Measures Taken to Hasten Oil Paint Drying Time

Hello,

I'm in a position of needing a recently completed painting to dry faster than usual under normal conditions for a coming exhibition, so I've been placing a heater in my studio overnight in an attempt to speed up the drying time. The temperature in the room gets to around 95F overnight.  It's clearly helping the drying process, but my question is --

Is there any chance of doing any harm to the oil paint by exposing it to hotter than normal temperatures? To be clear, the heater is on the opposite side of the 12' x 13' room, therefore the heat source is well distanced from the painting itself.

I've done some minimal research and the only thing I find in terms of potential harmfulness is when the proximity of the heat source is too close to the painting, but thought I'd check in here for any advice.

Thanks for any thoughts/advice. 




Comments

  • That should be fine, @Bucky. I've read of people putting their paintings in the car parked in the sun to dry. That might be a bit much.
  • I would express some caution based on what I'm picking up from those think in terms of conservation best practice. Since oil paint does not 'dry' in the sense of evaporation, it's an oxidisation process, when we attempt to speed it up it is apparently more likely that we create a dry film on the outside that may crack because of wet paint beneath.
    Virgil Elliot, author of Traditional Oil Painting:
    "There are long-term consequences to consider when we try to get oil paints to dry faster than it is their nature to do, and some methods and materials are more harmful than others. The technically soundest way to achieve fast drying is to choose the paints whose pigments themselves aid the drying of the binding oil. There is a list of them in my book. It isn't necessary to add driers if we choose our paints with their drying rates in mind. Heating with ovens, heat guns or hair dryers is not a good thing for the painting if we care about how long it will remain in good condition."
    "A better idea is to let the paint dry at its own natural rate in stable conditions. Ideal conditions are those in which humans are comfortable: around 70 degrees Fahrenheit and relative humidity around 50-60 percent."
    Re Cardany, Traditional Oil Painting site admin:
    "Direct sunlight and/or increased heat alters the paint chemistry oxidation process. Best practice is 65-75 degrees with a moderate humidity. 'Works for me' isn't the standard."
    "Don't force drying with direct sunlight, heat guns or direct fan wind. This may seal surface [dry to touch] that will crack eventually [because lower layers are not dry]. Paint oxidizes to dry. Best place is in a warm room with air movement like a ceiling fan or open windows."
    Annie
  • edited October 13
    Yes, I wouldn't recommend subjecting it to too much heat. Still, a room at 95 degrees F would be just like having it in a room on hot summer days. I doubt there would be a problem with that. I guess you would also want to ensure there is at least some ventilation, too, so that there is a continual supply of oxygen to oxidize the paint. 
    Annie
  • @Bucky – I think your painting will be fine. I am respectfully disagreeing with the Traditional Oil Painting advice given by @Abstraction.

    Here is why: It is true that reducing drying time by increasing temperature, sunlight, or dryness of the air will affect the robustness of the paint surface. However, what is not discussed by Mr. Elliot is the paint thickness which will be adversely affected by these practices. The thinner your paint layer, the less it will be affected by forced drying (with a few exceptions based on pigments).

    How thick is your paint layer? I paint in thin layers of much less than 1 mm. I am guessing less than 1/10 mm. I live in Phoenix Arizona USA which is hot and dry. For 5 months of the year, the temperature gets over 100F, and for 3 months it gets above 115F off and on. The relative humidity is usually less than 20%. Sometimes it gets below 5%.

    I routinely put out my paintings on my back patio, where it gets dry, hot, and breezy, to speed up the drying. No ill effects noted after years of this. This is because my paint layers are so thin that the expansion and contraction of the paint film as oxygen is taken on and then released is much less because the volume is much less.

    Don’t put your paintings in direct sunlight, as the UV will affect some pigments dramatically. This is the basis of lightfastness (LF) tests.  I stay away from modern synthetic pigments and lakes, and try to use mineral and metal based pigments. (I do sometimes put my paintings in direct hot sunlight to quick dry, but do not recommend this. I know my limited pigments.)

    Some pigments are also affected by heat: again, modern synthetic pigments and lakes. I don’t know at what temperature the different pigments and lakes will be harmed. I am guessing somewhere over 140F. But I don’t know.  I only know my limited pigments.

    Abstraction
  • PS – The difference in drying time based on changes to humidity and temperature can be dramatic. My personal experience: using the exact same pigment cadmium red, from the exact same tube: when I lived in a cool, humid, and cloudy place (Flagstaff AZ), the thin paint layer took 60 days to become tacky dry. In hot, dry, and sunny Phoenix AZ, inside in the studio, it takes 5 days.  When I put it out on the back patio, out of the sun, it dries in 2 days.

  • A well known painter from ages ago wrote in his book about this (sorry, can't remember his name).  He urged students to leave their paintings out in the hall (cold) at night to slow drying, or to take it in their room at night (warm) to speed drying.  Now that I think about it, I believe his name was Solomon J. Solomon.  Point is, Mr. Solomon would say you're on the right track.
  • Thanks very much @tassieguy @Abstraction @Desertsky @broker12. I think I'll Keep it going for a few more days seeing as the ventilation in the room is fairly decent and the 95F temperature is hot but not too extreme.  Thanks again! :) 
    tassieguy
  • @Bucky - Please keep ups posted as to results. 
    Bucky
  • Thanks @desertsky. Helpful clarification. Even as I wrote that I was wondering about the impact paint thickness although I hadn't seen any reference to it. I expect then it's the same with varnishing. If we don't have significant impasto created the waiting time to varnish would be less. I also paint thin layers and rarely paint wet into wet.
    Bucky
  • @Abstraction - the devil is always in the details. :) 
    Bucky
  • Does the general rule that a 10 degree C/18 degree F increase in temperature speed up drying by a factor of 2 apply to oil paint?  If so, you could come up with some kind of reasonable expectation as to the time it will take to dry your painting at 95 based on your normal practices.  Thereby see if you have a reasonable expectation that it will dry in time.  Of course people have shown wet paintings...  What a hassle though, and it wouldn't be good if there was an odor.
    Bucky
  • My understanding is that the formula you are using applies to drying by water evaporation. So since oil paint 'drying' is not evaporation of water, it's oxidisation of the oils, it would need different formula.  Heating does speed chemical reactions - as well as pose other risks if excessive - but there are many variables here - which oil(s) are in your paint, thickness of paint films, effects of pigments (some dry more quickly than others), additives such as driers and other things... And then there's touch dry vs sufficiently stable for glazing, etc. I can't see a formula that would allow us to handle all those variables.
    This is a good article from Golden that touches on the topic. The graph needs to be seen in context of the article as it's simply one pigment in one oil under one set of conditions. https://justpaint.org/weighing-in-on-the-drying-of-oils/
    httpsjustpaintorgwp-contentuploads2015053a-1jpg
    DesertskyBucky
  • @Abstraction – a very useful posting and link. In reading the details about the paint drying time, I see that the paint film was 10 mm thick! (That is 1 cm, or 4/10 of an inch) This is easily 50 -100 times thicker than my usual paint layer. I would be concerned about paint wrinkling, uneven curing, and cracks with that thickness.

    What thickness layer do you, or others, paint? 

    AbstractionBucky
  • I use stiff paint and rarely add medium  - I like a lot of texture in my paintings but even at it's thickest the paint would be no more than 2 to 3 mm. This is much thicker than what you would get with Geneva paints because of their thinness and leveling properties. My paint is touch dry within a day or two and dries solid within a week or less in my warm studio but I know that curing continues for much longer. I have paintings that are now about 5 years old and they have no sign cracking or wrinkling. It gets mighty hot in my studio in summer as there is no air conditioning  in there and in winter I keep it very warn with a heater ( cold causes my arthritis to act up) but this seems not to cause any problems with cracking. So, I think @Bucky would have no problems keeping a heater on in his studio overnight to get the painting dry enough to show. 
    Bucky
  • edited October 17
    Desertsky said:
    I see that the paint film was 10 mm thick! (That is 1 cm, or 4/10 of an inch) This is easily 50 -100 times thicker than my usual paint layer.
    That is such a profoundly unintuitive thickness that I didn't even notice it. I was mostly posting the article for the discussion of factors that influence drying and hadn't read through the test in detail.
    Bucky
  • No, it's not the same:

    A mil is a measurement that equals one-thousandth of an inch, or 0.001 inch. One mil also equals 0.0254 mm (millimeter). Thus a mil is not the same thickness as a millimeter. The term "mil" is not an abbreviation but a unit of measure.The chart below gives you an idea of mils to millimeters to inches. An every day trash bag ranges between 1.2 mils and 1.7 mils. A much stronger trash bag that offers better tear resistance is between 3 mils and 6 mils. A credit card is around 30 mils thick while a common deck of playing cards including the box is approximately .75 inches thick or 750 mils.
    AbstractionBucky
  • Gosh, I'm glad I used "mm" in my response and not "mils". 
    Bucky
  • @Richard_P – thank you! That had never occurred to me, and so – obviously – I have been puzzled over the weird way in which the paint tests were conducted. And I was wrong all along :)  Another reminder to check assumptions.  Now the draw down test make sense to me. This 10 mil = .0254 mm is about the same thickness as I paint, maybe a little thicker.

    @Tassieguy, I paint in a similar way, only I sometimes add a little alkyd with a drier in it. And yes, my studio and home are kept warm to keep the arthritis at bay.


    BuckyAbstraction
  • edited October 17
    Cheers, @Desertsky. I, too, sometimes add a little cobalt drier to colours that dry more slowly such as Titanium white and Ultramarine blue. I do this in an effort to get all my whole painting to dry at the same rate. I learned about cobalt drier here on the forum. In fact, I learned everything I know about painting here on DMP.  :)
    BuckyAbstraction
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