The Gardner Scale: Modern Applications for Colour Measurement of Transparent Liquids

The Gardner Scale allows for standardized colour measurement of yellow to brown transparent liquids and is instrumental in a range of industries. Image Source: Unsplash user Sonja Langford

In some transparent liquids, colorlessness is king and manufacturers rely on the APHA/Pt-Co/Hazen colour scale to ensure both chemical purity and aesthetic appeal of colorless and near colorless liquids. Other transparent liquids, however, have different colour properties, either by nature or by design, that require a separate, specialized colour measurement scale for yellowness beyond that offered by the APHA. Yellow to brown transparent chemicals, liquid pharmaceuticals, food products, and beverages are ideally suited for Gardner Scale measurement.

The Gardner Scale and Colour Consistency

The Gardner Scale was developed in the 1920s to classify the colour of drying oils, varnishes, fatty acids, polymerized fatty acids, and resin solutions. The scale originally consisted of 18 liquid standards composed of potassium dichromate, ferric chloride, cobaltous chloride, and potassium chloroplatinate and represented in sealed glass tubes for visual comparison, ranging from light yellow (1) to dark brown (18). These liquids were vulnerable to instability, colour variation, and reproduction difficulties, and were eventually replaced by glass filters in 1958 in an attempt to enhance precision. However, the Gardner Scale remained a visual evaluation method relying on the subjective human gaze, leading to inaccuracies and difficulties maintaining colour consistency.

Monitoring the colour of linseed oil allows manufacturers to produce the precise consistency consumers are seeking. Image Source: Flickr user Alsatian

Integrating Spectrophotometric Colour Measurement

With the advent of more advanced colour measurement technologies, the Gardner Scale became correlated with colorimetric scales, most importantly the CIE Chromaticity Coordinates. The articulation of this relationship led to the development of ASTM D1544, Standard Test Method for Colour of Transparent Liquids (Gardner Colour Scale).1 The conversion of the scale to precise colour data also allowed for integration with sophisticated spectrophotometric instrumentation. In 1997, a new spectrophotometer-based Gardner Scale methodology was introduced to provide even more precise colour measurement via spectral analysis. Using a 10 mm path length transmission measurement, the Gardner value is calculated by “triangulat[ing] the chromacticity coordinates and expand[ing] the values to include a decimal.” This measurement method is described in ASTM D6166, Standard Test Method for Colour of Naval Stores and Related Products (Instrumental Determination of Gardner Colour) 2

The detailed chromatic data offered by spectrophotometric analysis provides important information for manufacturers across industries. Divergences from colour standards serve as warnings that something has gone awry either with the raw material or in the manufacturing process, and can help operators pinpoint the variables creating undesirable results. Objective analysis allows for increased efficiency and accuracy, ultimately enhancing both your production process and finished product, whether you are manufacturing liquid pharmaceuticals, soft drinks, perfumes, or wood finishing varnish.

The Relationship Between the APHA and Gardner Scales

The APHA scale offers more precise measurement for colorless and near colorless liquids while the Gardner scale provides detailed chromatic information for liquids with more yellowness. However, the two do overlap at the top end of the APHA scale and low end of the Gardner scale; “an APHA/Pt-Co/Hazen 500 colour standard is considered to be just under a Gardner 2.” In some applications, a liquid that starts out requiring Gardner colour measurement by virtue of its high level of yellowness may necessitate APHA measurement as refinement processes remove contaminants and produce greater water whiteness. As such, the two standards can work hand in hand to cover a full spectrum of colour quantification throughout production.

HunterLab’s EasyMatch QC software allows you to easily generate Gardner colour measurement data. Image Source: Unsplash user Damian Zaleski

HunterLab and the Gardner Scale

Today, HunterLab offers a range of spectrophotometric instruments ideal for generating Gardner Scale colour measurement values in transparent liquids, including the Vista, a dedicated transparent liquid spectrophotometer, and UltraScan PRO, a versatile reference instrument that can handle a range of solid and liquid materials. Our user-friendly Easymatch QC software makes it simple to implement appropriate ASTM protocols to produce reliable Gardner Scale data while also allowing for APHA measurement, should it be needed. We have the expertise to help you select the instrumentation that makes sense for you and your industry. Contact us today to learn more about our colour measurement technologies and outstanding customer service.