Such three dimensional decorations were covered with gold leaf, while the rest of the picture was painted very smoothly. The painting technique remained very smooth, both in tempera and in oils, all through the subsequent periods. Still, it isn't unusual to find a thicker application of paint, where it serves a functional purpose. Rembrandt's Man in a Golden Helmet is a beautiful example, showing repousse work on armor done with thicker paint. In modern times, impasto, a heavy application of paint, has become a widely accepted technique.
Some artists make their impasto so thick that it looks like a veritable bas-relief.
Until recently, impasto was possible only in oil painting, since linseed oil dries very slowly and retains its adhesive power forever. The binders in watercolor, gouache, tempera, and casein, on the other hand, cause the respective paints to adhere to one surface in thin layers only. If you apply such colors in heavy piles, they'll soon crack and fall off. Polymer, although applied with water, is an exception. The plastic binders allow you to heap paint upon paint, without the danger of peeling off.