more common until the 16th century in Italy and the 17th century in Northern Europe. Mantegna and Venetian artists were among those leading the change; Venetian sail canvas was readily available and regarded as the best quality. Canvas is usually stretched across a wooden frame called a stretcher and may be coated with gesso prior to being used to prevent oil paint from coming into direct contact with the canvas fibres which would eventually cause the canvas to decay. A traditional and flexible chalk gesso is composed of lead carbonate and linseed oil, applied over a rabbit skin glue ground; a variation using titanium white pigment and calcium carbonate is rather brittle and susceptible to cracking. As lead-based paint is poisonous, care has to be taken in using it. Various alternative and more flexible canvas primers are commercially available, the most popular being a synthetic latex paint composed of titanium dioxide and calcium carbonate, bound with a thermo-plastic emulsion. Many artists have painted onto unprimed canvas, such as Jackson Pollock, Kenneth Noland, Francis Bacon, Helen Frankenthaler, Dan Christensen, Larry Zox, Ronnie Landfield, Color Field painters, Lyrical Abstractionists and others. Staining acrylic paint into the fabric of cotton duck canvas was more benign and less damaging to the fabric of the canvas than the use of oil paint. In 1970 artist Helen Frankenthaler commented about her use of staining: When I first started doing the stain paintings, I left large areas of canvas unpainted, I think, because the canvas itself acted as forcefully and as positively as paint or line or color. In other words, the very ground was part of the medium, so that instead of thinking of it as background or negative space or an empty spot, that area did not need paint because it had paint next to it. The thing was to decide where to leave it and where to fill it and where to say this doesn't need another line or another pail of colors. It's saying it in space. Early canvas was made of linen, a sturdy brownish fabric of considerable strength. Linen is particularly suitable for the use of oil paint. In the early 20th century, cotton canvas, often referred to as "cotton duck", came into use. Linen is composed of higher quality material, and remains popular with many professional artists, especially those who work with oil paint. Cotton duck, which stretches more fully