what the ratings mean

 
I made the ratings of paint attributes on numerical scales so that I could conveniently write them down and present them compactly on a web page. This page explains exactly what each rating measures and what the numbers mean.

The key to interpreting the ratings is that the name of the rating (such as "transparency" or "staining") is the name of the attribute being measured, and the number represents the quantity of the attribute in the paint. Complete lack of the attribute is defined by its opposite or negation ("opacity" or "lack of staining"). The pure attribute always get a rating of 4, and its absence or opposite attribute gets a rating of 0. (The middle value is 2.)

If a paint is rated 4 on "transparency," then it is completely transparent; if it is rated 0, then it is completely opaque. If a paint is rated 3 on "staining," then it is fairly staining, if it is rated 1, then it is close to nonstaining.

All the ratings are set up in this way: the only exceptions are the ratings for value range, hue angle, and hue shift.

To help you along, the comments under each pigment in the guide to watercolor pigments first summarize the average rating across all brands for that pigment. Thus, the comments for pyrrole red (PR254) begin: "Pyrrole red PR254 is a completely lightfast, semiopaque, highly staining, dark valued, intense middle red pigment."
 

Transparency was measured by drawing 2 closely spaced black lines on the watercolor paper with a broad tip Sharpie™ indelible pen, painting over the lines a brushstroke of paint diluted to approximately a 1:3 concentration with water, then drawing a third line across the paint swatch after it had completely dried. The rating indicates the difference in blackness or darkness between the lines underneath the paint and the line drawn across the paint, or between the lines underneath the paint and the lines as they appear on either side of the swatch. Note that "transparency" as it is defined among watercolor painters is actually called hiding by paint industry professionals: transparency is measured by how much a paint color completely disappears when the paint is applied over a completely black surface. Low transparency often indicates a "sedimentary" or thick paint formulation, but note that any paint can be made more transparent by diluting it with water. My ratings are conservative; many artists will find paints rated as "3" are more than transparent enough for excellent glazing results.

Staining was measured by applying a brushstroke of paint diluted to approximately a 1:3 concentration with water, allowing the paint to dry completely, then firmly wiping the five times with a wet cotton swab, then blotted dry. The rating indicates the amount of paint or discoloration that remained on the paper. In many watercolor tutorials, staining is equated with the tinting strength or dominance of the pigment in paint mixtures, which is inaccurate. Staining is commonly said to be an attribute of a pigment or paint, which is also false. In fact, staining paints is usually caused by smaller pigment particle sizes or a higher electostatic attraction between the pigment and paper fibers (pigment attributes), by a higher proportion of dispersant or humectant in the paint which diffuses the pigment particles deeper into the paper fibers (paint vehicle attributes), by unsized or lightly sized papers, by papers with a rougher surface finish, and by papers made with insufficiently macerated pulp (paper attributes). Staining is desirable in paints that will be applied as a background wash that will be painted over, since the color will not lift if rewetted, scrubbed or blotted. However, any staining paint can redissolve when wetted, tinting new layers of paint: this is usually indicated by the measure of blossoming.

To transform VR into the steps of a painter's 11 step value scale (from 0 to 10), divide by 8. The value range, tinting strength and dilution of a paint determine its dominance in mixtures with other paints (a dark, concentrated, strongly tinting paint usually has the largest effect in mixtures).

Granulation was measured as the visible or tactile graininess, clumping or texture of the paint when applied as a wash in dilute concentration. This rating includes granulation and flocculation or the apparent clumping of the pigment texture into tiny flakes or spots (for example, in ultramarine blue).

Blossoming was measured as the amount of movement in the pigment when applied as a single moderately diluted layer of paint that is allowed to dry until nonreflective, then rewetted with clear water. (Blossoms are also called backruns, blooms or oozles by some watercolor painters.) A blossoming paint will be redissolved by the added water and the pigment particles moved by the capillary movement of the water as it evaporates. The rating indicates how much the paint can be adjusted after it is applied, or will redissolve when glazed over by another paint. (The tendency of a paint to blossom is separate from how much it diffuses when applied wet in wet.)

Diffusion was measured as the amount of movement by the paint beyond the brush mark when applied to a premoistened area of watercolor paper (wet in wet). Greater diffusion is caused by a smaller size and lower specific gravity (weight in water) of the pigment particles, and by an increased density in the composition and concentration of the vehicle. Very small, light pigment particles, such as carbon blacks, iron blue and the phthalocyanines (see the table of pigment particle sizes), will diffuse slowly through osmotic dispersion in water. Rapid, shooting diffusion in soft or finely divided pigments such as cadmiums, chromium oxide, ultramarine blue or iron oxides ("earth" pigments) is caused by a soaplike dispersant or wetting agent (such as ox gall) used to prevent pigment clumping in storage or reduce milling time during paint manufacture.

The paint's masstone hue, expressed as the hue angle in counterclockwise degrees around the CIELAB a*b* plane. Measurements were taken with a GretagMacbeth Spectrolino™ spectrophotometer in CIELAB a* and b* units, and converted to hue angle by the GretagMacbeth software. After calibrating to the manufacturer's white standard, two measurements were taken of each paint swatch; if these differed by more than five units on either dimension, a third measure was taken and the average of all three was used as the result. I measured the masstone (full strength) color rather than the undertone (diluted) color because the masstone is usually close to the optimal paint dilution (has the highest chroma).

The shift in a paint's hue from masstone to undertone (from full strength to tint) defined as the undertone hue angle minus the masstone hue angle. (The measurement of hue angles is described above.) A plus HS means the paint hue moves counterclockwise on the CIELAB a*b* plane (from red to yellow to green to blue); a minus HS means the hue moves clockwise. See the page on color in the world for an explanation of why a hue shift occurs. A monochrome paint will present the same hue across a wide range of wash mixtures. A duochrome paint will shift, for example, from orange yellow in masstone to a pale yellow in undertone.

Finally: always treat lightfastness ratings as advisory only. Some pigments show highly variable lightfastness, and pigment manufacturers and art supply companies are continually changing their products, not always for the better. Your only assurance is to do your own lightfastness tests, and to treat cautiously any pigment with highly variable ratings across paint manufacturers (such as iron blue, PB27 or dioxazine violet, PV23), or any paint with ratings of 6 or lower on the blue wool standard used here. For more information, see the section on lightfastness tests and lightfastness with a grain of salt, and the comments in palette paintings.

2004 lightfastness tests in progress

 

SUITABLE FOR ARTISTIC USE: 8 = very lightfast: will remain unchanged for 200+ years 7 = lightfast: will remain unchanged for 100 to 200 years 6 = marginally lightfast: may alter in about 50 to 100 years UNSUITABLE FOR ARTISTIC USE: 4-5 = impermanent: will alter in 15 to 50 years 2-3 = fugitive: will fade in 2 to 15 years 1 = very fugitive: will fade in less than 2 years two lightfastness ratings are given for every paint, separated by a comma: • the first number is for the lightfastness in a highly diluted sample (tint) • the second number is for the lightfastness in a slightly diluted sample (masstone) as follows: Lf(tint),Lf(full strength) See this table for the blue wool levels divided into the ASTM lightfastness categories.

Individual paint chroma (roughly equivalent to saturation) is not provided in the numerical paint ratings because the visual impact of chroma (or saturation) is different across different hues and tonal values. However, the masstone chroma of all paint swatches was measured using a GretagMacbeth spectrophotometer, and the results of these measurements, averaged across all paints tested for a pigment, are reported in the text description of the pigment. For example, any pigment described as "very intense" had an average LCh chroma at or above 88; a pigment described as "intense" fell between 75 to 87; a pigment described as "very dull" fell between 36 and 25, and so on. In general, the apparent intensity or luminosity of a paint depends on its hue: a dark middle blue hue with a chroma of 87 will appear very saturated, while a light yellow hue at 87 will appear dull. Paints lose chroma (and saturation) as they are diluted toward tints, although there are exceptions to this among very dark paints and for some paints on the warm side of the color wheel. (See this page for a detailed discussion of chroma and saturation.)

Watermedia paints (watercolors and acrylics) change color appearance as they dry. Measurements were made in units of the CIELAB color space with a GretagMacbeth spectrophotometer of 2 cm wide paint swatches on Arches CP watercolor paper. The first measurement was made while the paint swatch was still glistening moist (about 30 to 60 seconds after application), and the second 4 or more hours after the paint had dried. This procedure was done on two separate swatches and the results averaged to reduce error. Shifts are reported for 75 commonly used watercolor pigments or convenience mixtures in this table, which can also be sorted by total color change, or change only on lightness (value), chroma or hue angle. Results are described in the notes to individual pigments as the percentage change in color lightness or chroma, using the quintile labels at right. These data show that loss of chroma (saturation) is consistently the primary change in color appearance, and many colors actually darken as they dry: the commonly repeated statement that "watercolors lighten as they dry" is inaccurate.

illustrative paint swatches

I've attached here four test swatches (for Daniel Smith and Winsor & Newton earth colors) to illustrate the general approach and specific examples of how the test samples were interpreted to derive the ratings.

• Transparency. The Daniel Smith raw sienna shows a clear masking of the two black lines drawn in indelible ink on the watercolor paper. This is also apparent by comparison to the third line drawn below, on top of the swatch after it dried. By comparison, there is no apparent masking of the lines on the paper (difference in lightness between these lines and the line drawn on top of the swatch) in the Winsor & Newton raw umber. Transparency ratings: DS raw sienna = 2 (semiopaque), DS raw umber = 3 (semitransparent), W&N raw umber = 4 (transparent), W&N burnt umber = 3.

• Staining. All the pigments clearly stain the paper, but vary in how much of the pigment can be removed with a web cotton swab. The Winsor & Newton burnt umber is the most extreme, but is not as staining as the most intensely staining paints. The raw sienna and W&N raw umber are both mostly lifted. Staining ratings: DS raw sienna = 1 (lightly staining), DS raw umber = 3 (staining), W&N raw umber = 1, W&N burnt umber = 3 (staining).

• Value Range. The color attributes of the test swatches were measured using a GretagMacbeth spectrophotometer and its computer color analysis program. The paint's CIELAB L* (lightness) was measured in the masstone area just below the black transparency test lines, as this is the maximum paint concentration typically used in most paintings. This lightness value was subtracted from 97 (the lightness of white paper), to give the value range. Value Range ratings: DS raw sienna = 37 (mid valued), DS raw umber = 66 (dark valued), W&N raw umber = 36 (moderately light valued), W&N burnt umber = 57 (moderately dark valued).

• Granulation. Inspection of the "thin" end of the swatch, the diffusion test below it, and the texture on the brush, all indicate these pigments have a liquid to powdery texture. However, the Winsor & Newton shows a slightly greater texture, noticeable as the diagonal darkening at the top of the swatch, caused by the difference between coarse and fine pigments. Granulation ratings: DS raw sienna = 1 (powdery), DS raw umber = 1, W&N raw umber = 2 (slight texture), W&N burnt umber = 1.

• Blossom. Blossoming is revealed in the amount of mottling, movement or discoloration of the partially dried test swatch under an application of clear water (at the bottom of each swatch). This is in general a test of the osmotic pull of the pigment particles, the solubility of the vehicle after it has dried, and the ease with which the paint can be modulated in washes. Blossom ratings: DS raw sienna = 3 (moderate blossom), DS raw umber = 2 (slight blossom), W&N raw umber = 3, W&N burnt umber = 1 (slight water discoloration).

• Diffusion. The amount of movement of the paint in a patch of clear water (painted horizontally at the bottom of the test swatch). This is measured as the horizontal distance the pigment travels from left to right. Most earth pigments are relatively inert when used wet in wet. Diffusion ratings: DS raw sienna = 2, DS raw umber = 1, W&N raw umber = 1, W&N burnt umber = 1.

• Hue and Hue Shift. The color attributes of the test swatches were measured using a GretagMacbeth spectrophotometer and its computer color analysis program. Readings were taken of the thin diluted wash at the bottom and of the masstone area just below the black transparency test lines; the difference between these two hues, in a*b color angle, is the measure of hue shift. I report the masstone reading (not the undertone) as the hue angle and value range of the pigment, as the masstone is closer to the typical concentration of paint in actual paintings. Hue shift ratings: DS raw sienna = +13, DS raw umber = +6, W&N raw umber = +5, W&N burnt umber = +10.

• Lightfastness. The manufacturers each rate their paints as a 4 (very lightfast), and this is consistent with the ratings given to the paints by third party publications, the ASTM, and the consensus testimony of art handbooks and pigment industry sources. However, I conducted my own lightfastness tests (in 1999 and 2004) on all single pigment paints and many convenience mixtures from each brand. The Daniel Smith paints are single pigment formulations; the Winsor & Newton are mixtures of two or three iron oxides. Lightfastness ratings: DS raw sienna = 8,8 (completely lightfast), DS raw umber = 8,8, W&N raw umber = 7,8, W&N burnt umber = 7,8.