Measuring Degrees of Yellowness with Gardner Index - AN-1004

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The Gardner Scale is a visual scale originally developed in the 1920s to describe the color of drying oils, varnishes, fatty acids, polymerized fatty acids and resin solutions. These liquids are generally a moderately-saturated greenish or reddish-yellow color in the raw form, and get progressively clearer at higher levels of processing. As the science of color measurement has developed, the Gardner Scale has been correlated with colorimetric scales. This application note considers the original visual scale and its relationship to instrumental methods ASTM D1544 and D6166.

Challenge: To relate visual Gardner color scale to a colorimetric method with an Instrument.

The original Gardner visual scale describes a degradation of near white or clear products due to age, exposure to light/chemicals and processing. The Gardner scale contains a set of 18 visual transparent standards made of potassium dichromate, ferric chloride, cobaltous chloride, and potassium chloroplatinate to set levels of yellowness. These solutions although sealed in glass, were unstable, faded with time and difficult to reproduce. These chemical solutions were eventually replaced with glass filters on a wheel (See Figure 1).

As the standards range from 1 to 18, the color goes from light to dark, increasing in dominant yellow saturation, and shifting from a greenish tint to a red tint.

With the advent of instrumentation, the visual scales were correlated with colorimetric scales. The first of these colorimetric scales provides a relationship between Garder Index and CIE Yxy (ASTM D1544). The second published method of comparison triangulates the relationship to chromaticity values (ASTM D6166).

The Gardner Scale was developed separately from other Yellowness indices such as ASTM E313, ASTM D1925 and APHA.

Gardner Scale vs. ASTM D1544 (Standard Test Method for Color of Transparent Liquids)

To relate the visual Gardner standards to a colorimetric measurement, the eighteen visual Gardner standards are compared to CIE Chromaticity Coordinates (Table 1) in a lookup table. Measurements are made on any colorimeter or spectrophotometer and read in transmittance using a 20 mm cell. The results are shown in Table 1 on the following page. (see attached file below)

Gardner Index vs. ASTM D6166

In June 1997, the ASTM D01.34 (Naval Stores) subcommittee approved another instrumental correlation to the visual Gardner Color Scale. This new instrumental Gardner scale is based on a 10 mm path length transmission measurement and is described in ASTM D6166, Standard Test Method for Color of Naval Stores and Related Products (Instrumental Determination of Gardner Color). This color scale triangulates the chromaticity coordinates and expands the values to include a decimal. This scale is thought to be more robust for off-hue samples.

Both of these scales are widely used today and are implemented in the EasyMatch QC software packages.

Summary

The Gardner D1544 using Chromaticity Coordinates, and D6166 indices are all effective for instrumentally measuring the Gardner Color of chromatic yellow liquids. These methods remove the subjectivity of visual measurements and the problem with reproducibility of making visual standards but still strongly relate to the original visual color scale.

References

Huebner, Fred E. and Harry N. Monck, “Measurement of Color in Resins and Adhesive Systems“Journal, September 1992.

ASTM D6166, Standard Test Method for Color of Naval Stores and Related Products.

ASTM D1544, Standard Test Method for Color of Transparent Liquids.

 

(See attached for the complete article)

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