Willie,
Here's an answer that I got from a manufacturer of boiled linseed oil .
<b>Dear Doug
Thank you for your enquiry via our website.
In answer to your question, traditionally double boiled linseed oil was manufactured for a longer period of time than boiled linseed oil and consequently had a higher specific gravity.
Nowadays, standard boiled and double boiled linseed oils are the same product and the other grades of boiled linseed oil that we have differ in their colour and type of siccative they contain. I have attached an information sheet showing the grades that are available.
I hope this explanation helps
Regards
Sue Pritchard</b>
If anyone would like to see the info sheet , reply to this thread and I'll post it .