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a=0.607n/D

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based on the facts that the glass that defines ASTM color number 0.5 is closely equivalent to 1.03 Saybolt disk (as shown on fig. 28) and that thickness on the ASTM scale has been changed to 1.25 in from 33.0 mm [7].

b. Vegetable Oils

In addition to the brown pigments found in petroleum products, edible vegetable oils (corn, cottonseed, olive, peanut, rapeseed, sesame, soy-bean) usually contain some chlorophyll. Although metamerism exists between the oils and Lovibond glasses [100], the oils are graded, for commercial purposes, by the number (N) of Lovibond red units, added to Lovibond 35-yellow, that are required to match a 5.25-in layer of the oil. Table 16 shows for values of N from 0 to 100 on the $35Y+NR$ scale the CIE source C chromaticity coordinates, $x,y$ , and the nearest chromaticity match on the ASTM scale and on the Gardner scale (the Gardner scale is described in the next section).

Table 16. *Lovibond 35-yelloiv plus N-red scale used to color-grade edible vegetable oils. CIE chromaticity coordinates*, x,y, *and nearest chromaticity match on the ASTM and Gardner color scales*
Lovibond 35Y + NR scale	Chromaticity coordinates		Nearest chromaticity match on:
Lovibond 35Y + NR scale	$x$	$y$	ASTM color scale	Gardner color scale
0	0.441	0.521	1.3g	9.2g
1	.455	.508	1.4g	9.9g
2	.469	.496	1.6g	10.7g
3	.482	.485	1.9	11.3
4	.494	.474	2.1p	11.7
5	.506	.464	2.4p	12.2p
6	.517	.455	2.6p	12.6p
7	.527	.446	2.8p	12.9p
8	.537	.438	3.0p	13.2p
9	.546	.431	3.3p	13.6p
10	.555	.424	3.5p	13.8p
12	.570	.412	3.8p	14.4p
14	.583	.402	4.0p	14.9p
16	.594	.393	4.3p	15.3p
18	.603	.385	4.5p	15.6p
20	.612	.378	4.7p	15.9p
24	.625	.367	5.1p	16.5p
28	.636	.358	5.4p	17.0p
32	.645	.351	5.6	17.4
36	.652	.345	5.8	17.7
40	.658	.340	6.0	18.0
45	.665	.334	6.1
50	.670	.329	6.3
60	.679	.321	6.6
70	.685	.314	6.8
80	.691	.309	7.0
90	.695	.305	7.1
100	.699	.301	7.3

Thomson [50] showed that spectral transmittance of oil at wavelengths 460, 550, 620, and 670 nm $r_{460}$ , $r_{550}$ , $r_{620}$ , and $r_{670}$ ) can be expressed in an index that gives excellent correlation with color grading by the Lovibond red units. This index,

{\begin{aligned}1.29\log(1/r_{460})&+69.7\log(1/r_{550})\\+41.2\log(1/r_{620})&-56.4\log(1/r_{670})\\\end{aligned}}

has been found useful for commercial grading of vegetable oils.

c. Rosins and Paint Vehicles

Gum rosin has been graded by color for more than 50 years. Up to 1914 the color standards were made of rosin itself in spite of the relative impermanence of its color, and from 1914 to 1936 standards composed of combinations of Lovibond glasses were used. Brice [15] has described the selection of the present twelve official standards composed of two components of colored glass combined with one component of clear glass, all three cemented together with Canada balsam. The various combinations are given letter designations denoting the grades of rosin delimited by them and have legal status under the Naval Stores Act. The cemented face of the clear glass in each combination is fine ground so as to duplicate the slight turbidity characteristic of molded samples of rosin, which commonly contain traces of fine dirt. The chromaticity spacing was adjusted by means of the uniform-chromaticity-scale triangle of figure 8 so as to progress regularly from small steps for yellow rosins to steps of about four times the initial size for reddish orange rosins. Osborn and Kenyon have studied rosin colors spectrophotometrically [124]. Table 17 gives the names associated with letter designations, luminous transmittance, $T$ , and chromaticity coordinates, $x,y$ , for CIE source C, together with the nearest chromaticity match on the ASTM, $35Y+NR$ , and Gardner color scales.

Many special color scales have been set up for the specification of paint vehicles (varnishes, linseed oil, tung oil, etc.). A solution of nickel sulfate and iodine [16] is used to define the darkest color permissible for spar varnish. A color comparator having eighteen glass color standards made by Hellige, Inc., has been used for similar purposes. The Pfund color grader compares a variable thickness of the unknown specimen with a variable thickness of an amber colored glass [131]. The standard is wedge shaped, and the cell for the specimen is likewise wedge shaped. The Parlin (or Cargille) color standards consist of a set of thirty-five solutions. The first ten are Hazen platinum-cobalt solutions, [52], developed originally to measure the color of natural water and still used for that purpose under the name of APHA (American Public Health Association) standards [4]. The remainder of the Parlin color standards are caramel solutions. They have been adopted by the ASTM (Designation D365–39) for testing the color of soluble nitro-cellulose base solutions. The Pratt and Lambert color standards are varnish mixtures calibrated against the Pfund color grader. The Du Pont, colorimeter employs six plates as color standards, together with a wedge of the same glass permitting a continuous variation of color between the standards. The Gardner color standards consist of eighteen combinations of the red and yellow Arny solutions. Gardner has determined the Arny and

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