Chalk resist: Chalk resist is a thick paste made from powdered chalk mixed with water. This paste is brushed onto the desired areas of a sheet; once it is dry, the thick layer of chalk protects the underlying paper from wet watercolor washes. The chalk can be easily removed by counter-rolling the sheet or by rubbing it gently with a knife to reveal the clean white paper underneath.

Chinese white: an opaque-white watercolor made from zinc-oxide pigment; “Chinese white” was the proprietary name given to zinc-white watercolor by the Winsor and Newton Company, which first introduced it in 1834. Bright, opaque, and completely stable, it was heralded as a substitute for unstable lead white watercolor (lead carbonate), which blackened upon exposure to atmospheric sulfur. Chinese white was considered a great example of the chemical industry’s contributions to the creation of stable art materials.

Fugitive pigments: Pigments that shift in color or fade as the result of a chemical change, especially ones that quickly lose color upon exposure to light.

Gouache: A watercolor containing an opacifying agent, commonly zinc white, which lends opacity, body, and brightness

Gum arabic: A colorless gum obtained from trees of the genus Acacia. Gum arabic is used in solution as a binding medium in watercolor.

Laid paper: paper made on a mold with a screen constructed from tightly spaced parallel wires attached at points to more widely spaced perpendicular wires. These wires leave a linear texture on one side of the sheet. A characteristic pattern of perpendicular lines is visible in transmitted light. Until wove paper was invented in 1755 all western papers were laid.

Lake pigments: an organic pigment prepared by precipitation of a dye onto a powdered, inorganic substrate

Madder lake: a red lake pigment popular in the 19th century that derives its color from the roots of the madder plant (Rubia Tinctorum L.)

Opaque watercolor: A broad term encompassing gouache, watercolors made from inherently opaque pigments without the addition of an opacifying agent, and watercolor rendered opaque through media manipulation and application methods

Photomicrograph: A photograph taken through a microscope

Resist (also masking agent): A resist is a material that temporarily covers and protects a selected area of paper over which a wash is subsequently applied. Once the wash is dry, the artist removes the resist, revealing the shape it preserved on the unpainted surface. This process was invented by the English watercolorist Francis Nicholson (1753­–1844).

Scumble: a quick overlaying of lighter, more opaque color on top of a darker underlayer

Size: A material such as gelatin or glue that is added to paper, either during manufacture or after the sheet is made, to make the paper water resistant. Size also controls the flow of paint or ink onto the sheet.

Twill paper: Wove paper with diagonal, twill texture imparted by the particular twill pattern of the woven wire screen that supported the pulp as the sheet was formed

UV photograph: A normal light photograph taken under ultraviolet light, which captures the visible fluorescence induced by ultraviolet light

Watercolor: a transparent paint made of finely ground pigment or dye, suspended in a solution of gum arabic

Whatman papers: high-quality watercolor papers made in England and named after the original maker, James Whatman. Through the early 20th century these papers were handmade from linen fibers and infused with gelatin size, which increased the strength of the sheet and decreased the rate at which washes were absorbed into the fibers. White, strong, and robust, these papers could withstand multiple rewettings and surface manipulations.

Wove paper: paper made on a mold with a woven wire screen. Wove papers are relatively smooth and display even fiber distribution when viewed with transmitted light. Wove papers were first produced around 1755 as an alternative to inherently textured earlier “laid” papers.

Note to Reader:
In order to accurately describe the materials and working methods that Winslow Homer employed in his watercolors and drawings, the Art Institute of Chicago used the following terms to describe works in its collection. Opaque watercolor refers to the appearance of any opaque aqueous media achieved by using opaque pigments, adding a white pigment or bulking agent (as in gouache), and/or manipulating the media and methods of application. Transparent watercolor describes transparent washes achieved by employing transparent pigments and/or controlling the degree of dilution. When it appears alone, watercolor refers to transparent watercolor and indicates the absence of any opaque passages. The term gouache is not used here, as it is not broad enough to encompass the full range of Homer's opaque effects. These definitions are based on the pioneering work of Roy Perkinson and Annette Manick (see "Notes on Media and Papers" in Reed and Troyen 1993.) The same terminology was adopted for
Masters of Color and Light: Homer, Sargent, and the American Watercolor Movement (Ferber and Gallati 1998). The terms and their use reflect a trend in the conservation and curatorial fields toward standardization of description.