The completed drawing was then transferred to the white panel by perforating the "cartoon", or a tracing of it, along its lines, then positioning it over the panel and slapping it with a pounce bag, or sock filled with charcoal dust. The stencil was then removed, and the drawing finished freehand. Another method for the transfer was to cover one side of a piece of tracing paper with charcoal, or with a thin layer of pigment and varnish or oil, which was then allowed to become tacky, and use it as one might use carbon paper. Once the drawing was transferred to the primed panel and completed, its lines were gone over with ink or very thin paint, either egg tempera, distemper (glue tempera), watercolor or oil, applied with a pen or small, pointed, sable brush, and allowed to dry. The drawing was then isolated, and the absorbency of the gesso sealed, by a layer of varnish. Sometimes a transparent toner was added to this layer of varnish, which was then called an imprimatura. The tone of the imprimatura set the key for the painting, making the harmonization of the colors easier, and allowing for more accurate judgment of values. A field of white primer tends to make everything applied to it appear darker than it is, until the white is completely covered, at which time the darks are sometimes seen to be too light. And when the darks are too light, generally the rest of the tones are too light as well. By toning the isolating varnish (a warm tone was most commonly used), to a tone somewhat darker than white, this problem could be avoided or minimized.
Once the isolating varnish or imprimatura was dry, painting commenced with the application of transparent glazes for the shadows. The paints used by the early Flemish practitioners were powdered pigments ground in walnut or linseed oil. There is widespread speculation regarding whether other ingredients, such as resins, balsams, and/or various polymerized oils were added, and the issue is not yet resolved as of this writing. All opinions on this subject must be understood to be guesswork until scientific analyses have been completed on enough paintings from this era to settle the issue. It is likely, though not definitely established, that the brushing characteristics of the paints might have been altered to a long molecular configuration by the addition of boiled or sun-thickened oils, and possibly balsams such as Strasbourg Turpentine or Venice Turpentine, and/or resins. Strasbourg Turpentine, sap from the firs growing in and around what is today Alsace Lorraine and elsewhere in Europe, is similar to Venice Turpentine but clearer and faster drying. Balsams and polymerized oils add an enamel like consistency to oil paint, changing its structure to a long molecular configuration. Long paint is easier to control than short paint, especially with soft hair brushes on a smooth painting surface, as in the Flemish Technique. Brushes used by the early Flemish oil painters were primarily soft hair rounds. Some were pointed at the tip; some were rounded, and some flat. Hog-bristle brushes were also used for certain purposes, such as scrubbing the paint on in thin layers for glazing and other effects. Painting commenced with the laying in of shadows and other dark shapes with transparent paint. In this method, the painting is carried as far along as possible while the paint is wet, but is usually not finished in one sitting. Large areas of color are applied after the shadows are laid in, and worked together at the edges. These middletone colors may be either transparent, opaque, or somewhere in between, depending on the artist's preference. The highlights are added last, and are always opaque. Several subsequent overpaintings may be applied after the initial coat is dry, if desired. Some Flemish artists also employed an underpainting of egg tempera, or egg oil emulsion paint, to help establish the forms before painting over them in oils.
The Flemish method, in summary, consists of transparent shadows and opaque highlights, over a precise line drawing, on wood panels primed pure white. The painting medium may possibly contain a resin and/or balsam, which increases clarity and gloss, or a combination of a polymerized oil with a raw oil, which takes on the most desirable characteristics of a resin when used together (i.e., sun-thickened linseed or walnut oil, plus raw linseed or walnut oil, mixed together), without the defects of natural resins. The innovations are the use of oil paint and the technique of glazing with transparent color. A glossy varnish is applied at least six months after completion. Paintings are generally limited to smaller sizes, due to the difficulties involved in constructing, priming, and transporting wooden panels of greater dimensions. It had its limitations, but was a vast improvement over egg tempera, both in ease of execution and in the beauty of the final result.
Although it originated in Flanders, word quickly spread of the marvels of oil painting, and it was soon adopted by the German artist Albrecht Dürer, who is known to have traveled to Flanders and to Italy, and by Antonello da Messina, who studied in Flanders, according to Vasari. Giovanni Bellini then learned it from Antonello, and taught it to Giorgione and Titian. The Flemish painter Rogier van der Weyden, who was adept at painting in oils, came to Italy around 1449 and influenced a number of Italian artists, including Piero della Francesca. The use of oil as a painting medium was adopted cautiously by some, and derided by others, as anything new always seems to create controversy. Michelangelo refused to paint in oils, and reportedly ridiculed Leonardo for adopting it. Titian (Tiziano Vecellio) recognized its merits, and soon added several innovations of his own.