Acrylics and Tempera Painting with  long molecules
On the face of it Acrylics and Tempera have little in common. One is from Medieval times and died out (most people think) because, well, it was simply superseded by better things.  Acrylics on the other hand are cutting edge products, space age and all that and simply do things better. Tempera in this theory was just not capable of big important works that mediums like oil and acrylic are capable of. Sorry to debunk myths here, but  it is not true. Botticelli's wonderful and famous Birth of Venus is in Tempera, as are many famous pictures of the Renaissance. Tempera actually went out of favor as artists primary medium because painting fashions changed. Tempera was perfectly suited to the needs of many artist's, but clients started demanding the flashier works of the artist's who were using oil paints at the time. The subtlety of the Tempera painter was forgotten in the rush for something new.

In fact Tempera never went completely away and many famous modern painters like Andrew Wyeth and Ben Shahn and Klimt used with these old time 'polymers'. Polymers are long molecules that are like spaghetti as they interlock during drying to produce the final paint film that locks the pigment particles in place. Before drying the polymers are suspended in water in what is called an emulsion. Acrylics are polymer emulsions, their molecules are very long and crystal clear. it is the way the molecules interlock like strands of spaghetti that gives the built in flexibility that enables the paint to be used for many surfaces and the clarity of the resin enables the full color of pigments to be appreciated in all their brilliance. Acrylics have quite rightly assumed a place as one of the most important ways for artist's to paint.
Mother nature had already produced it's own long molecules that bind together as they dry. They are called proteins. Egg yolks just so happen to contain proteins of a type that make a strong and flexible paint film. In the case of egg yolk it is called albumen, and the egg yolk is an emulsion of albumen and similar ingredients. It is at this molecular level that Tempera, which is made with egg yolks, and acrylics are very similar paints. Of course there are important differences too. Let's have a look at some of those.

Related Links:

Oils and Alkyds

Watercolors and Gouache

Encaustic and Fresco

Drawing materials

Tempera Ground

Differences Viva la difference!
Body and texture
Acrylic works well being built up in thin layers, but it can also be applied thickly. Substances like sand, glass beads, or chalk can be added to it to make all sorts of thick textural surfaces that will dry and stay firmly in place. Tempera on the other hand can only ever be applied thinly. Any layer thicker than 1 mm will not adhere permanently to any surface. Thick layers of paint can be made but only by applying many layers. Textural possibilities from including foreign substances are limited.
Acrylic makes a great glue. Gel medium is a thick adhesive that can glue all sorts of objects to various surfaces. Acrylic molecules like to stick to each other, but they are not fussy and will stick to virtually any other non oily surface. Tempera on the other hand is less adhesive. It is not recommended as a glue. The egg molecules have a far greater desire to stick to each other than any other substance. This explains why it can only ever be used thinly, because in thick layers it can freely stick only to itself to the exclusion of any thing else, whereas in very thin layers the molecules are forced to stick to other substances, so they do.
Drying rate
It is often said that Tempera dries more quickly than acrylic. That is because of two factors. Egg yolk is a mixture of albumen, fatty egg oils and lecithin (an emulsifier). The egg oils are naturally fast driers. Painting technique also plays a role, as Egg Tempera is applied in thinner layers that dry quickly anyway. As acrylics tend to be applied in thicker layers, it would normally take longer for that thicker layer to dry than for the thin layer of Tempera even if the actual drying rates were the same. This technique difference combined with the egg oil drying rate makes Tempera a very fast drying paint.
Ease of manufacture.
Tempera is unique in that it is the only artist's material widely used that can never be successfully manufactured in a factory. Because it is made with egg, it can spoil if left liquid for more than a few days unless loaded with preservatives. The freshness of the egg determines many of it's qualities also and tempera kept in a tube for an extended period loses much of its beauty. Consequently the only way it is possible to use good Tempera is if you make it for yourself. It is best to make only the quantity needed for one day's work, although in the modern age we have the luxury of using the fridge to keep some paint a little longer. Tempera is also one of the easiest of all paints to make. Acrylic on the other hand, while easy to use can present the maker of artist's paint in the studio with a number of potential difficulties. If you want to make a durable water based paint I would recommend Tempera over acrylic in most cases.
Surface preparation
Acrylic is a surface preparation all by itself. Few artists do not use acrylic because even oil painters prepare canvases with acrylic. Tempera on the other hand is less adhesive which means it is best applied to a traditional size and whiting ground. As preparing one of those grounds is not too different to making paint in terms of skills we include a description of it here.

Acrylic Paint characteristics
There is a subjective judgment involved here. Some artist's prefer a more structured acrylic paint that appears to have more body. Adding small amounts of ammonia can aid this process which is the reason for that slight ammonia smell that comes from drying acrylics. Some artists prefer a slightly more runny acrylic sometimes referred to as a 'flow formula'. The more flowing acrylic holds a higher percentage of pigment and so is more dense and is more intense. Depending on the artist's desired technique it is possible to add textured and thicker mediums to this flow paint just as easily as it is to make thicker paint more flow like. Acrylic is very versatile in this way. Acrylics hold less pigment concentration than oil paint does (typically up to 60% as opposed to up to 80% with oil) but since the acrylic resin is crystal clear the colors are still brilliant. The paint should have sufficient body to cover well and be sufficiently flowing to easily be manipulated and applied with the brush. As the acrylic dries it loses the volume of water that was keeping it liquid and so it tends to shrink as it dries. If acrylic is likely to give problems such as cracking it will ordinarily do so within a short period of drying. Versatility is it's chief characteristic.

Tempera Paint characteristics
Tempera is a simple mixture of egg, water and pigment. The proportion of egg solution to pigment is roughly half and half. This makes an opaque pint that will hold up well. The pigment concentration of acrylic and tempera paints is very similar. Properly made tempera should be about the consistency of fresh cream and should flow from the brush easily. The fact it is common for Tempera painters to use sable brushes gives a good indication of the liquid nature of the paint. As the paint dries very quickly and loses water rapidly while working it is normal to make up a solution of egg yolk and distilled water to add to the paint as necessary. Distilled water must always be used as impurities in tap water can affect the paint. Likewise palette knives must be rust free as rusty steel can adversely affect the paint. The traditional test to make certain the paint is correctly mixed is to dab a little spot on a piece of glass. When it dries it should come off in a strong film. If it is powdery or otherwise weak it needs more egg yolk.

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