The document outlines an enhancement to the existing ASTM D7918 methodology for evaluating grease color using colorimetric spectroscopy. It introduces a new spectrometer that allows for variable pathlength assessment, addressing limitations from excessive absorbance in standard testing. Proposed next steps include integrating a new colorimeter, revising the ASTM standard, and continuing research on grease-specific color relationships.

1. Variable Pathlength Colorimetric
Spectroscopy of Greases; an
enhancement to D7918
Rich Wurzbach, MRG Labs
York, PA, USA
12/12/18

2. Introduction
Grease comes in a wide array of colors
New and In-Service Grease can be evaluated
Methodology introduced in ASTM D7918-15
iLab, Spectroscopy Box, Thin Film Substrate of
grease

3. 3
CIE Color Space Chromaticity Diagram

4. 4
L*
40
50
60
70
80
90
a*
40
50
60
70
80
90
b*
40
50
60
70
80
90
CIE color space chromaticity diagram
ASTM E308 Defines CIE Color
a* is the change in color in the red spectrum.
a* is positive in the red direction and negative in the
green direction.
b* is the change in color of the yellow spectrum.
b* is positive in the yellow direction and negative in the
blue direction.
L* is the change in shade from white to black.

5. Existing D7918 Optical
Spectroscopy Box

6. Grease Color Examples
Three different
greases
Follows the patterns
identified by the
chromaticity diagram
Colors Sample 1 2 3
Light…dark
L* 48.6 63.5 61.3
Red…green
a* 42.5 13.6 25.8
Yellow…Blue
b* -14.0 -52.1 3.8

7. New Grease to End of Life
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
400 450 500 550 600 650 700
Wavelength (nm)
Transmission
New Fluid
In-Service Fluid
End of Life Fluid

8. Limitations of Method in D7918
iLab Spectrometer is called out in method
iLab Spectrometer is no longer manufactured
or supported
A review of 5+ years of testing shows that the
majority of in-service submitted grease
samples are “flatlined” in the visible spectrum
due to excessive absorbance at the 1mm
nominal thickness of die extrusion

9. iLab following D7918: limited
response, “black grease” #384

10. Grease under varying conditions
New grease (Vigo)
Significantly dark (3418)
Moderately dark (384)
Slightly dark (389)
Significantly dark (391)

11. Improvements to D7918
Introduction of a new spectrometer
Recognition of the limitations of method
due to excessive absorbance
Development of new method to allow
variable pathlength of the grease sample
Allows for a screening process for greases
without requiring die extrusion
High transmittance samples (light, new)
and low transmittance samples (used)

12. Sample 3418; current D7918 result

13. Sample 3418: 0.035” path vs.
0.010” path

14. Sample 384; current D7918 result

15. Sample 384: 0.035” path vs.
0.010” path
dE iLab 0.035 0.010
3418 139.1 50.0 38.3
384 209.9 45.6 13.8

16. Sample 2885; current D7918 result

17. Sample 2885: 0.035” baseline
vs. inservice – “Red” grease

18. Next Steps
Inclusion of new Grease Thief Colorimeter
in current ASTM Inter-Laboratory Study
(ILS)
Submit research report and ballot revision
to D7918, removing spectrometer-specific
references, and retaining spectrum option
Continue to find grease-specific data
relationships to color-influencing
parameter changes
Continue to evaluate and create new
color-based and grease specific criteria