My, all the interesting topics people bring up! Let me see if I can throw in my two cents' worth.
On the topic of reversing the darkening of paintings, let me quote from Ralph Mayer's The Artist's Handbook:
"Several much-quoted letters written by Rubens recommended exposure to sunlight for certain oil paintings that had in each case been boxed soon after being painted, and stored for some time. For centuries artists have known that freshly painted oils should be exposed to daylight under normal conditions, that continual or severe exposure to bright, direct sunlight is not beneficial [it leads not only to fading of the pigments but also to embrittlement of the paint film, particularly if there is any resin in the painting medium], and that the darkening of oil paintings from continued absence of light during the drying process is a reversible reaction that can be corrected by further exposure to daylight."
As for beeswax in megilp, Pam, you may not be confused (Confusion is my middle name). Although beeswax is not typically added to megilp (as in the recipes I cited in the post above), there is a famous exception. In
http://www.lavendera.com/paint%20tech/maroger.htm there are six recipes for maroger, including...
"Fourth Lead Medium (attributed to Peter Paul Rubens)
"This medium was allegedly based on the black oil of Giorgione with an addition of mastic resin, Venice turpentine and beeswax. One or two parts litharge or lead white, combines by cooking with 20 parts raw linseed or walnut oil. A little more that one spoonful of 'black oil' combined with even one spoonful of mastic varnish resulted in the 'jelly' medium thought to be Megilp (another name of Maroger mediums)."
As for evening out glossy and matte patches -- which I find often occur in areas of different pigments, with different capacities for oil absorption, or in areas where I have handled paint differently, particularly areas I've touched up (perhaps evidence of the "suede effect" attributed to the alkali-refined linseed oils typical of modern oil paints) -- there is this from a current Gamblin brochure:
"If you would like to unify the surface and saturate the colors without varnishing, consider a clear glaze layer of Galkyd or Galkyd Lite, thinned with 50% odorless mineral spirits."
That worked for me (at least so far, in the short run -- I will varnish in 6 months or more).
From Mr. Gamblin himself, I received the following in an e-mail:
"When the work is done, and all that is left to do is to determine the surface gloss, then we are in the domain of varnish and not medium. I would use the Gamvar [Gamblin's specially formulated varnish], but make a 'retouch' concentration. Add an additional two parts solvent so that you have a very light coating of the varnish. This is standard practice. If you use a painting medium then technically you have glazed the painting but the glaze has no color, except for any color change the medium goes through. If there is linseed oil in that glaze then it will warm up the painting over time."
Once again, after 6 months it is typically beneficial to varnish, although that, too, will change the overall look of the work.
As for the stability of paintings created with alkyd mediums over time, Gamblin states (
http://www.gamblincolors.com/newslet...sletter07.html):
"Galkyd painting mediums speed the drying time of oil colors and increase their flexibility. Galkyds will not yellow over time."
And I'm sure Winsor & Newton makes similar claims for Liquin.
As I've said, some impartial experts say it's too soon to tell; but the consensus seems to be that paintings done with alkyds are at least as durable as those done solely in oil (and probably more so than those done with mediums incorporating natural resins, even soft mastic).
I would be interested in seeing any sound, controlled scientific data (i.e. not just "anecdotal" evidence) for long-term or accelerated aging studies of oil paintings executed using mediums with alkyd resins.
Finally, on the topic of toxic materials, I must admit to using cadmium yellows and reds (I prefer their more "earthy" tints, tones, and glazes to those of modern pigments; and I value their lightfastness and, in many instances, their opacity). However, I use the colors from Gamblin, who claims that their cadmium colors are "made from chemically pure cadmium of low toxicity (no health warning labels required)." Here in California, however, with the strictest environmental standards in the country if not the world, there indeed is a standard warning label on all cadmium materials, although the indication seems to be that the major danger is from inhalation via spraying.
Nevertheless, from the official MSDS filed by Gamblin for their cadmium reds and oranges (there is a similar statement for the yellows) -- found at
http://www.gamblincolors.com/msds/cadredorago.html -- comes the following:
"OSHA has chosen to regulate occupational exposure to all cadmium compounds, including pigments, as a single category. The standard states that substances containing cadmium are a cancer hazard and can cause lung and didney [sic] disease.
"Cadmium and cadmium compounds are listed in the Annual NTP Report as carcinogenic to animals, but with only limited evidence of carcinogenicity to humans. This information is based on test results for cadmium compounds other than pigmentary forms.
"Although certain cadmium compounds are known to cause kidney damage in humans and has [sic] been shown to cause lung cancer in laboratory animals, no chronic health effects have been shown to result from exposure to cadmium pigments. Cadmium pigments have been shown to be significantly less biologically available and less active than other cadmium compounds...
"No known medical conditions aggravated by exposure to cadmium oil paints...
"Small amounts can be dried and disposed of as ordinary trash."
As every locality has its own standards and methods of trash and sewage disposal and treatment, I'm not going to touch that topic with a ten-foot pole.
Cheers!