Every paint is a mixture of microscopic pigment particles, which provide the paint colour, 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 adjust the visual appearance and handling attributes of the paint, and increase its shelf life in the art store.
Each paint manufacturer develops a proprietary backbone composition, a basic recipe of pigment and other ingredients, that is fundamentally designed to keep manufacturing costs under control and to get the best possible handling attributes for every pigment in the watercolour line.
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 medium consisting of:
Binder, traditionally and still commonly said to be gum Arabic but, in some brands, actually a synthetic glycol is used.
Plasticizer, usually glycerine, to soften the dried gum Arabic and help it dissolve.
Humectant, traditionally simple syrup or honey but now often inexpensive corn syrup is widely used, to help the paint retain moisture (especially in pan type paints).
Extender or Filler, such as dextrin, used to bulk out and thicken the paint without noticeably affecting the hue.
Manufacturing additives, in particular dispersants (to prevent clumping of the raw pigment after manufacture and to speed up the milling of the pigment and other ingredients) and a fungicide or preservative to suppress the growth of mould or bacteria.
Water, which dissolves or suspends all the ingredients, carries them onto the paper, and evaporates when its work is done.
The ratio of pigment to other ingredients 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” (non-calcinated) black and red iron oxides, zinc or titanium white, yellow quinacridones, benzimidazoles 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.
Unfortunately, watercolours 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 dissolve. In fact, early 19th century watercolours, 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.
Watercolours 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 the overall volume. Nowadays this is most often glycerine (glycerol), the trihydroxy form of alcohol. Glycerine 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.
As larger amounts of glycerine and gum Arabic are added to the paint — for example, in strongly tinting or finely divided pigments — the paint texture becomes stringy or toffee like, the gloss of the paint increases, and the paint bronzes more readily. These paints tend to lift (dissolve) too easily from the paper, which can lead to undesired blurring, bleeding or lifting of colour areas when new paint is applied over or alongside them.
To counteract these problems, many watercolour paints are formulated with a colourless, inert filler added to thicken the paint and to make the various pigment and vehicle mixtures within a watercolour 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.
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 and most of all, on the quality of the pigments.
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.
(4) Packaging the paint.
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