Wednesday, December 9, 2015

Piggy Blue, 10x12 and How Oils Dry

Trapped inside the day that nine feet of rain came down......
...... (Yes, that's correct.), we painted still life.

Because I was leaving Maine in a few days I switched to Gamblin's Fast Matte pigments so my paintings would be dry in time to be packed.  I don't think I got the hang of using FM paint.  After two paintings I abandoned trying because, even though they got tacky while I was painting them, in three days they still weren't dry enough to pack.  Plus they didn't exactly 'flow' like real oil paint.

This was my second attempt using them and was better than the first, which I may still post someday.  I confess that I did sneak in a little non-FM Quinacridone Red into this one  for that pinkish cloth.

Flying back I was thinking about a person who has taken several classes with me and is a retired chemical engineer.  Who better to ask about what happens when paint 'dries'?

His name is Chuck Witham and he spent some time writing up the following short description of what happens to paint on the way to being solid and ready for varnish.  At my request this is a simplified version for non-techie consumption.  Chuck said he could have gone into much more depth of how and why certain pigments dry faster than others or all the chemical changes that occur within the paint structure.  I didn't know, for example, that at one point it gives off hydrogen peroxide.

Anyway, for your interest and edification this is what happens.  Remember it the next time you are in a hurry to get a painting into a frame.  Thanks to Chuck.

Drying Oil Paint
Monday, November 30, 2015

Before we start, we need to understand what the paint on the canvas really is.
  * Pigment.  Finely ground particles of an insoluble material in a variety of colors.
  * Oil Medium.  Polyunsaturated fatty oil (linseed, walnut or other oils known as drying oils).
  * Thinner.  Any volatile organic solution (turpentine, Gambol, Terpenoid, etc.)
The pigment and oil is mixed together until a uniform mixture is achieved.  This mixture is placed in sealed tubes until used by the painter.  When we squeeze out an amount of paint, we add thinner until the desired consistency is reached and spread it on our picture.

Now we are ready to understand what happens as paint 'dries'.
First, we get rid of the thinner.  The thinner evaporates off the surface but the rest of the thinner within the paint mixture must travel to the surface before it can evaporate.  This travel is controlled by diffusion and/or capillary action and is drastically influenced by the thickness of the paint.  A paint film twice as thick will require four to eight times longer to get rid of the thinner!

Now the oxygen in the air can permeate the oil and start the process of curing the oil through oxidation followed by cross linking of the oil molecules.  The result is a firm flexible film that is stable in most normal environments.

Can I speed up the 'drying' process?
Room temperature air blown over the surface will help evaporate the thinner as it reaches the surface, and provide an ample supply of oxygen to start the curing.  Providing a gentle heat (100 degrees F) to the BACK of the picture will aid in the removal of thiner and increase the rates of oxidation and cross linking.  Do NOT blow hot air on the paint surface as this will cause the paint to 'skin over' and drastically INCREASE the 'dry' time.

There you have it.  Hope it helps.

Thanks for reading and checking in.

Back soon.

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