Doug,
My Rembrandt article for ASOPA was written several years ago, and is probably incorrect on the point of resins in his medium, according to more recent discoveries made by conservation scientists at the National Gallery, in London. I have since updated that article, excerpted from my book, and the updated version can be seen on the Art Renewal Center web site, at:
http://www.artrenewal.org/articles/2...rembrandt1.asp
That is the danger in committing technical information to print; it is subject to becoming obsolete when new information comes to light to supersede it. This is what has happened to Ralph Mayer's books, and those of all the other authors who wrote on the same subject before him. One must put forth a great deal of effort to stay abreast of the latest discoveries. It is not as simple as reading a book and trusting that everything it contains is the last word on the subject. Knowledge continues to advance.
I believe Mayer was right about mixtures of polymerized linseed oil and raw oil imparting resin-like qualities, and this may well have been done by Rembrandt. He also added ground glass and chalk to some of his paints for various reasons. See my updated article on ARC. I cannot give you the exact proportions, but I doubt they are critical. Too much polymerized oil will increase the gloss. Thus it is for each artist to discover what works best for his or her technique. While it is interesting to know what Rembrandt did and used, we must each paint our own pictures. Only Rembrandt was Rembrandt, and his unique genius was what enabled him to do what he did. The materials he used were incidental to that.
Velaturas can be done without resin ingredients in one's medium, simply by scrubbing the paint on very thinly with a stiff brush. Its consistency can be made more fluid by adding a drop or two of linseed or walnut oil to a pile of paint the size of a large coin, and mixing it in well with a palette knife, on the palette. Scrub a tiny bit of the oil onto the surface of the painting to be painted into beforehand, and wipe off as much of it as possible with a soft rag. The transparency/opacity of one's paints can also be controlled by choosing the pigments with an understanding of the natural degree of opacity or transparency of each. This is preferable, from a structural standpoint, to adding a lot of medium to the paint. Too much medium weakens the paint film. See my other posts for more on that.
One can easy waste much time in an anal-retentive obsession with sophisticated techniques, to the detriment of one's art by virtue of insufficient attention to the aesthetic aspects of art beyond painting technique. These are what should be seen as of paramount importance. The rest is just nuts and bolts.
Virgil Elliott