Oil finish gone rancid

[Dean Center]

I have never had this problem before and wanted to share, since it was a fairly well read thread not long ago, and also to ask for suggestions.

I made 8 personal salad bowls to give to relatives and wanted to speed the curing of the walnut oil finish so I could send them off sooner. I have used walnut oil for many years and never had a problem, but this time I did. I think I used the grocery store 'roasted' walnut oil for these, and in that recent thread, there were some negative comments about it, instead of a heat-treated oil like Mahoneys. To speed the curing, I also put the bowls outside on the patio table for a day or two, where it was warmer than in my 65 degrees year round basement. I got busy with other stuff and the bowls sat in the house for a few weeks and then I went to box them to ship. To be sure the finish was cured, I gave them a sniff and they smelled. It took a moment to realize it wasn't just residual oil, but rancid oil.

A quick internet review reveals that when oils go rancid, the double carbon-carbon bonds of the unsaturated oils are changed to single carbon bonds, usually by oxygen. Light and heat and oxygen make the rancid change more likely. Like sitting out in the sunshine on a warm summer day. I've never had walnut oil do this before, so I'm assuming I am the culprit, rather than the grocery version of oil, but both may contribute.

Does anyone have a fix for this problem? I can't give these away as they are. Thanks in advance for any successful suggestions.

 

[Webb Willmott]

At least you have several test subjects. I would give them all except one a good washing with dish soap and water. Then try a dilute vinegar soak on one, baking soda paste scrub on the second, and a lemon juice salt scrub on the third. After they are rinsed and dry, do the sniff test. You might even find that a combo treatment is the ticket. If you are lucky, just soap and water might be all that is needed.

The unwashed bowl will be baseline and you will be able to rate the results. Report back what worked the best.

 

[Steve Tiedman]

Hi Dean. Are they thick enough to safely turn down more and remove the oil-soaked wood, and still leave enough functional thickness? I am highly doubtful about pulling all that oil out by any other means. A single trace left in the wood is bad news.

Personally, I'm not comfortable with leaving any trace of bad oil on/in the wood. If it were my project, removing the oiled wood is my only option, and angrily trashing them when/if that doesn't work.

As far as oils go, and I know this swims against the current of what others may do/suggest, I won't use any oil that can go rancid, period. Tried & True Danish (straight heat polymerized linseed oil, no wax, solvents or chemicals), pure tung oil, or no finish at all, those are my vote for food-ware. If walnut oil is insisted upon, then only known, reputable products need apply for the job, like Mahoney's. Leave anything from the grocery store at the grocery store, don't bring it into the woodshop, no matter how romantic the notion. Our recipients simply won't care about the finish, unless we spend time going on and on about it.

I hope you can salvage them.

 

[robo hippy]

Well, I know vegetable oils can go rancid. Never heard of walnut oil going bad though. From bowls I have seen that have gone rancid, most of the time it is due to food residue left on the wood, and usually that takes a while. Once they go sour, it is time to burn them. I only use the Doctor's Woodshop oil, and it works fine. Not all grocery store walnut oils are equal. Considering how many bowls I still turn, like 300 a year or so, it is worth it for me to buy the walnut oil by the gallon.

robo hippy

 

[John Whitley]

I think I used the grocery store 'roasted' walnut oil for these

It's worth considering that the grocery store product may have been adulterated with some other, cheaper vegetable or seed oil. This problem has been widely reported with other grocery store culinary oils, e.g. extra-virgin olive oil. So the rancidity may not be a walnut oil problem, but rather a product purity problem.

 

[Randy Heinemann]

This may be a foreign concept and possibly not a very popular one concerning food safe finishes, but there was an article in a Fine Woodworking issue late last year written by a known expert in wood, including allergies and food safe finishes. I've attached a pdf of it. The basics are that no finish is safer than any food-safe finish. The reason this is true is that bacteria need moisture to grow and thrive. Given wood's properties just sitting and drying out after use will deprive the bacteria of the needed moisture. It's a bit more complicated than that I suppose, but that is what I got out of it.

Finishing a piece of wood which is used for a cutting board or, I would guess, a bowl used for food, will actually help seal in moisture in some cases and allow the bacteria to remain active. Hopefuly the attached file with the article is accessible to all. I can open it by clicking on the link and downloading it to my computer. It's possible, though, that I can do that because I have a subscription to FWW. If you can't get to it and want to, reply and I will attempt another way to allow access.

 

[Gord Cameron]

I am a great fan of Seri Robinson and there is a place for her no-finish philosophy. The patina of an unfinished and well-used wooden plate has its own attraction.

But one must be willing to accept the limitation that Dr. Robinson herself puts on her “bare wood” recommendation for food-contact wood items (I have read and heard her talk about this caveat several times): after use, the item (bowl, plate etc.) must be rinsed and then left to thoroughly dry (with all surfaces exposed) for 24-48 hours. It is during this period that the item self-cleans, through wood’s antimicrobial mechanisms.

So OK, I can do this with my own pieces, where I have back-ups to use for the 48 hour wetting / drying period, and where I can keep track of things and tell others in my household that the clean-looking plate in the drying rack has actually not finished its self-cleaning cycle, so should not be used.

But I don’t want to rely on others, to whom I give or sell my bowls and plates, to remember to follow this regimen (assuming they would even want an unfinished item after I have explained the process they have to follow.)

I have some unfinished pieces of my own, but for items going to others, I put on a finish—usually walnut oil. Wiping or gently washing finished wooden plates and bowls is natural to most people and deals with the hygiene issue without having to hope that everyone who might use the item will remember to follow Dr. Robinson’s wetting / drying regimen. I trust her that it works, but it will not always be a practical way for end users to treat their wooden bowls and plates.

 

[Randy Heinemann]

While I haven't read everything that Ms. Robinson has to say about finishes, my understanding is that she really doesn't recommend any finishes as truly "food-safe". At least part of the reason is that all finished surfaces on utensils, boards, and bowls get marred during use. Therefore, some bare wood is essentially presenting itself anyway, therefore requiring the drying period just to be safe. Refinishing periodically could have the effect of sealing in bacteria that has existed in the bare wood areas.

This whole dilemma is the reason that, when I teamed up with my wife to combine polymer clay trim with wood bowls, we specifically tried to make pieces which really weren't functional in nature. That, of course, limits the market which is why I don't sell bowls anymore. I just enjoy making them for friends, relatives, and the occasional person who comments on my work and seems like someone who would appreciate it. I realize that, sooner or later, everyone I know will have some turned object and that market is gone, but then I'll just do it for the challenge and only keep the best of my work.

The reason the whole "food-safe" finish thing became something I followed for awhile is because it has always been a dilemma which really doesn't have a solution which, across the board, is THE SOLUTION.

 

[Dean Center]

The bowls are about 1/4" thick, no room for turning away the surface. I hate to trash my entire stash of Mountain Ash, and the largest chokecherry blanks I've had in a few years. ANYONE have experience removing an oil finish???

 

[Gord Cameron]

I have "removed" an oil finish this way: scrubbing with solvent (I used citrus solvent but mineral spirits would work if you are OK with the toxicity) followed by isopropyl alcohol (DNA would be the same). The alcohol will fully evaporate and take some of the remaining solvent with it.

I put "removed" in quotations, because once an oil finish is in the wood, you are not really going to get rid of it unless you turn or sand it out. If what you have is a rancid oil problem, "washing" it in the way I described could reduce or eliminate that problem. Then, if you want a finish, you can refinish with something that you are confident will dry in your conditions. Some would not consider the end product "food safe", but that is a whole other discussion.

 

[Paul Vechart]

Maybe try some of the food safe orange oil solvent.

 

[Steve Tiedman]

I just do not ever see 100% of that bad oil coming back out of the wood. I don't know if the oils penetrate the cell structure itself, but if it has, you'll never get that out. And the oil has penetrated down into the grain of the wood some fraction of an inch, and some has cured down there, and you'll really never be able to extract the bad uncured molecules from the good cured molecules buried in the wood. And every moment that goes by, what is still liquid can creep deeper and what is going to cure is curing.

It's not much different than asking how I could extract the pizza grease from the cardboard delivery box. You can't. What's done is done. Even if you think you've extracted all the oil, how will you ever prove that you did?

I'm gonna go out on a limb, and I do not know the science behind this thought- heat. At some not-so-high temperature (mid 100F range I thought I heard?), wood starts to break down. Not burn, but it's permanently affected. But, would some level of heat, prolonged as needed, kill the bacteria and force the polymerization of the oil(s)? Would cycles in a microwave do something? (That would probably boil liquid oil pretty fast, and that internal vaporization expansion could crack the wood.) Plain ignorance on my part is on full display here, use caution.

 

[Webb Willmott]

Why not look at recommendations from wooden bowl companies on dealing with rancid wood bowls. As I previously said, you have multiple test pieces. Your goal is to emulsify and extract as much oil as you can and then neutralize what remains.

My personal preference for freshening up the large prep cutting board that was the top of a butcher block table which lives on the kitchen counter is salt and rubbed with half a lemon. Of course it gets washed after each use with a soapy dishcloth.

Old salad bowls have a much deeper penetration of rancid oil than does a bowl with one application and they can be made fresh. My money says you will have success. You just need to find out what works the best.

 

[Mike Zip Hamilton]

My chemistry is nearly 60 years old; so take this acordingly. I'm guessing that the combination of heat and light from the sun shine caused the oil to oxidize. Normal "curing", polymerization of the oil would have been accompanied with the oxidation reaction. Polimerization would have joined the oil molecules into long chains; your normal hard finish. The oxidation reaction would have broken the oil into smaller chemicals. Some of these would would be volatile giving your bowls its rancid smell. These chemicals are probably not toxic; just offensive. I think that any of the oil that oxidized could be extracted with solvent. I would take one bowl and submerse it in lacquer thinner (acetone) for several minutes. Towel it dry, and let it air dry. Then give it your sniff test. You could probably repeat this extraction. If any of the offensive chemicals were trapped in the polymerize matrix, I think they would remain trapped. Now I would try an additional coat of your Finnish letting it cure in your normal manner. If this bowl turns out to your satisfaction, repeat seven times.

 

[Timothy Hoyt]

If, more or less any oil is a finish to some degree and oil contributes to pieces becoming contaminated with difficult or impossible to clean bacteria. What happens with an unfinished salad bowl or to a lesser extent a plate in normal user? Many oils are used in food, some seeps into unfinished wood. Or is the small amount of food oil "cleanable" via soap and water?

 

[Steve Tiedman]

If, more or less any oil is a finish to some degree and oil contributes to pieces becoming contaminated with difficult or impossible to clean bacteria. What happens with an unfinished salad bowl or to a lesser extent a plate in normal user? Many oils are used in food, some seeps into unfinished wood. Or is the small amount of food oil "cleanable" via soap and water?

What would happen if one sample bowl was submerged in hot water (tap hot, not boiled) containing Dawn dishwashing detergent? Dawn is known as one of the best grease-cutting consumer detergents.

I once heard detergents described as having the function of "making water wetter", maybe giving water less surface tension and a better capacity to get surfactants into/under the crud you are trying to remove. Like crude oils from duckling feathers...

Just grasping at straws here.

 

[Dean Center]

Earlier today, many of the ideas mentioned above dawned on me. (sorry, Steve) So I've got an experiment going with alcohol, mineral spirits, and Simple Green, my go to degreaser. I had forgotten d-limonene and I'll add that to the trial.

I can't see giving these to the intended recipients in any case, so I've dug out a few lesser blanks from the stash and started finish turning. Dang I hate to lose a set of 8 matching bowls.

 

[Steve Tiedman]

Oh, that's okay, glad to hear similar crazy, but hopefully useful suggestions came up for you. Best of luck!

 

[Jon Rista]

I have never had this problem before and wanted to share, since it was a fairly well read thread not long ago, and also to ask for suggestions.

I made 8 personal salad bowls to give to relatives and wanted to speed the curing of the walnut oil finish so I could send them off sooner. I have used walnut oil for many years and never had a problem, but this time I did. I think I used the grocery store 'roasted' walnut oil for these, and in that recent thread, there were some negative comments about it, instead of a heat-treated oil like Mahoneys. To speed the curing, I also put the bowls outside on the patio table for a day or two, where it was warmer than in my 65 degrees year round basement. I got busy with other stuff and the bowls sat in the house for a few weeks and then I went to box them to ship. To be sure the finish was cured, I gave them a sniff and they smelled. It took a moment to realize it wasn't just residual oil, but rancid oil.

A quick internet review reveals that when oils go rancid, the double carbon-carbon bonds of the unsaturated oils are changed to single carbon bonds, usually by oxygen. Light and heat and oxygen make the rancid change more likely. Like sitting out in the sunshine on a warm summer day. I've never had walnut oil do this before, so I'm assuming I am the culprit, rather than the grocery version of oil, but both may contribute.

Does anyone have a fix for this problem? I can't give these away as they are. Thanks in advance for any successful suggestions.

Was the roasted walnut oil "refined"? If not, then it probably wasn't just oil rancidity, but potentially proteins as well.

With oils, the double bonds don't necessarily just become single bonds, but the double bonds can break entirely leaving you with lipid fragments (which are often actually highly radical and toxic when this occurs in the human body.) If you left these in the sun, then the UV light probably accelerated the process of rancidification as well...one of the reason oils are often kept in dark green bottles, it filters out a lot of the UV light so the oils go rancid in the bottles a lot more slowly.

Only resolution I can think of, is to do as has been recommended: clean up the surface level residue, deep clean as best you can, then finish with a properly refined oil that will cure with polymerization. That should create a barrier between any of the grocery store oil deeper in the fibers, and the food-contact surface. Outside of that, I would just avoid store bought food oils and make sure you use properly refined and purified oils designed to be a finish.