Here's a excerpt of the section on Brushes from my website www.surrencystudios.com in the artist reference section.
I hope you find it informative. There is more on watercolor and other types of brushes and reference material on the site.
Enjoy - Marc
Western brushes consists of six primary parts: the tip, belly, and binding of the bristle/hair/fiber, the ferrule, the crimp, the handle and handle adhesive.
The brush consists of bristles/hair/fiber that is bundled together and tied before being placed into the ferrule and glued to the handle with adhesive. Only about half of the bundled bristles/hair/fiber is exposed above the ferrule. The tip of the brush is the most fragile part of the brush. With round brushes the tip should come to a point. The tip is responsible for drawing up and releasing the paint.
Below the tip is the belly. The belly is the reservoir of the brush, where the paint is held. Brushes created for liquid paints typically have larger bellies and can hold more paint.
Beneath the belly is the ferrule, which is typically metal and is often chrome, brass, in professional brushes. In some cases the ferrule can even be gold in limited edition high-end brushes. The ferrule provides the structural support for the bristle/hair/fiber. Good quality ferrules are made out of a single piece of metal and should not have a seam.
The ferrule is crimped onto the handle. This crimp holds the ferrule onto the handle. Most handles are made of hardwoods. Some exceptions are watercolor brushes with acrylic handles. If the handle is wood, it is typically painted to protect the handle from paints and liquids. The only part of the handle not finished is the top where the bristles/hairs/fibers are attached with adhesive. No finish is applied in this area so that the adhesive may form a strong bond. Although not applying a finish to the tip of the handle promotes a strong bond with the adhesive it also leaves the wood susceptible to damage by paint, solvent, or water if the brush is mishandled, such as dipping the brush into water/solvent past the ferrule.
The ability of a paintbrush to hold and transfer paint is a result of the capillary action created by the bundle of brittles/hair/fiber and the surface area of the fibers. When the brush is applied to a low viscosity liquid paint, such as water color, the paint is drawn up into the belly of the brush by capillary action. When tip of the brush is applied to the paper surface the absorptivity of the paper and force of gravity pull the paint out of the brush. As the viscosity of the ink increases and it becomes more paste like, the paint no longer flows up into the belly on its own. It must be "loaded". That is, the paint is drawn up into the belly of the brush by pulling the paint up into the brush by physical action/pressure.
How much paint can be held in the belly, the control with which the brush releases the paint, and the ability of the brush to maintain its tip is a reflection of the number of, size, and shape of scales on each hair, which varies between species of animals and their environment.
The size of the belly and the length of the handle vary depending upon the viscosity of the intended paint. Watercolor brushes have fat bellies and short handles. Oil paint brushes have narrow bellies and long handles. The difference in the length of the handles is quite significant as illustrated in the figure to the left. The image right illustrates the difference between the slender belly of the oil round (a) with the fullness of the belly of the watercolor brush( b).
Oil Paint Brushes
Brushes used in oil painting must contend with great differences is the viscosity of the paint. That is, how easily or difficult the paint flows onto the canvas. How the paint flows is a result of many factors, but is chiefly a result of the combination of the pigment/oil ratio of the paint, the medium used, and the surface of the canvas/painting. Since oil paint can range from very stiff to very fluid, different materials (bristles/hairs/fibers) are often used to create brushes that can control the different viscosity paints.
Early in the process of painting, oil paints have less oil than they will in later painting sessions (fat over lean) and the painting surface is rougher at the start than it will be later in the process. This results is significant drag or resistance when the brush is drawn across the canvas or board. As a result the brush must have a head of bristles or hair that is fairly stiff so that it is able to retain its shape and transfer the paint despite the resistance of the thick paint. Most artists use a brush made of hog's bristles for this purpose.
As more paint is applied to the surface and if the painting is painted in layers, more oil is added to the paint decreasing the stiffness of the paint, also mediums may be added further changing the consistency. Additionally as more layers of paint are applied to the canvas or board, the surface of the painting becomes smoother. The result is less drag on the brush as the paint is transferred to the painting. The paint no longer requires the stiffness of the hog's bristles and other bristles/hairs/or fibers can be used. If more detail is required artists often switch to a hair that comes to tip such as sable.
For each bristle/hair/fiber type there are different different sources (i.e., animals, geographic location, etc.) that are considered more desirable than others due to their differing characteristics. One significant problem with these different sources is their nomenclature, that is how they are named and the lack of stringency and regulation on whether the name actually describes the hair in the brush. Luckily, with the exception of "sable" hair, most large manufacturers are diligent in ensuring that the hair in the brush is correctly reflected in its description.
The two most common materials for making oil paint brushes are hog bristle and hair, typically sable or a blend of sable and other hair or synthetic fiber.
Artists brushes made from pig, boar, or hog's bristles are stiff and intended for use with very viscous paints such a oils. The better bristles come from hogs found in the colder regions of Asia. These hogs have longer and stronger bristles than those found in more domesticated western countries. Another and perhaps more important characteristic is abundant flagging. Flagging is split ends found at the tips of fine bristles and responsible for better paint transfer and control. The bristles are boiled to remove some of the color and straighten the bristles. Currently the finest bristles come from the hogs found in the Chungking province of China.
With some of the curve still left in the bristles, the bristles can be aligned so that the bristles curve toward the center of the brush. The result in called interlocked and the brush will always maintain this shape and will have superior control. The photograph to the right shows a filbert (left) and flat (right) interlocked bristle brush. Note how the bristles curve inward toward the center.
Excessive boiling creates very straight and somewhat weaker bristles. These bristles are used to create non-interlocked brushes which have the bristles pointing in all different directions. These brushes are usually much less expensive and lack the control found in interlocked brushes.
Bristle brushes come in various shapes and sizes. The brush size is a relative designation and is not based on any actual metric (numbered measurement such as inches). The only exception is large flat brushes which are often given in inches. As a result, brushes that bear the same size designation, but are produced by different manufacturers may not be the same actual size.
The round (a and c) is the oldest style and was the only style available during most of oil painting's history. Thus, it was the style brush used by most of the masters. So what painting characteristics does this brush have? The start and end of the stroke is a point with the stroke widening with more pressure applied to the tip.
The filbert (b and d) is essentially a flattened round and provides painting characteristics similar to the round but with a broader stroke than the same size round and even with a round two or three sizes larger. The figure above illustrates the difference in shape and size found in a size 4 round (a) and filbert (b) and size 6 round (c) and filbert (d).
The figure above shows a large interlocked filbert (a, size 20), a large interlocked flat (b, size 20), and close-ups of a medium sized filbert (c, size 10) and medium size bright (d, size 10).
Flats have a sharp blunt start and maintain a constant width during the stroke and ends with a blunt lift. This brush is often used to paint buildings (as a subject or actual) where a hard chisel edge is desirable.
The bright is similar to the flat, but the length of the bristle is much shorter, only about half to three-quarters as long. The stroke of the bright is the same as the flat. The difference is in the execution. The bright is much stiffer and provides more control. However the shortened length of the bright holds less paint and therefore cannot create as long of a stroke as the same size flat.
Another style is the brush fan (below). The bristle version of this brush is often used as an "effects" brush and is used to create texture or create foliage.
Artists brushes made from animal hair or blends of animal hair and synthetic fiber are designed for use with low viscosity oil paints such as those used when painting detail or glazes. These brushes lack the stiffness to transfer stiff oil paints and can be severely damaged if they are used for this purpose. Although the quality of hair does typically not need to be as high as that used for watercolor, the hairs should have excellent spring (the ability of the hair to come back to its original position after being bent) and should maintain a sharp point at its tip.
The most prized hairs used for artists brushes come from the sable family. The term red sable was created by trappers and refers to a family of animals as opposed to one particular species. Here lies one problem in the naming process. The term sable can refer to the kolinsky, the marten weasel, and the Asian mink to name a few. It would be nice if the brush makers used the scientific name, such as Mustela siberica, for the Kolinsky, or the Martes zibellina for the marten weasel since both are "red sables" (Steven Saitzyk, 1987).
The most highly prized hair used for both watercolor and oil brushes is from the tail of the Siberian kolinsky's winter coat. It's tail has long hairs that have excellent spring, holds a great deal of water and paint, and releases the paint from the belly of the brush very consistently. The color of the hair is light yellowish turning reddish and darker toward the tips. The quality of these hairs are really only needed for watercolor painting - at least in my opinion. The only time that they might be needed in oil is for very detailed work with the oil thinned out with lots of medium. However, I believe that the red sable would do the job just as well.
I won't go into the background here, but because of the cold war, trade restrictions, and over trapping, the availability of true kolinsky sable has been an issue. In fact, the original animal to hold the name, a strain of mink that lived in the Kola Peninsula in western Russia, is almost extinct and no longer used for its hair. Currently, the term "kolinsky" denotes the Asian mink, Mustela siberica, found in northern Siberia (Steven Saitzyk, 1987).
As a result there have been numerous instances of mislabeling in the brush world. However with the end of the cold war, the availability of kolinsky hair seems to be increasing, with some brush manufacturers offering a selection between winter and summer and between male and female coats. This being said, there are still a lot of "kolinsky" brushes that have hair that range from red to black in color. The label for a kolinsky brush typically read "pure kolinsky," "kolinsky," or "kolinsky sable" and have hair as described above. One word of caution when buying kolinsky brushes, since the hair is so expensive its not unusual for unethical companies to bleach and dye weasel hair to make it look like kolinsky. Remember to always try out a brush before buying it. If a store won't let you try out a $100+ brush out with water, then shop somewhere else.
Kolinsky and red sable are really the same animal (most of the time), however the brush makers reserve the term "Kolinsky" for the finest mink hair.
The label "red sable" generally indicates that the hairs are not up to the same standards as the kolinsky, typically due to differences in climate that the animals are found in rather than differences in species (Steven Saitzyk, 1987). So the short of it is, the term "Kolinsky" is typically reserved for the better hair grown on the minks in northern Siberia, while the term "red sable" is used for seconds from the kolinsky brushes, the minks found in warmer climates and/or the marten weasel or a combination of the three. Most reputable manufacturers will tell you what constitutes their brush hairs. After all these are still not inexpensive brushes and they have pride in their work.
The hair of the red sable is similar to the Kolinsky, but lacks some of the responsiveness, and spring. It still comes to a sharp point like its cousin the Kolinsky (after all it might be from the same species - just a different season pelt). It's also typically less expensive, although the prices for these brushes can easily reach $100. The hairs of this brush are reddish and darken at the tip.
These brushes are often created from various species of marten weasels and/or left over remnants from red sable brush production and can vary tremendously in their characteristics. (Steven Saitzyk, 1987).
[Size and Shapes
Like bristle brushes hair brushes come in a wide range of shapes and sizes. The most popular are the rounds, filberts, and flats.
The brush size is a relative designation and is not based on any actual metric (numbered measurement such as inches). As with the bristle brushes, brushes that bear the same size designation, but are produced by different manufacturers may not be the same actual size.
With the round (a), the start and end of the stroke is a point with the stroke widening with more pressure applied to the tip.
The filbert (b) is essentially a flattened round and provides painting characteristics similar to the round but with a broader stroke. Often sable filberts (c) come to a fine point at the tip and are designated as "cat tongues." Flats (d) have a sharp blunt start and maintain a constant width during the stroke and ends with a blunt lift. The bright (e) has the same on canvas characteristics as the flat, but is stiffer and has more control. As with the bristles the fan brush (f) is also available in hair. However, the hair fan brush is used much differently than the bristle fan. The sable fan is used as a blender and often o manipulate direction of strokes to reduce glare.
Another type of hair brush is the blender. These are generally large brushes and are used to blend or fuse one color into another.
Brushes used in watercolor painting only has to contend with the low viscosity of the paint. Therefore the requirement for the stiffness of the oil brush is replaced with the need to be able to hold and control the release of a fluid paint. Additionally the oil in oil paints which helps to create and maintain a sharp point is not present so the brush must maintain its shape on its own. As a result, the quality or lack of quality is noticed more with watercolor.
Kolinsky / red sable
As with oil, the most sought after hair for watercolor brushes is the kolinsky followed by the red sable. The main difference between those brushes made for oil and watercolor is the shorter length of the handle and the fuller belly of the watercolor brush. The kolinsky and red sable brushes have superior storage capacity, control, and spring than the other available hairs and fibers. They also have a superior price tag. Luckily, if they are properly cared for they can last for decades. One warning. Never use watercolor Kolinsky brushes for oil. These brushes have fuller bellies which not only make them more expensive, but more susceptible to damage by oils. Additionally the types of oils used in oil painting often stick to the surface of the hair and change their ability to retain water. You'll ruin your watercolor brush if you use it for any other medium.
Synthetic fibers have found success in use with watercolor brushes. They sometimes have excessive spring and lack control, but recently have improved so that they are comparable to some of the lower end red sables. In fact, I know one artist (Mal Surrency) that prefers synthetics to his series seven sables. What they may lack in control and longevity (approximately 2 years), they more than make up with their low price tag. Synthetic fiber brushes are marketed under several different names including those easily confused with natural hair such as "white sable," "golden fleece," and "golden sable."
Steven Saitzyk, The Definitive Guide to Artists’ Materials 1987