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As discussed in an answer to the question about putting an oil paint behind glass

The reason for the delay after the painting is finished is that it can take this long for the painting to fully 'dry'.

And another answer

Believe it or not, oil paintings actually take years to fully dry.

It was also mentioned that there is no consensus about putting it behind glass while it is drying. Waiting until it is dried could be a safe bet.

Of course there are factors beyond time that would affect this e.g. environmental factors, thickness of paint used etc. If the painting being dry is important for whatever reason (like storage) it would be useful to know when the painting is free of moisture.

How can I tell if an oil painting is dry? Or perhaps it does not really matter.

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  • Oil paintings really do not need glass over them to protect the painting (unless it is somewhere people will mess with it) instead a coat of varnish is used once the painting is dry enough to take it. The varnish can then be cleaned without threatening the painting underneath.
    – rebusB
    Aug 17, 2017 at 18:19

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Before providing a way to test the oil painting is dry, it's useful to know how the drying process works and why it is important.

How does oil paint dry (cure)?

Oil Paint doesn’t really dry, rather it cures. The pigment is dispersed in oil (typically linseed) and generally contains a solvent (methylated/white spirit). The solvent evaporates away leaving the oil and the pigment.

Then over a period of time the oil & pigment slowly become less malleable through a process of oxidisation. The oil hasn’t evaporated away (dried) it has become hard (cured).

What can cause differences in the drying time?

As you have mentioned in your question there are several things that can affect the curing time of the paint.

  • Application thickness of the paint
  • Pigment colour:
    • Earth colours require less oil (ochre etc.), so cure quicker Earth Colours
    • Primary colours require more oil (crimson etc.), so cure slower Primary Colours
  • Brand/type of paint
  • Environmental factors such as temperature, humidity, airflow & sunlight

Most paint suppliers offer good advice on how long their paints take to dry. But some general guidelines are that most paints will be touch dry after 10-14 days and should be fully cured within 6 months. However, for those pieces where the artist has used a liberal impasto technique, then upto 2 years is possible!

Why wait?

If the artist intends to varnish, and there are several reasons for this:

  • Protection
  • Bring richness & vibrancy to colour
  • Ease of cleaning

Doing so before the paint has cured could lead to cracking and also it might not allow the paint to fully cure, and this can lead to weakness between layers. And, therefore, flaking.

When framing it is less important that the painting is fully cured. But there are risks as the piece could be knocked during the process and cause indenting, or squashing, of the paint. Also, it can slow down the curing process.

So to answer the actual question:

How do you know that the oil painting is dry?

There is a very simple test.

Take a lint-free rag and dip it into white spirit. Pick an inconspicuous area and gently rub. If any colour shows on the rag, it requires further drying time before varnishing or framing.

But I wouldn’t suggest doing this on your newly acquired Gerhard Richter.

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    Is that the only way to test it? Does that mean if you're painting, you should paint somewhere on the edge of the canvas that you don't intend to be visible, in order to test it later?
    – user24
    Jun 17, 2016 at 19:44
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    What I mentioned is the only practical way, other than intrusively poking or cutting into the paint, that I'm aware of. It's really about understanding the oil paint you are using and the thickness/method of application. There are quicker drying oil paints, typically student paints, but many feel the pigment isn't as vibrant. Basically it's down to experience and knowing your materials.
    – BeaglesEnd
    Jun 17, 2016 at 19:59
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    You can also add cobalt drier to speed drying time but this can weaken the paint if used too liberally. Usually this is done on the thin underlayers so the heavy work can start sooner. Usually I wait 2 - 6 months before varnishing, sometimes longer.
    – rebusB
    Aug 17, 2017 at 18:16
  • Stretch and prime all the way around to the back of the bars (or buy pre-primed canvases with primed canvas on the back.) Paint successive layers on the a small area of the wrapped area of canvas when you finish each layer. Test these areas in 6-12 months for adhesion and full cure.
    – Aaron
    Jan 23, 2018 at 19:16
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It will never really "get dry". If you paint with raw linseed oil --like many old paintings are painted with-- it might take around 100 years before it is really dry.

Touch-dry, however --so you can paint another layer-- could take from 1 day to 2 weeks, depends on the oil medium and paint you are using.

Light colors dry slower.

Test it with your brush: if no paint comes off, you can paint over it.

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