(First post on tempera grassa here). For some examples of great modern (20th century) paintings done in tempera grassa, check out the web page on Pietro Annigoni (1910-1988) at the Art Renewal Center.
Tempera grassa paint is a little like egg tempera (if you've tried that) and a little like oil paint, but mostly it is its own beast. It is much easier to control tempera grassa if you paint in thin layers and have relatively little paint on the brush when you make a stroke. It is, however, more flexible in application than egg tempera, and allows much more blending and re-working of the paint. To adjust the consistency of the paint, you can add as much water as you like, so long as the ratio of medium to pigment is correct. You can apply thin washes or heavy smears of oil-like paint. Never apply thick blobs of paint (impasto) as it can crack as it dries.
Small and medium-sized sable or soft synthetic brushes are best. Firmer bristle brushes can also be used—a bristle brush holds more paint and is good for initial lay-ins. Be very sure to clean your brushes afterwards, as the oil will take its toll on them if you are not careful. For cleaning, I’ve had good luck with wiping brushes first with cheap linseed (not vegetable) oil, then washing very thoroughly with soap and warm water.
The paint will quickly dry on the palette if it is not kept moist (a spray mister is good for this). After an hour or two, the paint is more difficult to work with; scrape it off and make a new batch. Working with tempera grassa will be frustrating until you come to understand the process by which it dries. Because it is composed of substances that dry at very different rates, the paint goes through multiple stages as you paint. By keeping these stages in mind and learning to work with them instead of against them, you will find that tempera grassa is extremely flexible, in some ways like a water-miscible oil paint.
In my experience, these seem to be the changes the paint goes through:
1. The paint is WET. As you first apply the paint, it is entirely workable. If you squeeze most of the paint from the brush, tempera grassa can be applied with drybrush hatching strokes, which tend to fuse together slightly (unlike egg tempera). It can also be gently blended with fingers or a soft dry brush. If the brush is loaded with more paint, it goes on loosely like any water-based paint. Once on the surface, it has a consistency similar to gouache and, depending on the recipe and the thickness of the paint, is workable for anywhere between one and five minutes.
2. The paint is TACKY. As the water evaporates out of the paint, there is a point at which it becomes sticky and difficult to work with. As you feel it enter this stage, leave it alone. It is not workable and attempts to manipulate it will pull up multiple layers of paint. If you accidentally dig a hole in the paint, stop and let it set before trying to fix it. It is possible to deliberately work with the paint at the tacky stage to create interesting textural effects, but this requires practice before the technique becomes controllable.
3. The paint has SET. The water has evaporated and the egg component is holding the paint in a semi-solid state. It feels firm but damp to the touch; rubbing will pull it off the surface. If the paint is applied thinly and the recipe is egg-rich, it will set a few minutes after becoming tacky (less if it was drybrushed on). If the recipe is oil-rich, it can take a long time to set (or there may not be enough egg for it to truly set at all). When the paint reaches this stage, you can paint over it, but only with soft brushes and a fairly delicate touch. Too much paint, or too firm a hand, will disrupt the surface just as if it were tacky. If the paint refuses to set properly after 20 or 30 minutes, a layer of thinned egg yolk (no oil) can allow you to paint over it without needing to wait for it to firm up.
4. The paint is DRY TO THE TOUCH. With egg-rich tempera grassa recipes, it takes anywhere from ten minutes to several hours before the paint feels dry and can be worked over without the risk of disrupting lower layers. If the recipe is oil-rich, it can take days to get to this state. Placing the painting in a sunlit room as it dries is a good idea—it won’t make the oil dry faster, but the actinic light will strengthen the egg component of the paint.
5. The paint is DRY. The oil component of the paint has oxidized to the point that the paint has hardened and is difficult to scratch with a fingernail. This can take anywhere from a day to a bit more than a week.
The working quality of these stages and the time it takes to reach them depend on the recipe you are using, how much you have diluted the paint, how thickly you are painting, how wet the layers underneath are, and local weather conditions. In order to paint easily with tempera grassa, you will need to constantly attend to the feel of the paint in order to get a sense of what stage it has reached. You can’t keep adding layers indefinitely, so you must recognize when it is time to stop working on a given passage and let it dry. Compared to oil painting, however, tempera grassa requires far fewer instances in which you must skip working for days at a time in order to allow the paint to dry.