The actual process is simple, but it has to be done right to prevent chalkiness or cracking.
|The Farm at Olana, by Carol L. Douglas|
“How should I treat older, unvarnished paintings that have become chalky?” a reader asked me. “And for that matter, what kind of varnish do you recommend?”
Chalky paintings are best revived with varnish. My personal preference is Winsor and Newton Artist’s Matt Varnish. It’s easily removable by a conservator, provides UV protection, and it doesn’t shine. (If you want more reflection, they make both gloss and satin finishes.) As with all varnishes, it shouldn’t be applied for a year after the painting has been finished, and it should never be used as a painting medium.
But learning to use medium properly is a better protection against chalkiness. (If you’re new to oil painting, consult my basic rules of oil painting.)
|Mamaroneck River, by Carol L. Douglas, in its finished form right before it went on the auction block.|
There are several families of oil painting mediums out there:
- Traditional mediums are made from a slow-drying oil (poppy, linseed, etc.) a resin, and a solvent, sometimes with a siccative like Cobalt Drier added to improve the drying time.
- Alkyd mediums are made with a slow-drying oil reacted with an alcohol and an acid. They dry faster, making them suitable for multiple glaze layers, but they carry less pigment. They should never be used on top of traditional oil mediums.
- Solvent-free mediums do not include any petroleum distillates. All unmixed slow-drying painting oils are inherently solvent-free, being plant-based.
- Wax based mediums include cold wax and encaustic.
|Mamaroneck River, when the second layer was done but the top layer hadn't been started.|
For field painting, I prefer a traditional medium: Grumbacher’s Oil Painting Medium II. It retards the drying time and allows me to work wet-on-wet longer. In particularly dreary conditions, I’ll use their Medium III, which has a drier. For studio work, I sometimes use their Medium II, because it dries to a matte finish. I buy this brand because I prefer a very thin medium. There are many good brands out there, and I would trust any medium made by a reputable paint house.
|Grain elevators, Buffalo, by Carol L. Douglas is an example of a cold-wax medium painting. It gives tremendous latitude in effects.|
Back in the last millennium, there was a popular recipe of linseed oil, turpentine, Damar varnish and a few drops of Cobalt drier. I stopped making it when I realized that so many 20th century masterpieces were degrading terribly. Chemists understand chemistry better than painters do.
|The drawing layer is always done with solvent, whether in the studio or in the field. That's Sandy Quang, back when she was my studio assistant.|
Neither medium nor solvent is a substitute for good, open paint. If you’re poking a skin off your paint and extending it with solvent, it’s time to clean your palette and get new paints out.
The actual process for using medium in alla prima painting is simple: fat over lean. Your first layer—where you’re drawing the picture on the canvas, called the underpainting—is cut with a small amount of solvent only. Use too much and you’ll have muck above. Your middle layer should be as close to pure paint as you can get, applied thinly. Your top layer should be paint with a small amount of medium, and here’s where you can get as impasto as you want.
|Clip this and paste it in your pochade box until it's a habit.|
Medium doesn’t belong in the bottom layers of a painting. The more oil in a layer, the longer the binder takes to oxidize. This keeps paints brighter and more flexible. However, oil also retards drying. Using too much in underpainting will result in a cracked and crazed surface over time.
Use too much medium and you’ll have a soup that dries to a plasticky finish. Use too little, and the paint will drag as you paint and cloud as it dries. I generally just touch the tip of my brush to the medium before mixing it into a brushstroke’s worth of paint.
This traditional method is tremendously variable and gives great control. Practice it.