paints

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, and sometimes

• a brightener, transparent or "white" crystals that lighten the value and increase the chroma of the dried paint

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

• plasticizer, usually glycerin, to soften the dried gum arabic and help it redissolve

• humectant, traditionally simple syrup or honey but now often inexpensive corn syrup, to help the paint retain mosture (especially in pan paints)

• extender or filler, such as dextrin, used to bulk out and thicken the paint without noticeably affecting the color

• 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, and

• water, which dissolves or suspends all the ingredients, carries them onto the paper, and evaporates when its work is done.

These ingredients are described below.

Pigment. 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, just as the Mississippi is a suspension of sand, clay, agricultural chemicals and effluent. All professional quality tube and pan watercolors are made with pigments.

In contrast, a dye is completely soluble (dissolves) in water, and binds directly with the materials it contacts (though a mediating chemical called a mordant must often be present to make this bond happen). Some brands of liquid watercolors or "brilliant" watercolors are made with dyes.

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:

• As the same mass or quantity of pigment is divided into smaller and smaller particles, the total surface area of all the pigment particles increases proportionally, which increases the proportion of vehicle or water necessary to completely wet or disperse the pigment.

• Strongly tinting pigments — especially very dark pigments such as the phthalocyanines or dioxazine violet — must be diluted with vehicle or extenders to increase the color chroma and reduce the tinting strength, so that the paint's color and handling attributes are comparable with other paints in the line.

• Paints made with softer pigments (such as ultramarine blue or the cadmiums) or finely divided pigments (such as alizarin crimson, iron blue and the phthalocyanines) tend to cake or clump during storage or milling, and sometimes manufacturers use more dispersant to accelerate the mixing of pigment and vehicle; this causes the paint to diffuse more aggressively when used wet in wet.

Pigments that are all three — finely divided, strongly tinting and expensive — are usually formulated with the largest proportion of vehicle and filler.

The proportion of pigment to vehicle in tube paints generally ranges from less than 10% to around 20% of total volume for a finely divided, strongly tinting pigment such as the phthalocyanines, red quinacridones, dioxazine violet or alizarin crimson; from 20% to 30% for prussian blue, carbon black, the "raw" (uncalcinated) black and red iron oxides, zinc or titanium white, yellow quinacridones, benzimidazolones and most other synthetic organic pigments; 30% to 40% for the yellow iron oxides, viridian, ultramarine blue, ultramarine violet and the finer grained cobalt pigments (blue, cerulean, turquoise, green); 40% to 50% for the weakly tinting cadmium yellows, cobalt violet and "burnt" (calcinated) red and yellow iron oxides; and 50% or more for cadmium orange, the cadmium reds, manganese violet and manganese blue. These proportions are illustrative; specific recipes vary across paint brands and depend on the quality of pigments they use.

Brightener. 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.

In the past, brighteners were commonly found in oil paints and house paints: their increasing use in watercolors is a reflection both of consumer preferences for bright color and competitive cost pressures. The best brands, if they use such additives, balance operating costs, profits, paint handling characteristics, consumer preferences and finished color in developing their formulations.

Binder. 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.

schematic backbone composition of a modern watercolor paint

In European painting since the 18th century, the binder of choice has been gum arabic, made from the solidified sap of thorny, shrubby acacia trees (species Acacia arabica or Acacia senegal, shown at right). Gum arabic was originally exported from Middle Eastern sources via Turkey, but in the recent era most commercial gum harvesting has been done by subsistence farmers in arid regions of North Africa — the Sudan and Chad alone provided roughly 85% of the total world supply, hence the alternative modern names gum sudan or gum kordofan for the product. Gum senegal is considered superior but it is currently produced in limited quantities and is hard to identify by appearance alone.

Gum is a pH neutral salt of acidic polysaccharides (which are types of sugar or carbohydrate); the gum may include potassium or magnesium, but the primary component is calcium. Gum arabic is sticky when wet and quite hard and transparent when dry — in that respect like household sugar — although pure gum arabic dissolves in water more slowly than ordinary sugar.

In raw form, gums are sometimes sold as yellow or brownish glassy beads or "tears," about the size of lentils. Most art wholesalers and retailers sell dried gum as a powder or coarse grains, which are easier to dissolve in water. The best gums have a pale honey color in solution, with very little or no visible sediment or residue. Impure or low grade gums will contain significant amounts of sediment and may have a darker color. All gums are filtered before use in commercial paints, where they are have a darker color because they contain less water than the filtered solution.

As is true with industrial pigment manufacture, watercolor and pastel manufacturers use only a tiny fraction of global gum arabic production. Gum has been widely used as an emulsifier or digestible coating in the soft drinks, processed foods, cosmetics and pharmaceutical industries. But recent civil wars (in Darfur, south Sudan and elsewhere), drought, and firewood harvesting have substantially reduced native acacia forests and have disrupted gum production. This has more than doubled the price of high quality gum arabic on world markets and created unpredictable variations in suppy.

In response, some manufacturers have shifted to alternative binders. Very satisfactory watercolors can be formulated entirely of synthetic materials. A recipe patented in 1953 by Binney & Smith consists of approximately 85% water soluble, waxy polyethylene glycol, 4% stearyl alcohol, 6% polyhydric alcohol and 5% water by volume. These synthetic vehicles appear clear and completely colorless if they separate from the pigment in the tube. You can usually examine the manufacturer's pure vehicle in commercial paints such as cobalt violet, viridian or cadmium red that tend to separate from the vehicle in the tube; beads of excess vehicle sometimes extrude from the crimp at the end of the paint tube.

Gum arabic is a relatively weak binder, and will not adhere to or can be easily scraped off of most surfaces. However it can be dissolved again in water, even after it has completely dried. This is why watercolors can be rewetted after they have dried on the palette, or blotted or lifted if they are rewetted on the paper, which allows the artist to manipulate the finished color. Oil or acrylic paints must be either scraped off or painted over once they have dried.

Plasticizer. 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. In fact, early 19th century watercolors, formulated with gum arabic only, were sold as small resinous bricks that had to be rubbed out each morning — laboriously dissolved by rubbing them on a shallow saucer or mixing cup containing a little water — before the paint could be used.

Watercolors made with a high proportion of gum binder also will bronze (appear darkened, shiny or leathery). And dried paint will crack or flake if it was applied as a thick or undiluted layer, or had pooled in the depressions of cockled paper.

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. 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. If too much honey is used in a paint, thick or concentrated paint layers will remain sticky after they dry, and may reabsorb moisture on humid days, damaging the painting. Used in excess, the sugars will also attract insects or mold.

Filler. 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.

The presence of dextrin is indicated by a "short" or stiff paint consistency: squeeze out a small amount of paint, then shear it off by scraping a palette knife against the edge of the tube nozzle. If the paint on the knife has a clean, flat edge and retains a cylindrical shape, then it has a short consistency. Poster paints, student paints and gouache usually contain much higher quantities of dextrin than professional grade watercolors.

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. When this occurs a finely powdered, transparent filler (such as kaolin or china clay, calcium carbonate or gypsum) may be used as well.

Are paints that contain fillers inferior to paints that don't? Not necessarily. In some cases — pigments that are dark or intensely staining, or pigments that tend to darken and dull in heavy concentrations (such as the cadmiums) — the additives can enhance the handling attributes or color appearance of the paint. But it's also true that they are often used to cut product costs, and can degrade color appearance, producing a whitish, thin or bland color.

farming the gum acacia tree in Senegal

Other Additives. Binder, plasticizer and humectant are standard vehicle ingredients — even paints you make yourself will contain them. Most commercial paints also include what I call manufacturing 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.

Pigment manufacturers — those big smokestacks on the horizon where the colored powders are born — may include other additives to increase the shelf life of the bulk pigment when the pigment is shipped as a water dispersion. These help to mix the raw pigment particles in water, prevent the pigment solution from "kicking out" or precipitating, retard the hardening, clumping or skinning of the solution, inhibit the growth of mold, and so on. They can improve the consistency or handling of finished watercolor paints, or can accelerate the separation of pigment and vehicle in the tube or the degredation of the pigment color, especially after the paint has spent many months or years hanging undisturbed in a retail rack. In any case, the paint manufacturer can't remove them, so they get passed along to you.

Many modern watercolors also contain a small amount of preservative or fungicide to inhibit the growth of mold in the tube and on the palette or the finished painting, especially when significant amounts of sugar or honey are used in the formulation.

Although alcohol is not a standard watercolor ingredient, it is sometimes added by artists to improve the wetting action of washes or shorten the drying time in damp or cool conditions. (The English artist Paul Sandby was especially fond of gin for this purpose.)

A few artists keep diluted solutions of gum arabic, glycerin, and ox gall on hand to adjust the attributes of commercial paints to suit the painting conditions or their painting preferences. For example, glycerin or ox gall can be added to paints in especially dry or hot weather conditions to delay the drying time and smooth the appearance of washes. I prepared these additives, diluted one part to six parts water, in small plastic squeeze bottles, and I almost never use them.

Water. Finally, tube paints contain about 15% by volume of water — the miraculous substance that gives life to you and unpredictable energy to your watercolors ... and to the Mississippi.

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.

handmade watercolors

A very effective way to learn about paint manufacture is to mix up some paints yourself, by hand. The ingredients are readily available from online art materials suppliers and the experience will improve your critical appreciation of commercial paints.

Recipes are available in most painting handbooks or online from some pigment suppliers. The following recipe is adapted from Mayer using information from several additional sources. The raw materials (pigments, gum arabic, dispersant, fungicide) are available from suppliers such Kama Pigments, Kremer Pigmente and Sinopia Pigments; glycerin (glycerol) is available at any pharmacy; humectant and dextrin must be made from food ingredients available at most supermarkets.

It is worthwhile to try this recipe to see how much manual labor and fine tuning of ingredients is necessary to produce a decent watercolor paint. The main problem is that different pigments require different proportions of vehicle ingredients and different proportions of vehicle to pigment.

The few painters I know who are committed to handmade materials have all mentioned the difficulty of working up a really desirable handmade paint, and after "roughing it" for a while most of them has gone back to using commercial watercolors. However, I highly recommend you experience the process and decide for yourself.

A very useful online resource for paintmaking supplies and procedures is Tony Johansen's paintmaking.com ... check it out!

pigment types

All pigments can be classified according to two criteria: whether the pigments are (1) natural or synthetic, and (2) inorganic or organic.

The term natural means that the pigment molecule is extracted from a mineral, plant or animal source that occurs in nature, and is only modified by grinding, washing, filtering or heating; synthetic means that the molecule was originally assembled or significantly modified by an industrial chemical process. Natural pigments have been largely replaced by synthetic compounds of superior permanence, color and consistency.

Inorganic means that the pigment is a mineral compound, typically an oxide or sulfide of one or more metal or rare earth elements; organic means that the pigment is a molecule of carbon in combination primarily with hydrogen, nitrogen or oxygen. (Note that many modern organic pigments are not found in living things, excepting artists who eat their paints.)

The two criteria can be combined to define four pigment categories (the links take you to the page describing the major pigment types within each category):

1. natural inorganic, metal or earth pigments extracted from natural mineral deposits. With a few exceptions, natural inorganic pigments are no longer used, primarily because they are uneconomical to extract and do not produce adequate color consistency.

2. synthetic inorganic, metal or earth pigments created by combining raw chemicals and ores through industrial manufacture. These comprise approximately 80% of world pigment manufacture.

3. natural organic, pigments made as extracts from animal or plant matter. With very few exceptions, natural organic pigments are no longer used, primarily because they are not adequately lightfast.

4. synthetic organic, carbon based pigments, often made from petroleum compounds, that mimic the chemistry of plant and animal colorants.

Most pigments show some alteration after long exposure to direct sunlight, but the change depends on the type of pigment. As a rule, the organic pigments dull and fade under prolonged light exposure, and some disappear entirely; the modern synthetic organic pigments are generally much more durable than the natural organic pigments. By contrast, the inorganic pigments either gray or darken under the effects of light, typically because of oxidation or a chemical reaction to impurities (such as sulfur) in the pigment.

Nearly all natural organic pigments (with the exception of carbon blacks) are chemically unstable and deteriorate when used as pigments. Synthetic organic paints were a major chemical innovation of the second half of the 19th century, but many of these first colorants are too impermanent for artistic use. Modern synthetic pigments, almost all developed in the 20th century, are far more durable and provide the most intense and varied colors.

Today, with very few exceptions, all commercial artists' paints use synthetic pigments. Reserves of most natural inorganic pigments, like reserves of tropical hardwoods or sculptor's marble, have been exhausted over time by unrelenting consumer demand; others cannot be mined or processed because of severe environmental impact. The development of synthetic inorganic pigments was perhaps the major technical advance in painting during the early 19th century, and has evolved since then into an amazing array of durable, brilliant colorants of every hue.

The Society of Dyers and Colourists (UK) serves as the international clearing house for commercial pigment information, as publisher of the standard pigment color index names, and as a registry for commercial pigment manufacturers of every pigment or dye.

The authoritative source on synthetic inorganic pigments is Industrial Inorganic Pigments, edited by Gunter Buxbaum (Wiley, 1998). The sister source for synthetic organic pigments is Industrial Organic Pigments by Willy Herbst and Klaus Hunger (Wiley, 1997), billed as "everything there is to know about organic pigments." A summary of the same information (by the same authors) is available as Ullmann's Encyclopedia of Industrial Chemistry (Wiley, 2000). (Ullmann's discusses the very important phthalocyanine pigments as a separate chapter, and has a chapter on "Artists' Colors.") All these references will be available at any good chemistry library.

Historical pigment information for natural inorganic or organic pigments is scattered across several sources. An excellent starting point is the compact but somewhat dated Painting Materials: A Short Encyclopedia by Gettens and Stout. A more selective and exhaustive treatment of specific pigments is the four volume Artists' Pigments: A Handbook of Their History and Characteristics edited by Robert Feller (Volume 1, dealing with indian yellow, aureolin, barium sulfate, the cadmiums, red lead and minim, green earth, zinc white, chrome yellow and other chromate pigments, lead antimonate yellow, carmine), Roy Ashok (Volume 2 on azurite, ultramarine blue, lead white, lead-tin yellow, smalt, verdigris, vermilion, malachite, calcium carbonate whites), Elisabeth West Fitzhugh (Volume 3 on egyptian blue, orpiment and realgar, indigo and woad, madder and alizarin, gamboge, vandyke brown, prussian blue, emerald green, chromium oxide greens, titanium dioxide) and Barbara Berrie (Volume 4 on carbon blacks, cobalt and cerulean blue, earth pigments, arylide yellows, organic browns), all published by Oxford University Press (1994-2006). There are many other primary sources available: consult a bookstore or library for more information.

You may also want to check out this interesting web site on pigments in paintings. If you can read German, then the pages on Alte Pigmente (up to c.1780) and Moderne Pigmente at Volkert Emrath provide an interesting, gallery style overview (with pigment microphotographs).

pigment manufacture

Paint manufacturers such as Winsor & Newton, Maimeri or Daniel Smith are dependent on a range of suppliers for paint raw materials. To a large degree, the quality of the paint depends on the quality of the ingredients that go into it — most of all, on the quality of the pigments.

Pigments used in modern art materials are manufactured by dozens of chemical companies such as BASF, Ciba-Geigy or Clariant GmbH (Germany and Switzerland), Hays Colours, ICI, or Holliday Pigments Ltd. (UK), Bayer, DuPont, Sun Chemical Corp., Hoechst Celenese or CPMA (USA), Sanyo Color Works Ltd. or Dainichi Seika Color & Chemicals (Japan), Sudarshan Chemical Industries (India), Sinochem Liaoning Corp. (China), and so on. Many companies based in Europe or North America also have manufacturing subsidiaries in Asia.

These companies make and sell pigments in bulk: as powders or fine grains, compressed into dry cakes (presscakes), or as water based pastes or liquid dispersions — especially used for pigments with low dispersability or that would irreversibly clump if packaged in dry form.

There are also several small pigment manufactories that cater directly to the artists' market for pigments and even supply some well known paint company brands: see for example Kama Pigments, Kremer Pigmente, and Sinopia Pigments. These suppliers typically trade in much smaller quanitites and tend to emphasize historical, inorganic and in particular cadmium, cobalt and iron oxide pigments.

All modern colorants, no matter where you buy them or how much you pay for them, are synthetic compounds made from a variety of basic ingredients, including recycled industrial wastes. The single chemical comprising roughly two thirds of total global pigment manufacture is titanium dioxide (PW6), which provides the white base for housepaints, primers and the like; next come the incredibly versatile and lightfast iron oxides, roughly 20% of world production. Nearly all the remaining colorants are synthetic organics, especially the many types of azo pigments, phthalocyanines and quinacridones, which are all manufactured from the complex interactions of petrochemicals and acids, sometimes at high temperature or pressure.

From a chemical point of view, exactly the same colorants are used in many manufacturing applications — to make housepaints, automobile finishes, plastics, printing inks (for paper and textiles), colored leather (for shoes, handbags or jackets), building materials (the colors in cement, stucco and bricks), carpets and synthetic floor coverings, a variety of synthetic fibers and textiles, rubber, paper, cosmetics, ceramics, pharmaceuticals, foodstuffs and even wood stains, dental ceramics and tattoo inks. The major difference among these products is that they are formulated to the specific lightfastness, hue, chroma, particle size, medium (water or oil based), purity — and cost! — suitable for different industrial or manufacturing applications. Thus, you can buy a chemically identical phthalocyanine blue (PB15) for $4 or $28 a pound, depending on the manufacturer and intended end use of the pigment.

For these reasons, different manufacturers offer the same pigment under separate trademarks, primarily to distinguish formulations designed for different industrial applications. For example, arylide yellow 10G (PY3) is sold by many different manufacturers under trademarks such as Eljon Yellow 10GE, Acosil Yellow 3, Dalamar MA Yellow YT-828-d, Hansa Yellow 10G, Solintor Yellow 10G, Pintasol Yellow E-L1, Monolite Yellow 10GE HD, Kenalake Yellow 10G, and so on — and these are just some of the brands registered as suitable for art materials manufacture! Obviously, the quality, hue and texture of a specific pigment such as "cerulean blue," "cadmium yellow" or "quinacridone rose" can vary significantly, depending on the grade of pigment obtained and the manufacturer that produced it. Yet artists must take this product decision on trust.

Despite the braggy marketing put out by some art supply manufacturers about their special formulations and unique colors, the art materials market is much too small to influence the industrial production of pigments. With few exceptions, the range and quality of pigments available to artists depends entirely on the color requirements of large consumer products and manufacturing companies — manufacturers of foods, house paints, inks, automobiles and plastics in particular.

Artists may enthusiastically buy paints touted as containing that same pigment used by some famous 19th century painter, when actually the pigment was designed, manufactured and priced to tint ceramics or aluminum siding! The best chemical manufacturing companies, such as Sun, Ciba-Geigy or Clariant, do produce a range of pigments that includes artists' grade colorants of very consistent and very high quality, but the pigment attributes and even its availability are specified by an industrial end use, not by the art materials market.

Case in point: quinacridone gold (PO49), a lovely, deep yellow pigment offered by several watercolor lines, is no longer manufactured. Why? Because automotive manufacturers stopped buying the color. Watercolor manufacturers will continue to manufacture paints using remaining stockpiles of the pigment.

An interesting comparison across generic pigments is the average retail price of dry pigment powders, shown in the table below. This is only a crude reflection of the actual cost to manufacturers: pigment costs can be driven down by choosing lower quality (less lightfast or less intense) pigments, or by adding more filler to the paint. Many of the expensive synthetic organics are much more cost effective than comparably priced synthetic inorganics (such as cobalt violet) because their tinting strength is so high — a little goes a long way.

In addition to pigments, art supply manufacturers must also buy agricultural products such as gum arabic, glycerin, glycol, corn syrup, honey or dextrin, as well as chemicals such as fillers, brighteners, surfactants and fungicides. Variation in the quality or availability of these materials within and across watercolor brands occurs as well.

Most pigments are manufactured to a specific particle size which usually cannot be modified by additional milling by the paint manufacturer. The pigment particle size affects the color and handling attributes of the paint, so it is very useful to know whether common watercolor pigments are usually coarsely or finely divided.

Particle size is responsible for several important differences in pigment or paint characteristics. Across different watercolor pigments, smaller particle sizes usually characterize pigments that are:

• higher in tinting strength, because the smaller particle sizes produce a greater surface area in the same weight (mass) of pigment, producing a more intense color in the same volume of water

• more transparent, because the higher tinting strength permits a thinner application of pigment on the paper, and

• more staining, because the smaller pigment particles more easily penetrate into the spaces between paper fibers.

Within the same watercolor pigment, smaller particle sizes (down to a limiting size of approximately 0.5µm or the wavelength of light) tend characterize pigments that are less saturated and lighter valued, because the increase in surface area produced by the smaller particle sizes increases the total surface scattering from the same quantity of pigment. In the synthetic organics smaller pigment particles are also generally less lightfast than larger particles of the same pigment; in mineral pigments this effect is less pronounced.

In oils and acrylics, smaller particle sizes make the pigments more transparent and up to a point more saturated, as the particles are entirely embedded in the dried paint vehicle, which reduces light scattering at the particle surface. In watercolors, pigments in smaller particle sizes are more transparent because the same number of particles cover less of the paper surface area, allowing more of the paper (or paint layer underneath) to show through. Even opaque paints can be made more transparent by diluting the color, which creates more visible spaces between the pigment particles applied to paper. (See this discussion of the luminosity myth for the differences between oil and watercolor paint layers.)

Typically the hue of the pigment also changes with particle size, sometimes dramatically. To see this, mix a small amount of cobalt teal blue (PG50) in a mixing well, let stand overnight, then drain off most of the liquid. The apparent surface color will be a grayish mid valued blue: this is the color of the smallest particles which settled out of solution last as a layer on top. Dig into the pigment with a brush, and you'll find the heavier pigment particles underneath, which have the original (greener and more intense) turquoise color.

Finally, particle size affects the handling attributes of the paint. Usually smaller pigment particles are more susceptible to backruns. They also require the addition of a dispersant such as ox gall to completely wet (disperse) the pigment powder in the paint vehicle during milling, and this additive causes the pigment to diffuse aggressively when applied wet in wet.

Each paint company has its own standards for pigment size, not least because more finely divided pigments can have a more brilliant color, but also are the more expensive grades, require more electrical energy and human labor to mill, and are often less lightfast. This is why (despite what Michael Wilcox assumes) pigments with the same color index name can have different lightfastness.

These watercolor brand differences are noticeable in the "coarse" pigments listed above: some manufacturers choose pigments that are very coarse, to emphasize the traditional pigment texture, while others choose pigments that are more finely divided, to get a modern homogeneity across all the paints in their line.

paint manufacture

Once a paint manufacturer has assembled the necessary raw ingredients, the methods of mixing paints have remained relatively constant for over a century. Today larger machines and manufacturing lines can produce greater quantities of paint, but four basic steps remain the same: (1) finishing the pigments by added grinding, (2) premixing the ingredients, (3) milling the premixed paste, and (4) packaging the paint. These can be illustrated with the small batch methods using a three roller machine.

The pigment is first premixed with the vehicle ingredients (gum arabic, water, glycerin, corn syrup and any other additives) in the proportions necessary to make a thick paste. These proportions vary with the pigments used: absorptive or finely ground pigment particles require more vehicle (because the total surface area of pigment increases as the average size of the individual particles gets smaller), and strongly tinting (or, in some brands, very expensive) pigments are more diluted with vehicle and filler.

Next, the premix is thoroughly milled under large stone or metal rollers, turning in opposite directions at different speeds. (In paint manufacture "milling" means mixing pigment and vehicle, not crushing or grinding the pigment particles.) There are usually three rollers on these machines, and they mix the pigment with the vehicle through the act of crushing, smearing and folding the paste. (Think of how you crush and smear globules of cocoa powder against the side of a pan to mix them with milk.)

a three roller paint mixing machine

In the traditional three roller machine, the premix is poured into the reservoir or space between the two first rollers (A). Because it is so viscous, most of the premix simply swirls around in this trough. But a small amount is pulled down into the opening between the rollers, where it is sheared and crushed. As it comes out the other side, it is pulled apart as the rollers separate, which pulls open undissolved pigment clumps. Sticking to the rollers, half is carried back around to the reservoir at A, and the other half to a second accumulation trapped between the second and third rollers (B). Here most of the paint again swirls in place, but a small amount is crushed and sheared through the smaller opening between the second and third rollers. Half of this paint travels back to the reservoir at A. When the reservoir at B gathers enough paint, it is scraped off the third roller by a floating metal tray (C).

This milling can take anywhere from several hours up to several days, and involve successive adjustments of the roller spacing. Water or other ingredients may be added during the milling to make the paint less viscous as the roller spacing is reduced.

After milling, the best manufacturers allow the paint to age in large containers for anywhere from a few days to a few weeks to make sure that the pigment has completely stabilized with the vehicle. Aureolin (PY40), for example, will continue to expand if packaged too quickly after milling — a problem that has caused tubes of Blockx aureolin to burst open in inventory, or squirt when the tube is first opened. Other pigments, especially the cobalts, will continue to soak up vehicle after mixing and will harden in the tube if packaged too soon.

Finally, the mixed paint is poured into tubes or pans. Paint manufacturers buy metal paint tubes with the bottom end open and the cap already screwed on; they fill the tube through the open bottom with a dispensing machine, then crimp fold the end of the tube to seal the paint inside and apply the appropriate label as an adhesive paper or plastic film.

Dry pans are filled by pouring the pigment into the pan or by shaping the paint (usually mixed with less water into a claylike paste) that is extruded as a long stick and broken or sawed into segments. Poured paints often have a characteristic drip around the lip of the pan, and may have a dimple in the center of the pan where the paint has shrunk during drying.

Most manufacturers develop a different formulation of paint paste exclusively for dry pan products. Schmincke boasts that they require two or three pourings to fill the pan; this is merely a sign that they are using the tube paint formulation for the purpose.

There is a brief but interesting tour of paintmaking operations at the M. Graham & Co. web site. For an illustrated tutorial showing how to prepare your own paints by hand, see the Kama Pigments web site. An excellent text on the basics of paint milling, manufacture and rheology (flow characteristics) is the classic Paint Flow and Pigment Dispersion by Temple Patton (Wiley Interscience, 1968).

Last revised 08.I.2015 • © 2015 Bruce MacEvoy

Raw pigment powder

A three cylinder paint
milling machine

Applying labels to tubes
filled with paint