Beyond various waxes and shellacs, reactive oils and varnishes probably have the longest historical pedigree. Both have been used for centuries. Books have been written about these products; their strengths and weaknesses, their application, and their ultimate quality as a woodworking finish. I've read many of them. But, I've also used them. I believe in them. They do something to wood that enhances wood's beauty.
Reactive oils are broken down into true oils and their boiled variants. They are reactive in that the two oils mainly used in woodworking (discussed in further detail below) react to oxygen to polymerize into a hardened film finish. While neither holds up like modern chemical finishes, natural oils impart a warmth, at least to me, that modern chemical finishes simply can't match.
True Oils - Linseed and tung oils are natural products; linseed oil comes from flax seeds while tung oil are from the nuts of the tung tree. Both are drying oils, but can take considerable time to do so. This time can be weeks to months depending on environmental conditions like temperature, humidity, etc,. Their labeling as "true oils" is a marketing mechanism to distinguish them from their boiled derivatives. While purists may use true oils, most woodworkers today simply don't have the time or patience to wait.
Boiled Linseed Oil - Long ago, someone discovered that boiling linseed oil changed its viscosity and significantly reduced the time required for it to dry. Again, purists may still obtain "polyermerized" linseed oil, or boil their own raw oil. But, most woodworkers these days get the commercial boiled linseed oil. Oddly, most commercially available boiled linseed oil today is actually raw oil with chemical mechanical dryers added.
Boiled Tung Oil - Like linseed oil, tung oil comes in raw and boiled varieties. And, like linseed oil, the Chinese figured out hundreds of years ago that boiling it would decrease drying time. Both are still available today, but raw tung oil is rarely used as the drying time is very significant. Tung oil dries harder than linseed oil, and it imparts less "color" when dry.
A Couple of Notes
Both, linseed and tung oil are penetrating oils, meaning that the oils will penetrate the wood's surface fibers. Once dry, they harden giving the wood some surface protection. Applying either of these penetrating oils is quite simple... wipe it on, let it penetrate for 15 or 20 minutes, wipe off any residue, and let it dry. Don't skip the wipe off residue part. Puddled oils will NEVER dry, and will become a gummy, sticky mess.
A Safety Note - When curing, reactive oils produce heat. This exothermic reaction can cause discarded rags and materials to spontaneously combust. Spread out rags to let them dry outside. It is rumored that King Tut was wrapped hard and put away wet... 3500 odd years later, they discovered his charred remains.
A Final Thought...
Of the two, I prefer boiled linseed oil. When applied, it "pops" the grain and gives wood a warmth that I find to be wonderful. Typically, I let the first coat dry over night and apply a second coat the next day. The second coat will get to dry for at least two days before the next step in my finishing process. Linseed oil will darken slightly over time, but it eventually tones out to that classic look we all know and love.
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