(ASTM D 1544, ASTM D 6166, AOCS Td 1a, MS 817 Part 10)
The Gardner Colour scale as specified in ASTM D1544 is a single number, one dimensional, colour scale for grading the colour of similarly coloured liquids such as resins, varnishes, lacquers, drying oils, fatty acids, lecithin's, sunflower oil and linseed oil.
The scale ranges from a pale yellow to a red in shade and is described in terms of the values 1-18. The glass standards used with the comparator can achieve a resolution of 1 unit, automatic PFX and PFXi instruments 0.1.
To comply with specifications a 10mm path length must be used.
The light yellow Gardner colour numbers (1 to 8) are based on potassium chloroplatinate solutions, numbers 9 to 18 on solutions of ferric chloride, cobaltous chloride and hydrochloric acid.
In 1958 The Tintometer Ltd were instrumental in the development of the master glass standards that were utilised when the current Gardner scale was specified in 1963; the earlier 1933 and 1953 versions are available upon request in the form of Comparator 2000 discs.