For broad areas, however, it is obviously unsuitable to work with a small sharp point. For large pictures, it is valuable to have some good bristle brushes; for they can be made with a broader, blunter point than the sables, and so produce a stroke better proportioned to a large scale of detail. It is not easy to obtain suitable bristle brushes, however, Artists' oil brushes are, for the most part, worthless for tempera. Up to the middle of the nineteenth century, oil colors were generally used fresh, ground by hand, with a good proportion of oil, and were in general fairly liquid. Brushes in those days were designed to hold enough of this liquid paint to make a long, smooth stroke. When colors began to be ground by machinery and packed in tubes, and kept, the proportion of oil was soon reduced; for manufacturers found that the pigments settled out of the oil, and their patrons complained. Cutting down the amount of oil did not solve the difficulty, and it was found necessary to put in wax and other materials, such as alumina and aluminium stearate, to [p. 93] Keep the color in suspension and to make it "set up." This sort of oil paint, however, did not flow from the brush. It had to be pushed into place on the canvas. And consequently the form of artists' brushes changed. Long bristles were an nuisance in handling this soft, plastic material; and round brushes spread it less neatly than flat brushes. The artist's bristle brush has changed within a hundred years from a pencil to a spatula. The thin, flat, straight, short-bristled brushes which are common today are as different as possible from the plump, long, springing brushes of the eighteenth century. There are, however, fortunately a few brush-makers who still offer well-made brushes with a good length of bristle.
Bristle brushes for tempera should be round, and very long in the hair in proportion to the diameter: as much as two inches of bristle is not excessive in a brush the size of a lead pencil, or a little larger. For the point to have any shape, a brush of this sort must be made from bristles of high quality, carefully graded, and accurately toed. This means that it must be expensive; and brushes of this sort are expensive. But with care and washing, they will last a lifetime, and pay rich dividends in satisfaction. They improve with use, and the more they are washed with soap and water, the better. The points may be rubbed with sandpaper or emery cloth if they do not give a smooth stroke; for the "flag" [the divided end of the bristle] is of no value except as an indication that the brush has not been made from cut bristles. [pp. 93-94]
[Thompson, Daniel V., Jr., Research and Technical Adviser, The Courtauld Institute of Art, University of London. The Practice of Tempera Painting. New Haven: Yale University Press. 1936. Fourth Printing, 1946.]