Alkali refined linseed oil (undergone refining processes to remove impurities)
Alkali refined linseed oil is a pale oil extracted from the seeds of the flax plant. When mixed with oil paint, linseed oil will increase the gloss and transparency of the paint and will thin the paint so that it flows more easily and does not hold brush marks.
It will also slow the drying time of the paint. Linseed oil is also highly suitable for use with dry ground pigments.
Raw linseed oil is linseed oil that has been extracted without any processing. It takes much longer to dry than refined linseed oil.
Linseed oil comes from flax seeds which are harvested from the same flax plant that produces the fibers used to make linen. Artists have been using linseed oil for centuries to make paints, mediums and varnishes. It is widely used today, not only in artist materials, but also for a wide range of industrial applications.
Linseed oil dries relatively fast and produces a durable, flexible paint film. The term alkali refined, when used in relation to oils, refers to the way the oil is extracted from the seed and subsequently clarified.
Note that while it might only take 3-5 days for refined linseed oil to dry when mixed in these proportions, as you increase the amount it may take longer, so if you're planning on doing a lot of layers, you may want to use a fast-drying medium (and remember to use these first; let's not forget the fat over lean rule!).