PAINT MAKING

We all love colors, some even like to paint with them. But ever wondered how our favorite colors are made? Well this article focuses on the various techniques on their manufacturing process.

This page discusses standard paint ingredients and manufacturing methods. The material is presented in four sections: (1) the ingredients and recipes used to make watercolor paints, (2) the generic historical and modern pigments that provide the color in paints; (3) the manufacture of modern pigments; and (4) the manufacture of watercolor paints.

Information on these topics is scattered across a wide range of sources, from chemical engineering texts to art conservation studies. In some cases I was only able to obtain information by querying experts or manufacturers directly. Each source has its own perspective and professional traditions, and they sometimes disagree on specifics. I've made editorial judgments based on all the facts I could gather, and regret any inaccuracies that remain.

 

PAINT INGREDIENTS

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, 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, 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.

Schematic backbone composition of
a modern watercolor paint

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.

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.

Farming the gum acacia tree in Senegal

Last revised 08.01.2005 • © 2005 Bruce MacEvoy