Another recent addition to the Spurgeon article I found worth saving. As this idea has come up a lot in discussion on MN.
Here it is...
heat refining debunked
There is a linseed oil refining method that sometimes surfaces that involves heating the oil to 600F briefly to 'burn off' the mucilage. Simple, done, why do anything else? I've always thought this could not work, mostly because no one, from all the older refining methods, to the modern refining method, does this. All these methods involve water. Because the mucilage is water-soluble. But this came up again recently from a correspondent who simply wanted to know if it was true, and I asked my friend Roland what he thought about it. Roland is a chemist by trade and both thinks about and researches these things several levels beyond what I can. First, he found out that the 600F method has its origin in print in a 20th century industrial method to bleach oil. But he didn't find anything about using this method to refine the oil, so he decided to do it and see what happened. He heated the oil as quickly as possible on a hot plate with a stirbar to 600F, then let it cool, and washed it with water and salt. And there is the mucilage, changed, like the oil, by the heat, but present. And will therefore make the film weaker, and attract water that makes the film darker. But, gasp, note that the oil is lighter! Oh my, this must mean it is better! But the 'wet' colour of the oil and the 'dry' colour of the oil are a function of two different processes. They have nothing to do with one another. The wet colour is always fugitive unless the oil has been carbonized. (This is in fact part of the strange appeal of the leaded oil called black oil, the nefarious wet colour 'magically' disappears.) The dry colour is a function of chemical changes that can occur as the fatty acids in the oil are broken down and rearranged into smaller molecules by interacting with oxygen. Everyone thinks that the wet and dry colour are linked. Even Ralph Mayer thought this. But they are not. No, no, a thousand times no. Yet, people want to believe it, so they do. I went through this myself for many years before finally realizing that two different things were involved. Isn't it weird how easy it is to be fooled by appearances? Anyway, this procedure might be useful after the oil has been refined for a fine style, but it doesn't refine the oil.
Microscope photo from Roland of the oil above. Round bubbles are water, wrinkled bubbles are mucilage coagulated by heat.