susceptible to scratches during the curing process, but will become much more durable after curing. Avoid framing egg tempera paintings in glass, as the glass can trap moisture and lead to the growth of mold. Tempera grassa Adding oil in no more than a 1:1 ratio with the egg yolk by volume produces a water-soluble medium with many of the color effects of oil paint, although it cannot be painted thickly. Pigments Some of the pigments used by medieval painters, such as cinnabar (contains mercury), orpiment (contains arsenic), or lead white (contains lead) are highly toxic. Most artists today use modern synthetic pigments, which are less toxic but have similar color properties to the older pigments. Even so, many (if not most) modern pigments are still dangerous unless certain precautions are taken; these include keeping pigments wet in storage to avoid breathing their dust. Application Tempera paint dries rapidly. It is normally applied in thin, semi-opaque or transparent layers. Tempera painting allows for great precision when used with traditional techniques that require the application of numerous small brush strokes applied in a cross-hatching technique. When dry, it produces a smooth matte finish. Because it cannot be applied in thick layers as oil paints can, tempera paintings rarely have the deep color saturation that oil paintings can achieve because it can hold less pigment (lower pigment load). In this respect, the colors of an unvarnished tempera painting resemble a pastel, although the color deepens if a varnish is applied. On the other hand, tempera colors do not change over time, whereas oil paints darken, yellow, and become transparent with age. Ground Tempera adheres best to an absorbent ground that has a lower oil content than the tempera binder used (the traditional rule of thumb is "fat over lean", and never the other way around). The ground traditionally used is inflexible Italian gesso, and the substrate is usually rigid as well. Historically wood panels were used as the substrate, and more recently un-tempered masonite or medium density fiberboard (MDF) have been employed; heavy paper is also used. Pre-made paints Apart from the traditional process of mixing pigment with egg yolk, new methods include egg tempera sold in tubes by manufacturers such as Sennelier and Daler-Rowney. These paints do contain a slight amount of oil to enhance durability within the container. Notable egg tempera artist and author Koo Schadler points out that because