But first, a more important question, why oil your flute at all? There is a mistaken idea that oil prevents a bamboo flute from cracking and it may but only indirectly. Oiling a flute creates a barrier layer which slows the absorption and/or loss of moisture. To be clear, what we're talking about here is a vapor barrier. What we're concerned with is water in vapor rather than liquid form.

Bamboo varies greatly in 'crackability' depending on growing conditions, time of harvest, soil nutrients and probably a host of other factors. Which is to say, bamboo varies greatly in it's dependency on balance of internal moisture to retain it's structure. So it's hard to know if your flute will crack until it does. A proper oil coat serves as a barrier to vapor transmission. Also it reveals and heightens the beauty of the wood, along with protecting it from stains, bumps, etc. A good oil finish makes your flute shine and has a wonderful sensual feel.

Daubing on some olive oil when your flute looks dry is probably a bad practice. As you'll see below, olive oil is a poor barrier oil and oil in general doesn't replace moisture. It's a mistake to mentally equate moisture and oil. As far as oil and moisture go, oil is like a plastic raincoat--something to stop (slow?) the transmission of vapor.

A number of vegetable oils have the property of drying to form tough, adhesive films either by themselves or when assisted by the action of added ingredients (catalysts). These oils do not "dry up" in the ordinary sense of the evaporation of a volatile ingredient, but they dry by oxidation or absorption of oxygen from the air. The drying process is accompanied by a series of other complex chemical reactions, and the dried oil film is a new substance which differs in physical and chemical properties from the original liquid oil; it is a dry, solid material which cannot be brought back to its original state by any means.

The drying properties of most oils are due to the presence of glycerides of linoleic and linolenic acids, which have the property of combining spontaneously with atmospheric oxygen to start a chain of reactions which ends in the conversion of the oil to the tough, durable, insoluble, waterproof, plastic material know as linoxyn.

Oils:

The word 'dry' as applied to vegetable oils is a euphemism for hardening and oils differ in their ability to dry. In the table below oils are listed with their Iodine Value which is a chemical indication of hardening ability. Non-drying oils never harden satisfactorily and will remain forever soft and gummy.

For our purposes the top three oils are linseed, tung and walnut--in order of toughness, hardness, and reflectivity. Linseed is the shiniest of the three and is the primary oil used in artistic 'oil' paints and painting. It is the preferred oil in respect to art durability, luminance, and archival quality. Tung oil is used mostly as a wood and furniture finish. Our current home has soft pine flooring finished with tung oil which has held up admirably well over many years. Walnut oil is used in wood finishing and consumed as a salad oil. Wood finishing oils are usually sold with added catalysts (or as partially polymerized) which speed drying time.

Non-catalyzed oils: It takes some searching to find linseed in a non-catalyzed form. Tung oil is readily available in both forms and walnut oil is usually only available as non-catalyzed. Although all three oils are edible, walnut oil is the only one available in the oil section of your grocery store. Basically, the only difference between catalyzed and non-catalyzed oils is the time it takes for the oil to dry, harden and cure. Catalyzed oils dry in 10-24 hours, non-catalyzed will take 75-90 days. That walnut oil from Hain you may have been applying to your flute takes three months to mature, during which time oils go through sticky to gummy to semi-hard to hard phases. Should you have any doubt about the drying time of non-catalyzed oils pour out a thin layer into a lid (or other thin flat container) and observe the hardening process over its course.

These three oils can be mixed in any proportions you wish, both by type and catalyzation. If you're mixing catalyzed and non-catalyzed you may need to do some tests to determine drying time. The basic chemistry of all three oils is, for all practical purposes, identical.

Non-catalyzed oils need light in order to fully polymerize. Flutes oiled and then stored away will never mature. The ideal conditions are room temperature, normal humidity and substantial (but indirect) light--for three months. The presence of normal or diffused daylight is necessary for the correct drying of an oil, but any prolonged exposure to the direct rays of the sun is harmful, leading to cracking of the oil coat by reason of too rapid or violent drying action.

Applying oils with an Iodine Value of under 140 is an exercise in futility because the oil won't harden sufficiently to create a vapor barrier. Skin oil (sebum), for example, never achieves hardness. And without adequate hardness the oil won't be reflective enough to create a warm beautiful finish. Stick with the top three oils.

You know the old warning about being careful with oily rags--that they can spontaneously combust if stuffed in a drawer? Well, here's the reason: when drying oils dry (harden), the reaction is exothermic--it gives off heat as oxygen is being consumed. And with greater surface area in the rag and so on the stuff can ignite. Now you know--it's about oil polymerizing.

Now for the bad news!

Of the three oils mentioned, only tung oil comes anywhere close to creating a suitable vapor barrier and only after several coats. Even then it is substandard compared to other finishes. Linseed and walnut oils are both inferior when it comes to creating a vapor barrier--to the point of being essentially nonexistent. Both linseed and walnut oils will dry, but pass vapor easily. Compared to a varnish (alkyd, phenolic, or polyurethane) oil finishes aren't even in the running as far as providing a suitable vapor barrier!

So the question again: Why are you oiling your flute? For appearance or as a vapor barrier? Tung oil does admirably well for appearance and will provide some protection as far as vapor transfer. A good varnish will serve both purposes.