Yellowing of Oil Paints
In my experience, most of the yellowing we see in old oil paintings is in the old varnish, and once that is removed, the yellowing of the paint itself is usually negligible, unless the painting has been recently stored in a dark place.
Linseed oil paints do yellow somewhat, but the yellowing is a temporary phase they go through that eventually reverses if the painting is exposed to light. That being said, the Old Holland Cremnitz White samples on my test panels yellow more than the others on the same panels, and remain yellower for a longer period of time. My guess is that this is due to their stabilizer, hydrogenated castor oil, which the manufacturer doesn’t want us to know is in there.
My personal feeling is that concerns for yellowing of oil paints are overblown, because the initial yellowing of linseed oil paints reverses with exposure to light, as I mentioned in my book. If the painting is kept in darkness, the yellowing will return, but will bleach out again if the painting is returned to the light. How fast these changes occur depends on the strength of the light the painting is exposed to.
The Separation of Oil and Pigment in Paints
Several Facebook users enquired about the runny oil they sometimes see when opening a new tube of paint.
The best oil paint brands (Natural Pigments, Michael Harding, among others) exercise restraint in the adding of stabilizers or they add none at all. Too much stabilizer causes oil paints to become too thick for good control with paintbrushes, and could possibly cause other problems as the painting ages.
In paint brands that have no or a minimal amount of stabilizer, the oil and pigment may eventually separate. The older the tube of paint, the more time the oil and pigment have had to separate. This means that sometimes there is more oil in the paint that we squeeze out [at the top of the tube], and other times there might not be quite as much [at the bottom of the tube]. And in some paints, the percentage of oil present may be higher than average.
“This is a simple matter to deal with. When the paint that comes out has more oil in it than we want, the thing to do is put it on a paper plate or the back of a business card just long enough for the excess oil to soak out of it and into the paper, and then transfer it to the palette. When the paint coming out of the tube is too stiff, we can mix a tiny drop or two of linseed oil into it and mix it in thoroughly with a palette knife or painting knife. By a tiny drop, I mean off the tip of a toothpick dipped in the oil jar. A little goes a long way.”
Oil paint in tubes needs no special way of storing. As long as the cap fits tightly enough to keep air out and there is no air bubble in the paint inside the tube, the paint will remain usable virtually forever. Store them in a temperature range of about 50 – 70 degrees F. This range is optimal for most sealed tubes.
Some artists store their tubes with the cap down so that separated oil will float to the crimped end. This may result in the paint coming out of the tube being too thick from the loss of oil, but in that case you can just add a little linseed and mix it in well with a palette knife.
The images below illustrate how some artists store their paints caps down: on peg boards, in plastic shoe holders, in test tube holders, and on clips.
Using stretcher or corner keys to tighten loose canvas on stretcher bars is not a best practice.
Removing Creases from an Unstretched Oil Painting on Linen
Symptom: Is there a way to remove creases from an unstretched oil painting on linen? My husband laid something on top of one of my paintings and somehow it got folded over and now has a large diagonal crease.
I’d suggest stretching it tightly enough to straighten the crease out of it, using metal-head push pins as tacks, and leave it that way for at least a few days. Then remove the painting from the stretchers and glue it to a rigid panel with a bracing framework glued to the back, and tack the edges of the canvas to the braces, stretching it while the glue is still liquid. I’ve done this enough times by now to be able to do it without an assistant, but if you have someone who can help you, it will be easier and less time-consuming.
The adhesive I use is BEVA 371 film, which is reversible with heat in the 130-140-degree Fahrenheit range. It requires heat to liquify it. When I do this, I first adhere the BEVA film to the panel, then carefully position the canvas on it and cover the canvas with a fairly thick cotton sheet. I use a regular laundry iron adjusted to the cotton or wool setting, and start ironing in the center of the canvas, keeping the iron moving constantly, going first from the center to the middle of one of the long sides, stretching and ironing at the same time, and then putting a push pin in. Then move to the opposite side and do the same thing, keeping the iron in motion constantly while it’s on top of the sheet to prevent it from scorching the painting. The procedure is the same as stretching on stretchers, but with the added task of ironing at the same time; essentially a juggling act.
The benefit of using this adhesive is that you can remove the canvas and redo it if it doesn’t turn out well on the first attempt, and in your case, the iron will help flatten the crease. There is a potential downside as well, in that the painting could be scorched if the iron is not kept moving or if it’s hotter than it needs to be to liquefy the adhesive. I scorched one of my paintings the first time I did it, but I fixed it by scraping out the scorched part and repainting it. Anticipating that possibility, shoot good photographs of the painting before you begin, so you can use the photos as a guide in case you need to fix it after scraping out a scorched part.
You might try doing this with a blank canvas first, in order to gain experience that will help you do it successfully when you’re doing it with one of your paintings. I would only do this with my own paintings because I can fix them if I mess up. With other people’s work, we shouldn’t risk screwing it up. Professional conservators are who should be entrusted with this task.
For the future, I suggest not painting on loose canvas. Stretch it or mount it before you start putting paint on it.
Mold and Bloom on the Canvas
Symptom: White spots on a canvas that has been stored in a damp studio. Could it be bloom or mold? How should one deal with it?
The white patches could be bloom instead of mold. This is something conservators have started finding recently in oil paintings from the 1960s and newer. It’s suspected as being caused by aluminum stearate, a stabilizer that most brands add to their oil paints to prevent the oil and pigment from separating in the tube while the paint is waiting to be used.
If there is any moisture in the paint when the varnish is applied, it can cause bloom, a cloudiness that interferes with the viewing of the painting. If you don’t see this distortion looking at the painting straight on, then it probably isn’t bloom.
If the spots are mold, try brushing them off with a clean, dry, stiff hog-bristle brush, while wearing a mask to prevent inhaling any mold or spores that become dislodged. The best advice, however, is to take it to an AIC-certified conservator.
Resource: American Institute for Conservation
Storing Your Artwork
Storing your art in circumstances with extreme changes in temperatures and/or humidity will cause adverse effects to an oil painting. If the substrate you used is on stretched canvas, the impact will be even worse than other supports because canvas will expand and contract with heat and humidity changes. The paint will crack and may be pushed off the surface.
Do not leave your artwork in an unheated area. Cold temperatures will slow down the curing process for drying paint and may allow for condensation of moisture build up in the layers. This can cause fogginess (a.k.a bloom). If the layers dry unequally, the cold can also lead to long term cracking. Do not store your work on a concrete floor either since concrete transmits cold and moisture
Keep in mind that with oil paintings, the damage done by changes in the environment may go undetected until the painting is clearly in a bad condition.