Converting whiskey barrels for furniture
Whiskey barrels

Whiskey barrels are flamed at Antonio Páez Lobato cooperage before shipping to distilleries and wineries.

Photo By Antonio Páez Lobato, S.A.

Editor’s note: Gene Wengert received the following question about whiskey barrels from a client. His answer actually has several ideas that will apply to various furniture operations. So, read on.

Q. After a whiskey barrel, made of white oak, is used, it cannot be reused for whiskey. However, we can recondition it and use it for wine. This requires the removal of the char inside the barrel. (A newly made barrel, called cooperate, is burned on the inside. Most notably, the whiskey itself is clear when manufactured, so the char adds the brown color expected. Charring also affects flavor.). When we remove the char inside, the staves become thinner, as we have to remove the char and a little discolored, solid wood. How much does this weaken the stave as we do not want over $400 of wine on the floor?

A. First if you take the hoops off the barrel after the first use, the staves will not return to the original flat shape, but will be curved. 

The curve in the barrel is called the bilge. So, what this means is that the high stress experience when first made has been relaxed or relieved — not all stress, but some. 

The greatest stress on the wood and the greatest chance of breaking a stave is when first manufactured and put into use.
As the stress from bending relaxes, this gives the barrel the ability to safely hold more weight or liquid without breaking. 

Simplified, this second-use barrel is stronger now than when first made, assuming it was not damaged in manufacturing or the first use.

From a furniture point of view, the greatest risk of damage is soon after the piece is manufactured. This risk drops as months go by partly due to relaxation.

Second, we are concerned about the weakest stave, as that will break first. We are not concerned about the average or above average pieces. 

Unfortunately, the books about wood strength, including white oak, give us average values for clear, knot free, average growth rate, etc., wood. I do not know how anyone can relate these average values to the weakest stave in a barrel. 

In fact, in the old days, when developing useful strength values for a piece of wood in use, the old way was to use 1/6 of the clear wood value for design. I suspect no one did that for cooperage, but coopers did learn from experience what strength they needed.
Third, as you indicated, MOR (modulus of rupture) is the critical number for a stave breakage. 

The MOR is very sensitive to both width and thickness, as well as length. For thickness alone, the relationship is that twice as thick is four times stronger. (Consider a 2x4 on edge and flat-wise.) So, the removal of solid wood when “cleaning” char from a used  barrel will have a big effect on residual strength. For example, a 1” thick solid piece machined 1/8” thinner will lose nearly 25% of its MOR.

That brings us to the question: Which piece is mostly like to fail? 

The answer is a piece that already has a failure or damage. 

Also, growth rate is a factore; slow growth (more than 10 rings per inch is weaker than fast growth (4 rings) in white oak. 

Because we know that flat grain, versus quarter grain, is much more likely to suffer damage when making a barrel, it makes sense that grain that is not quite close to perfectly quartersawn is going to be weaker.

In summary, all wood is not the same. A visual inspection for damage, grain and growth rate should help in identifying risky staves and barrels. 

Minimal wood removal to maximize thickness is important, small fractions are important. 

With these steps, the customer of the remanufactured barrels will know that you have done the best possible to assure quality. 
Maybe a statement from a lawyer about liability is also prudent.

Q. I am buying green un-steamed walnut from a mill and having it custom kiln dried. After drying, the wood has a green cast to it even after surfacing. Please help.

A. The greenish cast is normal for fresh walnut lumber. The greenish-colored chemicals then oxidize and turn walnut brown. 
Steaming before drying does indeed create very fast oxidation. 

Even kiln drying at temperatures above 155F also helps. Overall, the color change is easier and faster at higher moisture content.
Once dry, exposure to sunlight will darken the surface slowly, but I cannot guarantee that the darkening will go very deep — probably several 1/100” only. 

A string oxidizer like ammonia will also darken the surface of dry wood.

The best cure is to pre steam at 195F to 208 F at 100% RH prior to drying. 

Air drying does NOT encourage darkening of walnut, so the sooner the lumber is in the kiln, the better the color, if the kiln is run correctly.

Q. I want to put in a solar drier to dry my wood, but I do not have electricity available in the sunny site. I assume this is okay but might take longer. Comments?

A. I will be honest with you. From a technical point of view, the lumber will dry, although very slowly. 

Slow drying means more warp, especially cup. It also means mold and mildew, plus other discolorations. I expect the top layers of lumber in the pile will over-dry. So, i cannot support this approach.

Also, consider that the price difference between dried lumber and kiln dried lumber is often over $500 per 1,000 BF. 

I would argue that with such a large increase in value with proper drying, you cannot afford to be careless or slipshod when drying. The nice profit is there, so do it right.

Q. It is a long story, but we have some artist-painted cypress benches that we are using to raise funds for our foundation. Unfortunately, the paint is coming off now a month or so after the benches were painted. It is not every board, but overall it is a disaster. Help.

A. After discussing this in detail with you on the phone, it is clear that some of the lumber used was from trees that were bacterially infected. 

The bacteria, active only in the living tree, created some chemicals (among other things that they do when alive) and these chemicals interfere with the attachment of finishes to the wood where the bacteria had been living.  

In short, this is the wrong species for this project. 

There is no cure for this undesirable characteristic. 

Note that we see an abundance of these bacterial effects in lumber from the bottom log of the tree in many species, especially those grown in wet sites. Hemlock is another problem species.

Q. I am using quartersawn white oak and red oak. The problem I am having is that the color of,the heartwood varies quite a bit, from time to time. This variation is causing appearance problems. What gives?

A. One of the beautiful aspects of wood is that no two pieces of wood are the same. 

So, small variations in color of the same species will occur. I suspect that soil nutrients play a role.

Further, we know that drying speed affects the heartwood color as the chemicals in the heartwood oxidize to different color depending on drying temperature and drying speed. This coloration is hard to predict ahead of time.

In addition to this natural variation, you have to appreciate that there are 20 commercial species of red oak and 20 species of white oak. 

So, the red oak lumber group has 20 various species of oak trees; the same for white. 

As a small example, within red oak we can have southern red, northern red, cherry bark, pin, black, and more. Each oak species has its own particular leaves, acorns, and heartwood color.

Here in Georgia, my backyard forest has four species of oak. 

Sawmills do not separate the species, other than red and white groupings.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.