There is argument over yellowing, brittleness and so forth...however, I have struck up a relationship with Ron Garrett whose family and family before him have made genuine copal medium going back to early American Impressionist times. Living in New Mexico...and a tedious process. Great stuff...expensive.
There were past personal riffs that brought up untruths about copal use here in this country.
I tried a bit of Garrett's copal about eight/nine years ago...was hooked and have used it religiously since.
It is true that it makes the paint hard and is best used on a hard support such as board, or canvas glued to board and so forth. Yet I only use about a drop per inch of paint anyway, which is not overuse even on stretched canvas.
Resin imitations in art stores that are quite cheap by comparison are no measure of what real copal is...
Real genuine copal is a natural siccative, a drier that molecularly attaches itself to paint pigment throughout the paint and dries the paint uniformly. IF you mix it right...when it is dry to touch to the fingers, it is dry throughout, and some of the old American Impressionists that then used "Oil of copal" felt free enough to varnish their work when it felt dry to touch. Personally, I would like to see some of their
works today and see how they held up.
At any rate...with the few drops I use...my paintings are relatively dry to touch in 3-4 days time.
Real genuine copal is also used to add more billiance to color, which as a plein air painter attempting to imitate nature's light for me anyway is critical.
Lastly...my first experience with it felt like the paint was painting itself. It went on to my canvas easily, sat where I put it, held shape and form (I paint finishing strokes with buttery consistency). My experience was like going from a loud Volkswagon Beetle to a plush cadillac. The paint was a cruise ride!
that's my story...and I'm sticking to it! ...and no, I'm not a distributor of Garrett's copal, nor do I receive royalties!!!!
Real genuine copal is also a medium used by some as final varnishing because it is tough and protective.
Copal's traditional use in the days of Rennaissance and Baroque was to paint a light coat film over a dry stage of the painting, then paint fresh paint directly into the film, and let dry. Coat again a fresh film of copal over that, and paint more paint directly on.
I don't believe exactly what medium Rembrandt used has yet been fully understood...but as example, he had as many as 30 such transparent layers with pigment suspended throughout the layers and accounted for the rich depth and rendering he achieved. Dragging a brush across whatever medium he used as it was dryng would create a texture for which he'd then glaze pigment over to settle into pock marks from the dragging. His methods were quite contemporary and in dispute in his day.
I read once where someone inspecting his painting very closely in attempt to figure out what he was doing, was chewed out by him. He told that individual he had no right to "smell" his painting!