Linseed Oil, Tung Oil, Shellac, Varnish, and Darkening of Cherry
Advice on tweaking an oil-based furniture finish formula, and on the tendency of wood and finish to darken over time. March 9, 2010
Question
Over a long period of time, how much impact does linseed oil have on the final color of cherry? For example, if I used BLO then shellac on a piece of cherry and used only shellac on a second piece will the former be much darker than the latter several years from now?
Forum Responses
(Finishing Forum)
From contributor C:
Yes - use a non darkening oil such as tung oil. This will not darken anywhere near as bad as BLO, and will cure harder also.
From contributor R:
Tung oil produces a beautiful look and it’s easy to apply too. If I’m not mistaken it also offers more protection than a BLO will.
From contributor U:
Oils like linseed, tung, and etc are finishes on their own, so why would you put shellac on top? Shellac doesn't give much protection, so perhaps you should just use oil as a finish.
From contributor D:
It was asked that if curing oils such as boiled linseed oil or tung oil are used, why use shellac on top of them? The answer is that curing oils are not film-forming finishes and shellac is a film-forming finish. Using an oil under the formed film of finish adds the look of depth to the wood. The oils penetrate into the wood fiber and the formed film behaves similar to the way that a glass panel does when placed on a wood surface.
The word chatoyance is often used when describing this iridescent quality to wood. The word moire is more accurate, but since usage makes right when it comes to language, talking about a wood's chatoyant qualities and the use of oils to enhance this look of depth wins the day. Moire will stay relegated to the world of velvet.
Back to boiled linseed oil and tung oil. If you add varnish to their mixes then you can have both worlds: oil finishes with film-forming qualities. The more varnish there is in the mix the more of a film-forming finish you have. The more varnish, the less soft and less brittle that the film-forming finish is going to be.
Always remember that curing oils generate heat as they oxidize. Rags that are not disposed of in air-tight metal cans filled with water can burn down your shop as they combust spontaneously. Laying them out flat to dry is not a 100% safeguard. The closed air-tight metal can that is filled with water is your safe approach to disposal.
From contributor C:
Tung is a film forming oil, other oils, if enough coats are applied, could also be considered film forming oils though they have no good film properties such as tung has. Second point - mixing varnish or urethane's with oils defeats the main purpose of using oil, especially tung. To apply a fresh coat of tung would require removal of the synthetic coating applied either in combination or over the oil. In tungs case, I believe it is best to just apply as many coats of a good polymerized oil to build up a nice low/medium sheen surface and re-apply as necessary.
Shellac or other evaporative finishes,(nitro, acrylic, and etc.) do not have the water resistance or heat resistance or ease of repair that a straight tung finish has. If polymerized tung oil is burnished properly with each application, a very nice satin or even low gloss sheen can be achieved, which should suit the purpose of most finishers or clients. Consider what it is you’re trying to achieve, and make samples till you get the desired effects. Then start your actual project and you will be extremely pleased with the outcome. Pay close attention to the flammable warning given!
From contributor R:
I’m curious about how you burnish between coats of tung oil. I have been told to use #2 steel wool. Does this sound right? I was considering using #3.
From contributor C:
Actually a grey Scotch-Brite pad works better than Steel Wool of any grade.
From contributor B:
Can you use just tung oil on a kitchen table? Will that be enough protection from a family with kids and spills?
From contributor C:
For over 25 years I had my own D. table (walnut) done in tung oil. It got plenty of damages, rings, cig burns, stains, etc. Why I like it is if such things happen it’s very easy just to sand/scrape/ or otherwise remove the damages and re-apply more oil (again as long as you have no other film forming resins added to it). In and of itself tung is pretty darn water resistant and heat resistant (not hot resistant). If you care for it with half the care you would give other films it will last a life time and then some.
From contributor B:
Hhow is a combination of tung and varnish ? I’m looking for something a bit more maintenance free. If that sounds like a good combo how much of each should be mixed?
From contributor C:
If you must use poly or varnish with your tung oil, then look at Tage Frid and Sam Maloof's recipe's. Keep in mind though both of those will add color to the surface (yellowing), and also tung by itself can be burnished to a high gloss also! It just takes a less thinned coat and a good RO sander with a white Scotch-Brite pad and a little more time. I have done gun-stocks this way and a few other customer items when gloss was desired. To me it looks to plastic. I like the wood to appear more natural like it has been sanded with 400-600 paper when viewed from a low angle across the surface.
