitive to the UV content of the light source.

Recommended Color Scale	CIE L*a*b* as a full color descriptor
Recommended Single-Number Indices	YI D1925 (2/C), YI E313, WI E313 for indication of yellowness and/or whiteness
Recommended Illuminant/Observer	D65/10°. C/2° may also be used.

Measurement Method

1. Configure your software to read using the desired color scale, illuminant, and observer.
2. Orient the LabScan XE with the port facing down using the port-down stand (HunterLab Part Number D02-1009-350). Replace the regular port insert with the 1-inch port insert. Standardize the instrument for the 1-inch area of view using the black and calibrated white standards that come with the instrument.
3. Insert the holder base of a Zeiss powder press** into the round side assembly. The holder will provide the constant background for your powder samples.
4. Place the press's barrel into the holder base.
5. Weigh a fixed amount of powder into the barrel. Using a sufficient amount of sample each time powders are measured ensures that the powders are effectively opaque for reflectance measurement.
6. Place the press lid over the press.
7. Screw the lid down firmly to press the powder into a plaque.
8. Remove the press lid, then remove the holder base from the round side assembly. A smooth, flat surface is now available for measurement.
9. Place the pressed powder in its holder at the sample port. The powder should be facing upward, flush at the port.
10. Take a single color reading of the plaque as viewed through the port insert. Empty the press, reweigh more sample, repress and remeasure it at least two times from the same batch. Average the two color readings for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
11. Record the average color values for the sample batch.

**The stainless steel hand powder press pictured here may be obtained from Analysen Technik Gerate in Germany at www.atg-gmbh.com using item #507882 0000 000. Other types of powder presses (such as the Carver Mini C manual press) may be ordered from Carver, Inc., www.carverpress.com.