Varnish tends to darken with age which is why many old paintings have required cleaning. The Sistine Chapel ceiling fresco by Michelangelo is an example. Synthetic resins are now used. Originally, artists applied oil paints to wooden panels. Since wood has a tendency to warp or split canvas made either of linen or a linen-cotton blend was generally preferred.
Acrylics are water-based paints made from acrylic resins.
Many painters prefer them to oils. Acrylics are quick drying, durable, and adaptability to almost any surface. They can be cleaned from brushes simply with water. Watercolors are either transparent or opaque. Transparent watercolors come in cakes, tubes, or pans.