Rembrandt's paintings question

I first want to state that what follows is oversimplified for the sake of answering your question in a manageable length as volumes could be written about this subject. In the conservation of old master paintings, there are many, many examples that fall between the two dipoles that I describe. This is more the norm than the exception.
A good deal of the yellow-brown seen in images of Rembrandts is the presence of a yellowed, degraded varnish. This can be because the work has not been treated for a long time or that the conservator/curators decided to only thin the vanish and not remove it wholly. The later often occurs because the painting has changed in some manner (previous damage, fading of pigments, increased transparency due to lead soap formation, etc.) or due to aesthetic biases on the part of the curator. The remaining varnish lends harmony to a work that may have become disharmonious due to the above. BUT this is not the original appearance of the work.
However, when you view a recently treated Rembrandt, the overall appearance is quite different. Yes, Rembrandt utilized a lot of brown, but this were generally contrasted with passages of cooler gray. The presence of a yellowed varnish does two things, it makes darks more turbid, therefore lightening them, and makes the lights darker and more yellow/brown. This compression of color and value may be helpful to reestablish balance of harmony to a damaged or compromised work but is misleading in terms of the artist’s intention on paintings that can be cleaned.

​Brian, Years ago, I visited a traveling exhibition featuring some works by Rembrandt which were in excellent condition, and I remember being struck by the crispness of the cool greys you describe. These were a long way from the heavily “sauced” images one knows mostly from older reproductions. In fact, a lot of younger artists probably have a different impression of historical paintings than my generation because of easy access to high quality scans. A lot of public libraries used to have very old art books when I was starting out, the ones with glued-down color plates taken goodness-knows how many years earlier.

There are a few reasons why the paintings have not been cleaned in addition to what I stated above. Some of it comes down to simple economics, treatments cost time and money. Additionally, some well regarded museums still think that, despite their tendency to quickly degrade, natural resin varnishes are just more aesthetically pleasing. I do not subscribe to this due to the accelerated frequency of treatment that results from this decision.  

It is true that simply removing yellow from the whole image does not result in an accurate digital cleaning. This is because dirty varnish does not distort all colors and values equally. Yellow, red, and orange ares will hardly change unless they are quite dark. Blue, whites, and green to a lesser extent, are dramatically distorted.

Additionall, your removal of yellow does not address darkening of lights or the lightening of darks as actiually occurs when paintings have a degraded varnish. 

Brian Baade