13 September 2011

Dr. Science plays with wood finishes - Part 3

Today's entry is about how changing wood media can change your perspective on what a finish does.  The board change-over to one with darker tonal qualities with different hue for the grain was proposed to offer a different perspective on what each oil and oil mixture would do.  As the gunstock I am about to finish has a variety of wood grain densities, getting something that would evenly enhance those tonal qualities and yet at the same time flatten them has been a hard thing to do.  Tung oil has a light yellow cast to it when dry, BLO has a deeper yellow cast to it at the same point and Walnut oil has a light brown cast to it.  Adding Cedar oil tends to dampen hue shifts, increase contrast, thin out the above oils and has an aromatic quality to it.  Unfortunately if what I have read of Cedar oil is correct, when used as a mixture with other oils, there is then the requirement to have a small amount of it in layers going over that layer.  This is not bad if going it alone with just the pure base oils, but if using something like a pre-mixed oil there will be a further need to experiment with that pre-mixed oil solution with Cedar oil.

On to the tests so the results of a large set of test applications can be seen.  The board used was the back of luan board (not the front, but the side with the dark brown and red grain).

Columns of application mimic that of the original tests, in that they each vary the oil to solvent proportions (Straight, 3:1, 2:1 and 1:1), and then go from the Straight oils on the left, to a 3:1 then 2:1 base oil with Cedar oil mixture, then a final set of four columns of Tung oil and BLO to Walnut and Cedar at 2:1:1 and 2:2:1.

There are many problems in doing such a test, not the least of which is that as oils get lower viscosity they tend to flow with the wood grain (normally a good thing).  I had left separating columns between the Straight and then Cedar admixture columns, but could not do that for the last Cedar admixture and base oil/Walnut oil/Cedar oil mixture.  Thus final analysis is done using a relatively small area within each box to determine shift in tonal qualities and grain popping.

Three pictures are taken to demonstrate the problem of how a simple shift in lighting angle changes what you see on the board.

Recent_Arrivals_12SEP2011_ 001

Recent_Arrivals_12SEP2011_ 002

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And then an attempt to get even lighting on the board:

Recent_Arrivals_12SEP2011_ 005

Due to the nature of the board it is very difficult to do measurements as a minor change in lighting direction changes the underlying reflectance of the wood structure, thus changing what can be perceived in each sub-test area.

Grain spacing varies widely across the board, which was desired, from nearly 35 grain lines per inch (of over 75% of grain doing an edge to edge crossing) to 12 glpi in the less dense areas.

As the grain coloration is red, it has shown great contrast enhancement even with straight BLO, unlike what the white wood tests would indicate.

Yet, for all that, the initial white wood tests hold firm in that straight BLO does add a distinctive yellow hue which also tends to flatten wood contrast.  Tung oil has a light yellow hue shift to it and adds only minimally to contrast enhancement.  While Walnut oil does add a brown hue to its test areas, they are hard to differentiate as the background inter-grain color is also a light red-brown.

These changes are very hard to discern on the board, however, and one must have a constant light source that is near to the board and magnifying glass when using 1.5" x 1.0" strips to apply finish to.  In particular the reflectance of the red grain tends to wash out minor tonal variations, which makes the sparseness of it in some sections of the board vital for examination.

The final variable area on the right of the board, broken down into four columns for the Walnut and Cedar oil admixtures, does indicate that even at a 2:1:1 set of ratios that the grain does darken moderately to the brown and has somewhat better contrast enhancement.  At 2:2:1 some of the contrast enhancement is lost and there is an overall darkening of the wood grain both in growth and inter-growth areas.



This test is more confirmatory in nature and demonstrates that there is a fall-off of what Cedar oil can do for reducing gloss and increasing contrast while reducing the hue shift of other oils.  In all cases it does decrease the gloss of other oils, even at the least concentration (2:2:1) with other oils.  At that lowest concentration the hue shift of other oils starts to re-appear as a factor for consideration.


Overall Conclusions:

Finding a oil finish for beech gunstock with a variety of grain densities and tonal gradations across the stock is difficult to do.  While the most common gunstock finishing oil BLO is preferable for the look it brings to a gunstock, that is more via recent tradition and extreme ease of application than one of bringing out the natural qualities of the wood.  Once you begin to see military gunstocks from a wide variety of Nations, you begin to see that it is the oddballs that are not finished with BLO that appear out of place.  Yet with the Mosin-Nagants a simple change in growth area for the gunstock between Russia and Finland will mean a very stark contrast between the look of the gunstocks.  They are finished with shellac, not BLO, and thus have a better opportunity to show the character of the underlying wood than does BLO.

Testing relatively non-traditional oils for finishing demonstrates a better opportunity for bringing out characteristics in the wood that may then have a possibility to being visually more appealing while containing much of the flattening effect of BLO via later layers of finish.  The ability of Walnut oil to bring out the browns in wood grain is one that is both startling and pleasing, and it already has the start of the darker brown to red-brown seen in many military BLO finished gunstocks.  Tung oil, being less reactive and having a lower hue shift with it means that a base or sanding coat(s) will then modify the later final appearance to one that is lighter than normal BLO finished gunstocks.

Of particular surprise is the changes that Cedar oil brings to wood finishing.  Typically used on floors for a 'natural' looking finish, it brings very few changes to the hue of underlying wood when applied straight.  When mixed with other oils it does multiple things simultaneously: flattens the gloss, improves contrast and lightens hue shifts.  If used to make a hunting friendly firearm the utilization of Cedar oil at some level to all the finish coats requires a re-thinking of what the final characteristics of the gunstock will be.  It may be very hard to utilize Cedar oil to get its desired effects and get something close to a standard BLO or arsenal finish.  It is unknown what the effects would be of having a Cedar oil mixed topcoat on top of BLO or Tung oil coats would be, although surface adhesion should not be a problem what it will do with grain characteristics is problematical without finishing a full gunstock in that fashion.

From my initial set of parameters to examine results it is #5 of being visually appealing for beech wood that is key.  I do not mind a non-standard finish if it performs this function well as a sanding and/or base coat.  For this BLO cut with Cedar oil is preferable due to the ability to make grain pop, have high contrast and yet have a low gloss finish.  To completely finish that, however, means that #4 is impaired as once you start with Cedar oil you must either cut concentration per layer or risk having the next coat not properly adhere to it.  Aromatic oils are just that - they allow distinctive hydrocarbon chains to come off of their structure and that is something that further coats must allow.

Thus to retain #5 a relatively straight oil mixture must be used and that indicates Tung oil at 1:1 with solvent for sanding coat and inletting coat.  This dries quickly, is thin and otherwise can be easily sanded (that from experience).  At such a low concentration it does very little to shift tonal qualities, unlike BLO.  A 2:1 mixture can be used for a base coat or more of 1:1 for a thin base coat can also serve.  The first sanding coat will be sanded at 320 grit and then the base coat will be done at 400 grit.  All other coats will get a 600 grit sanding between applications.

After the sanding and base coats the major coats will either be BLO or the commercial article Linspeed.  The latter tends towards more of an arsenal or traditional finish color shift and darkening at a faster rate than does BLO.  As its name implies, it is faster drying than BLO which is a plus.  On the downside it is a commercial mixture with many unknowns involved if trying to modify it for use with other oils on application.

A hunting friendly coat can be applied last either as a Cedar admixture oil layer or mixing Cedar oil with a thinned wax.  A wax may be preferable as it can be re-applied as needed and removed in a relatively easy fashion if the Cedar oil scent is not wanted for storage or use elsewhere (at the range, say).

Some variation to this can happen if the need to bring out browns is needed between coats, and a thinned layer of walnut oil can be used for that.



- Typical amounts used for application were in the 5ml range for oils, and adding solvent reduces viscosity.  A typical acid brush can hold 5ml easily but cannot coat four areas of 1.5" x 1.0" when only Straight oil with no solvent is used.  At all other ratios, 5-10ml of finish can easily coat that much, and at the high end there is plenty left over as the application fluid is so thin.

- Luan board is not a good substrate for testing finish types to examine tonal gradations.  Doing a comparison test piece examination over the board strains the eyes and counting the grain lines is a difficult process.

- Pipettes were failing during this test as the bulbs on many of them were breaking apart.  By the end of the test only one of the original four pipettes had survived the testing situation.  A bulkier pipette from a Testors paint kit was substituted and washed frequently.

- It is possible to get stainless steel cups squeaky clean.  This is required between each test set-up.

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