It is clear from what I can google that rags soaked in linseed oil and similar must be taken special care of because they could catch fire while drying. Is this also the case for rags, etc., with oil colors straight out the tube?
3 Answers
Yes, there is still a risk, as the hazard is in the oil; as the oil dries, it emits heat (an exothermic reaction) which, if it's in a pile of other flammable objects--such as other oily rags, papers, etc--can cause them to combust.
From the Wet Canvas forum (the thread also contains some personal experiences with combusting rags):
Remember it is the linseed oil that heats slightly as it dries (oils dry via a slow combustion process) and if the heat is trapped fire is a possibility, especially combined with highly flammable substances such as solvent. Whether you use solvent or not, soak your oily rags/paper towels in water or spread them out to get air.
And a blog from Mutual of Enumclaw gives an additional citation to rags with oil-based paint on them being a hazard (as well as tips for handling them):
Simply put, rags that contain residue of oil-based paints and stains, paint thinners, varnishes, or polyurethane can spontaneously combust and catch on fire. Here’s what happens: When oily rags begin to dry, they produce heat. Combined with oxygen they turn into combustible cloths that can quickly cause trouble.
(All emphasis above is mine.)
It is clear that the oil is the risk, and as the paints "straight from the tube" contain oil, it is best to treat them as a risk as much as you would the other items.
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Thank you for these resources. I also checked the composition of the particular colors I have and the binder is in fact linseed oil.– einarMar 28, 2020 at 11:22
I think was a problem of long ago. Although it can still happen today, it is now much less common. The risks were more common in the past when someone like a professional painter would collect a large box of oily/solvent/paint soaked rags. Back before water based paints were so common, and before disposable paper towels were available for cleaning, and before various cleaning products were available. A can of solvent type liquid is no problem; the problem is a collection of rags where there is good oxygen access over a large surface area and the center of the pile is thermally insulated by the surface rags.
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1The references I found made it clear that this is clearly a modern issue, and that paper towels (and any other paper) are also risky, not just "rags." Mar 27, 2020 at 19:30
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Water base paint does not predated oil base unless you are calling white-wash ( lime in water ) a paint . Paint contains a pigment, a vehicle ( oil, water, alcohol, etc.) ,AND a resin/polymer. Mar 29, 2020 at 15:19
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I am referring to a modern technical definition of paint as ASTM would use. The Wiki definition is not good ; but they do call the resinpolymer a "binder or film former". No doubt people have been painting on rock with muddy water,etc, for many thousands of years. Mar 29, 2020 at 17:08
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1There have been many binders before oil was used: beeswax, glue, (Arabic) gum, egg yolk (of course, the history off oil-based paints is still lacking, so this also has to be taken with a grain of salt). And I would consider 'white-wash' paint when used for drawing imagery that is to have a decorative, ceremonial, religious, or expressive function. Also, 'muddy water' is a little derogatory, since the paintings that survive us were drawn with intentionally pigmented substances, and great care and skill.– Joachim ♦Apr 12, 2020 at 9:40
No the oil paint alone will not combust but if paint thinner or mineral spirits that the oil paint brushes are cleaned with can.
