10 September 2011

Dr. Science plays with wood finishes - Part 2

Today was the examination of results from yesterdays tests started with combination of oils for a first coat, sanding sealer finish for a beech wood gunstock.  The tests have been done on anonymous 'white wood' bought in 1x6 boards from the local Home Depot, and yesterday's tests gave some good indications on some general directions to go in this.  One of the minor points for the finish (making it woodland scent compatible) meant the utilization of Cedar oil as one of the finish types.  On its own, while having a nice aroma, it doesn't do much, but does leave a very thin finish on the wood.  From that the question of what happens when both Tung and Boiled Linseed Oil are mixed with Cedar oil was to be examined.  Also started was a new oil typically not used for modern gunstocks that will be presented a bit further on.

All methodology remains the same, save that both boards used for the test got a 2 hour blotting to get excess oil off of them, so that there wouldn't be any gumming as had happened with BLO in the previous test.

Two ratios of main oil to Cedar wood oil were done: 3:1 and 2:1.

Oil to solvent continues to be varied by: Straight oil mix, 3:1 oil to solvent, 2:1 and 1:1.

Lighting remains similar, which is poor.

Humidity has varied in a range between 40 and 60%, but temps have been constant around 70 F.

All other apparatus and items remain the same.

Now to the board:

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The top two rows are Tung oil and BLO with a 3:1 ratio to Cedar wood oil, and the bottom two are at 2:1 in the same order.

In the previous test BLO has not been applied to fine grain wood and that happened in this test in the 2:1 ratio, bottom row.

Now the straight and the mixture boards side by side:

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Some of the reflection off of the BLO middle row on the left is part of what the finish looks like.  It has a somewhat glossy finish when used straight with solvent.

This is in contrast to being mixed with Cedar oil at both 3:1 and 2:1 ratios.  There is less of the noticeable yellow cast from BLO in rows 2 and 4 on the right and the long, sparse wood grain is now clearly popping out visually.  Additionally the fine grain on the bottom shows that BLO mixed with Cedar wood oil improves definition and contrast of closely spaced wood grain without adding a yellowish cast to it (at 2:1).  BLO and Cedar wood oil seem like a very good mix of oils to gain high contrast in light woods while dulling the gloss of the finish.

Tung oil has behaved similarly in losing some of its cast from the finish and having a higher definition of difference between the light and dark colors of the wood grain.

What Cedar wood oil brings to both Tung and BLO is to increase contrast, decrease any yellowish cast to the finish and generally improve the color that is brought by the oils to the wood grain.  For percentages of 25% and 33% (1 part in 3 and 1 part in 2) this is fascinating.  Cedar wood oil is a contrasting agent, gloss flattener and reduces native oil viscosity of Tung and BLO, so that even when used straight it both oils flow easily off of the brush and not like syrup.

Next up is the third oil being used:

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This is Walnut oil used to finish salad bowls and some wooden eating utensils and has previously been used with cabinetry and even gunstocks before the modern era.  Here the test is with straight oil to solvent in the top row, 3:1 to cedar oil in the middle row and 2:1 in the last row.  The columns continue to be Straight (no solvent), 3:1 oil to solvent, 2:1 and 1:1.

Another shot to reduce some of the reflection on the finish:

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Walnut oil for this light wood is a different experience than either Tung or BLO as it introduces a deeper brown to the grain, while Tung brings a yellow-brown cast and BLO a yellow cast to it.  On both tight grain and widely spaced grain, Walnut oil has a deep effect on coloration and contrast when dried.  As with both BLO and Tung oil, Cedar wood oil brings a higher level of contrast when used with Walnut oil.  For light woods this is a very visually appealing finish at one coat, and it is food safe as well at least for the straight oil.

Now a comparison of Walnut oil to the other straight oil board, with the rows being Tung oil, BLO and Cedar wood oil.

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The browns brought out in Walnut oil are similar to that brought out by Tung oil, but without a very light yellow cast that Tung oil has.  The cast is, instead, light brown which tends to flatten contrast with Walnut oil even while making the grain appear deeper.  The starkest contrast is the 3:1 Walnut:Cedar blend and straight BLO with similar wood grain: where BLO does little to enhance the contrast and brings a distinctive yellow cast and gloss to the finish, the Walnut/Cedar mix deepens the browns and has a flatter gloss to it.  At the highest proportions of Cedar oil and solvent (at the lower right) Walnut oil is losing some of the stark popping of the grain as seen in all other parts of the Walnut treated board but still compares favorably to straight Tung oil at high solvent mixture (1:1).

Now for all three boards side by side and a few shots to try and get some perspective on conditions:

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There has been no color compensation done to these pictures nor to ones in the previous post. Most of the deep yellow cast comes from lighting conditions, not the finish or wood itself, at this point, and tonal variations only show up on close inspection.

Conclusion: The one defect of this set of tests is that they are done on 'white wood' and not old grade beech stock.  Tentative conclusions are just that, and not indicative of what will happen on a darker wood with less natural contrast to it.

Some general conclusions about Cedar wood oil as an additive to regular finishing oils can be made, however.

First - It is a very light oil for finishing wood, and does not leave a thick finish like BLO does.

Second - It imparts very little tonal shift to the wood it is on as a finish when used straight.

Third - As an additive it performs many functions to the finish of other oils. It is not a simple 'does one thing additive'.

Fourth - When added to Tung oil, BLO or Walnut oil, Cedar oil tends to dull gloss in the finish.

Fifth - Also when added to the above it reduces yellow and yellow-brown tonal shifts.

Sixth - Also when added it enhances tonal shift on the dark part of wood grains.

Seventh - Taken cumulatively as an additive it is a contrast enhancer and gloss flattener.

Eighth - When added to other oils it decreases viscosity and drying times of the other oils tested.

If pressed at this point I would go with either a Tung oil or BLO mix with cedar oil at 2:1 concentrations as a sanding coat, and then add small amounts of cedar oil to all further coats for adhesion.  Walnut oil might do for a second to last finish coat if browns needed to be deepened and brought out visually.



When prepared in tiny batches of <10ml in size, Cedar wood oil has a pronounced change to the viscosity of other oils.

Areas prepared with Cedar wood oil mixtures dried faster even when no solvent/drier was added to the mixture, although after two hours there was still visible oil on the surface of all samples, they were beginning to be puddles.

At concentrations above 25% of Cedar oil in a mixture and 1:1 solvent ratios, the drying time was approximately 20 minutes.  At 2:1 they were upwards of 60 minutes, after which only droplets were left on the surface.

Walnut oil is of lower viscosity to start with and runs easily with even a small amount of solvent added to it.

When making a Walnut oil mix with citrus solvent, one gets hungry as they are reminded of salad dressing.  Prepare only after meals, not before, as it is distracting.

A thorough set of tests will need to be done on luan board on the dark wood side as it is a good representative of old beech wood, save the ring grain is red, not brown.  This will take a couple of days to set up.

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