Every paint is a mixture of microscopic pigment particles, which provide the paint color, mixed in a liquid paint vehicle that holds the pigment in suspension and allows it to be applied with a brush, then dries to bind it to the support (paper, board or canvas). The vehicle also contains other substances that reduce manufacturing costs, adjust the visual appearance and handling attributes of the paint, and increase its shelf life in the art store.
The Backbone Composition
Each paint manufacturer develops a proprietary backbone composition — a basic recipe of pigment and vehicle ingredients — that is fundamentally designed to keep manufacturing costs under control and to get the best possible handling attributes for every pigment in the watercolor line.
The manufacturer then tweaks the exact proportions of this recipe from one pigment or paint color to the next, so that the texture and color of each pigment is put on best display and the differences in pigment dispersability, tinting strength or staining across the different paint colors are minimized.
The backbone composition is the foundation of the manufacturer's brand style and quality standards. It usually includes most or all of the following ingredients:
One or more pigments:
Pigments are chemical compounds with appealing or useful color attributes and that do not dissolve in water. Paints are a dispersion of tiny pigment particles suspended in the vehicle. All professional quality tube and pan watercolors are made with pigments.
Brightener, transparent or "white" crystals:
These substances lighten the value and increases the chroma of the dried paint. A few watercolor brands add one or more highly refracting substances as a brightener, to adjust or enhance the lightness or chroma of the finished color. These traditionally include alumina trihydrate (aluminum trihydroxide), titanium dioxide, or micronized barium sulfate (blanc fixe), but newer, more effective compounds are available. The particle size and specific gravity of brighteners is usually similar to the pigment, so they do not separate from the pigment when the paint is mixed with water.
Excessive amounts of brightener can impart a whitish or sparkly appearance to the dried paint, or can form a thin, whitish coating on top of dried paint applied as a juicy brush stroke. They often can compromise the lightfastness or permanence of the color. The most reliable method to assess paint formulations is the tinting test, which directly reveals the proportion and the quality of pigment used in the paint by dissolving it water or a large quantity of titanium dioxide.
The above are dispersed in a vehicle or medium consisting of:
Binder:
Traditionally and still commonly said to be gum arabic but, in some brands, actually a synthetic glycol. Pigment particles are dispersed through milling in a liquid vehicle that consists primarily (about 65% of vehicle volume) of a transparent binder. The binder carries the pigment particles as a viscous liquid so they can be applied with a brush; it binds the pigment to the watercolor paper; and it produces a brighter color by holding the pigment particles on the surface of the paper, rather than letting them be pulled by capillary action deep between the paper fibers. A diluted solution of gum arabic can be applied as a varnish or top coat to dried paint to reduce surface scattering and give the paint a deeper, richer color.
The binder usually determines the name of a medium — linseed oil for oil paints, acrylic polymer emulsion for acrylic paints, egg white or yolk for egg tempera. Watercolors are named instead for their solvent (water) and historically have used a variety of gums, starches or animal glues as binder. As is true with industrial pigment manufacture, watercolor and pastel manufacturers use only a tiny fraction of global gum arabic production. Some manufacturers have shifted to alternative binders. Very satisfactory watercolors can be formulated entirely of synthetic materials.
Plasticizer:
Usually glycerin is used to soften the dried gum arabic and help it redissolve. Unfortunately, watercolors formulated only with gum arabic and water have significant drawbacks. Excess paint in the mixing well will dry to a hard, glassy block that is very difficult to redissolve. To counteract these problems, the gum arabic is buffered with a carbohydrate plasticizer, usually 20% or less of vehicle volume. Nowadays this is most often glycerin (glycerol), the trihydroxy form of alcohol. Glycerin reduces the native brittleness of the gum arabic and minimizes the cracking or chipping of dried paint. It also helps the gum arabic to dissolve in water more quickly, and inhibits hardening (drying out) of the paint in the tube.
Paint manufacturers can also use methyl cellulose, the binder commonly used in pastels and chalks, as a plasticizer, because it is very flexible when dried. In paints it also acts as a mild binder and solvent or dispersant.
Humectant:
Traditionally simple syrup or honey were used but now often inexpensive corn syrup, to help the paint retain moisture (especially in pan paints) is adopted. Unfortunately both glycerin and gum arabic will dry out relatively quickly, even if stored as tube paints. So some other substance is necessary to retain water or act as a humectant. Since the middle 19th century paintmakers have softened watercolor paints with a carbohydrate moistener, either a sugar syrup (nowadays glucose, in the form of corn syrup) or honey.
Like gum arabic, these sweet carbohydrates are hygroscopic — they tend to absorb and retain water from the atmosphere — which makes the paints considerably easier to redissolve once they have dried, and extends the life of the paint in the tube. Humectants also extend the paint drying time so that washes can be manipulated more easily, and they may increase the staining effect of watercolors by prolonging the capillary action that pulls small pigment particles deep between the paper fibers.
Honey is more effective than corn syrup at retaining water (in fact, honey will crystallize but never dries out), but it is also roughly 14 times more expensive. Used in excess, the sugars will also attract insects or mold.
Extender or filler, such as dextrin:
These are used to bulk out and thicken the paint without noticeably affecting the color. As larger amounts of glycerin and gum arabic are added to the paint — for example, in strongly tinting or finely divided pigments — the paint texture becomes stringy or taffylike, the gloss of the paint increases, and the paint bronzes more readily. These paints tend to lift (redissolve) too easily from the paper, which can lead to undesired blurring, bleeding or lifting of color areas when new paint is applied over or alongside them.
To counteract these problems, many watercolor paints are formulated with a colorless, inert filler added to thicken the paint and to make the various pigment and vehicle mixtures within a watercolor line of similar consistency. Filler is also used to subdue intensely tinting pigments such as the phthalocyanines or quinacridones, or simply to reduce the proportion of costly pigment in the paint.
The most commonly used filler is dextrin — a clear, gelatinous processed wheat or corn starch — which thickens the paint, alters the taffylike vehicle to a smooth, buttery consistency, and reduces surface gloss or bronzing in the dried color. Sometimes conservation grade, water soluble adhesives, including carrageenan or funori (a gelatinous polysaccharide extracted from a type of Japanese seaweed [genus gloiopeltis]), are used for the same purpose. Dextrin also acts as a binder in combination with (or, in poster paints, in place of) gum arabic.
Dextrin can also be used as an extender, to bulk out the paint and cut down on the amount of costly pigment used, especially in cobalt and cadmium paints. However, if too much dextrin is used, the paint will dry to a dull, matte finish and will be prone to flaking.
Manufacturing additives:
In particular dispersants (to prevent clumping of the raw pigment after manufacture and to speed up the milling of the pigment and vehicle ingredients) and a fungicide or preservative to suppress the growth of mold or bacteria. Binder, plasticizer and humectant are standard vehicle ingredients — even paints you make yourself will contain them. Most commercial paints also include additives, which are put into the pigment batch when the pigment is packaged in bulk, or are put into the paint during milling, and are passed along to the artist with the paint.
Most common is a dispersant or wetting agent that accelerates and improves the milling (wetting and mixing) of the pigment in the water based vehicle, much the same way as dishwashing soap divides and dissolves greasy dirt. Dispersants can be used as a labor saving shortcut in any paint, but they are more common in finely divided or water repelling synthetic pigments such as carbon black, phthalocyanines, alizarin crimson, transparent iron oxides and prussian blue; and in soft pigments that can compress or cake during milling, such as the cadmiums or ultramarine blue. Ox gall (the yellowish extract of dried bovine gall bladders) was and still is commonly used for this purpose, but synthetic surfactants are sometimes used instead.
The painter notices the presence of wetting agents in the paint because they reduce the time it takes the paint to dissolve, cause the paint to stain papers (especially absorbent papers) more readily, and make the paint diffuse aggressively or shoot outward when applied wet in wet.
Water:
Water dissolves or suspends all the ingredients, carries them onto the paper, and evaporates when its work is done. Tube paints contain about 15% by volume of water — the miraculous substance that gives life to you and unpredictable energy to your watercolors.
Paints are manufactured with excess water in the vehicle, as this reduces the viscosity of the vehicle and decreases the amount of time (labor) and electrical energy necessary to mill the paint. This water mostly lost through evaporation during milling, but also after milling when the paint is left to sit and age or stabilize. Some pigments or fillers absorb water very slowly, causing them to expand: these are the paints that "explode" or squirt from the tube when it is first opened, because they were not aged adequately before packaging.
Creating an effective watercolor vehicle is a complex balancing act. Each ingredient contributes its own benefits and drawbacks to the formulation of the paint, and the best formulations are based on considerable manufacturing experience and consistently maintained quality controls.
The manufacturer's cost considerations aside — and those are usually a major consideration in commercial paint design — the pigment particle size, tinting strength and dispersability primarily determine the adjustments made to the backbone formulation.
This week's watercolour discovery is Irap Ashish who won the outstanding watercolour painting at the October 2014 BoldBrush Painting Contest.
Have a great week
Danielle
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