Because they are harder and last longer.
yes
Tempera won't stick to gold leaf. It will crack off as it dries out.
underpainting
"GOUACHE", paints, (pronounced 'g-wash') are water soluble paints in the watercolor family. Unlike traditional watercolor paint, which is translucent, gouache is an opaque medium, with a matte finish as opposed to a lustre. It can be mixed with traditional watercolor, and painted OVER watercolor, for interesting effect.
You may be referring to portraits made in "egg tempera." Egg Tempera was a European method of painting that used ground pigments mixed with egg and applied in fine thin layers. Egg Tempera portraits are most always small in size and rendered in fine and delicate detail. Many will crack over time.
Yes, with enamel paints.
put a cover over the over paint then you get get the two paint mixed
Egg tempera is a terrific medium with many advantages. It is safe, non-toxic, and permanent. Unlike oil paint, it will not yellow, change in color, or grow transparent over time. Unlike acrylic, it has a proven track record going back hundreds of years. Egg tempera shows the beauty of pigments off to great advantage. Colors are clear, bright, and pure. This is a terrific site that explains what-why-and how to make your own: * http://www.alessandrakelley.com/eggmedium.html
Alloys are useful improvement over pure metals because they tend to be stronger.
He was an improvement over both the Monarchy and the Republic.
Not really. You need something to clean the oil. Turpentine works, but there are also organic substitutes for it. You can find them at most art supply stores, usually near the paint itself. *Beware that the agents that clean oil paint (propanols, naphtha, mineral spirits) are also very likely to remove color pigments from whatever you clean.
Andy Warhol.