Dan,
Based on information I have gathered over the past couple of years from you, Eric K, and Mad Monk, I started to do my own work with boiling linseed oil. I have come up with a method to boil 100 to 200 milliliters at a time so that I can experiment with driers, boiling temperatures, boiling times, etc., without making up a gallon at a time. I test drying times on a glass plate. The dried films on the glass also give me a method to asses the elasticity, color, hardness, "stickiness", etc. This small batch testing is also much quicker and less dangerous than boiling a gallon at a time.
During these experiments, I ran into exactly the problem you note, namely, that the precursor material is often not the same batch to batch and has completely unknown constituents in it that obscure the results of the experiments. I have begun using two linseed oil bases for the tests to help eliminate this problem. One is organic flax seed oil, the food supplement kind. It is cold pressed "linseed" oil and is unrefined. As a food supplement, it has no driers of any kind in it so I have a consistent starting point with each batch. The second linseed oil base I have been using is the "Varnish Maker's Linseed Oil" from Wood Finishing Enterprises:
(
http://www.woodfinishingenterprises.com/coating.html).
This also has no driers in it, so it gives me another known starting point. So far I have had good luck with both but have not been able to produce the "ruby red" oil that Monk talks about in his book. However, the "coffee" colored oil that I have produced seems to dry rapidly and produce a tough, elastic film. Not done yet, but this small batch process with a known starting oil has been working out well so far.
